Micro-Cavity Fluidic Dye Lasers

M.Sc. Thesis Bjarne Helbo Student Number: c960336

Supervisors: Anders Kristensen and Aric Menon

Mikroelektronik Centret (MIC) Technical University of Denmark (DTU) November 2002 Abstract i

Abstract

The work described in this masters thesis deals with development, fabrication, and optical characterization of micro-cavity fluidic dye lasers. The wide band fluorescence of organic dyes make them suitable as the active gain media for tunable dye lasers. Decreasing the laser cavity size down to the micron level makes dye lasers suitable for integration with existing bio/chemical microsystems. Theory for organic dyes is presented together with basic laser theory. The theory is used for explaining the behavior of the fabricated devices. Two types of micro-cavity fluidic dye laser devices were fabricated based on two different micro-fabrication schemes. The devices were vertical emitting with fixed lasing wavelength. The initial device was a microfluidic channel defined with an KOH etch of silicon. The bottom (100) plane of the etched silicon channel was used as a deposition surface for a gold/chromium mirror. The etched surface was not smooth enough for mirror purpose and the following anodic bonding of a glass lid on the top made the surface even more rough. Due to thermal heating from the anodic bonding process the metals diffused into the silicon substrate and left the surface dull and rough, not suitable for optical mirrors. A cw argon ion laser (488 nm) was used for optical pumping of a Rhodamine 110 dye dissolved in ethanol, which was pumped through the microfluidic laser cavity. The optical spectrum emitted from the device only showed fluorescence. The second generation device was based on a SU-8 photoresist. The microfluidic channel was defined by UV-lithography and development of the 10 µm thick SU-8 photoresist layer. The SU-8 was spin-coated on the top glass wafer with the semi-transparent gold/chromium mirrors. The bottom glass wafer with a zero transparent gold/chromium deposited mirror was spin-coated with a SU-8 layer used for polymer bonding. The SU-8 polymer bonding technique was optimized for ensuring a better bonding quality. The SU-8 laser device was optically pumped by a pulsed frequency doubled Nd:YAG laser (532 nm). Rhodamine 6G dissolved in ethanol was used as active laser medium and was pumped through the SU-8 fabricated microfluidic channel. Lasing was obtained from devices containing dye concentrations ranging from 10−2 to 10−1 mol/L. The lasing peak wavelengths ranged from 570 to 581 nm depending on the dye concentration. The lasing wavelength was increased with increasing concentration in agreement with existing theory. Resumé ii

Resum´e

Denne kandidat afhandling beskriver arbejdet med udvikling, fabrikation og optisk karak- terisering af mikrovæske lasere. Organiske farvestof molekylers brede fluorescens spectrum gør dem brugbare som det aktive forstærkningsmedium i farvestof lasere med regulerbar bølgelængde. Skalering af farvestof lasere ned til mikro niveau gør det muligt at integrere dem med eksisterende bio/kemiske mikrosystemer. Teori omkring organiske farvestof molekyler og lasere er præsenteret i rapporten. Teorien bruges til at forklare opførelsen af de fabrikerede laser komponenter. To typer mikrovæske lasere er blevet fremstillet med hver sin fabrikations procedure. De fremstillede komponenter er vertikale lysende lasere med en fast bølgelængde. Den første type komponenter bestod af en mikrovæske kanal defineret med en KOH æts i silicium. Kanal bunden, som er en (100) plan, blev dækket med et guld/chrom lag som optisk spejl. Den ætsede kanal bund viste sig ikke at være jævn nok til at fungere som spejl. Den efterfølgende anode forsegling med et glas l˚ag viste sig at ødelægge den optiske overflade af metallerne. Den tilførte varme under anode forseglingen, fik metallerne til at sive ind i silicium substratet, hvilket efterlod overfladen meget ru. En kontinuert lysende argon ion laser (488 nm) blev brugt til optisk at stimulere det optiske farvestof Rhodamine 110 opløst i sprit, som blev pumpet igennem mikro laser kanalen. Det udsendte optiske spectrum fra komponenten indeholdte kun fluorescens. Den anden generation af komponenter var baseret p˚a fabrikation med den foto lito- grafiske polymer SU-8. Mikrovæske kanalen blev defineret i et 10 µm tykt lag SU-8 ved hjælp af UV-litografiog derefter blev SU-8 laget fremkaldt. SU-8 laget var spundet p˚a top glas pladen, som var belagt med et delvist transparent guld/chrom spejl. Bund glas pladen belagt med et ikke-transparent guld/chrom spejl, blev dækket med et tyndt lag forseglings lim best˚aende af SU-8. SU-8 laget blev brugt som lim i en forsegling mellem top og bund glas pladerne. SU-8 polymer forseglings metoden blev optimeret s˚aledes at forseglings kvaliteten blev bedre. SU-8 laser komponenten blev optisk stimuleret ved hjælp af en frekvens fordoblet pulseret Nd:YAG laser (532 nm). Rhodamine 6G farvestof opløst i sprit blev brugt som det aktive laser medium, som blev pumpet igennem mikrovæske kanalen fremstillet i SU- 8 polymer. Laser lys blev udsendt fra komponenten, n˚ar farvestof koncentrationen l˚ai omr˚adet fra 10−2 til 10−1 mol/L. Bølgelængden, hvor laser lyset havde sin spids værdi, l˚ai bølgelængde omr˚adet fra 570 nm til 581 nm afhængig af farvestof koncentrationen. Laser bølgelængden viste sig at stige med stigende farvestof koncentration i overenstemmelse med den eksisterende teori. Preface iii

Preface

The M.Sc. thesis was carried out at Mikroelektronik Centret (MIC) affiliated with the Technical University of Denmark (DTU). The thesis title is ”Micro-Cavity Fluidic Dye Lasers”. The thesis finishes the education for the civil engineering degree and equals 50 ETCS credits. The work done was under supervision by associate professor Anders Kristensen and professor Aric Menon. The thesis describes the work, which has been carried out the during the last year. The work includes a theoretical study of dye lasers, experimental cleanroom work for micro-processing, and optical characterization. The work has ended up with an accepted abstract for the international conference IEEE MEMS 2003 held in Kyoto, Japan. Further a manuscript was submitted to the Journal of Micromechanics and Microengineering for revision. I would like to thank Anders Kristensen and Aric Menon for giving me the opportunity for doing my masters thesis in the MEMS group at MIC. Further I would like to thank Anders Kristensen for always having time to creative inputs especially during the writing of the manuscripts for the publications. Then a thank goes to my girlfriend Maiken for supporting me and for reading through my thesis. Also I would like to thank my brother Carsten and his girlfriend Lene for also reading through my thesis. Then I would like to thank the MEMS group, the lab staff and other MIC employees for always being very helpful. Last a thank goes to the students in the studentroom for making things a bit more fun. The graph shown on the frontpage is a critical inversion ratio surface determining the required concentration of excited molecules to reach lasing in a dye laser. The critical inversion ratio is a fundamental concept for dye lasers and is dealt with in this thesis.

Bjarne Helbo Contents iv

Contents

1 Introduction 1 1.1Motivation...... 1 1.1.1 Microtechnology Meets Biotech and Analytical Chemistry . . . . . 1 1.1.2 Dye Lasers for µ-TAS...... 3 1.1.3 Initial Considerations for a Microﬂuidic Dye Laser Device ...... 3 1.2GoalsfortheProject...... 4

2 Theory 7 2.1ChapterOutline...... 7 2.2HistoryoftheLaser...... 8 2.3BasicPrinciplesofLasers...... 8 2.3.1 TheActiveLaserMedium...... 8 2.3.2 TheLaserPumpSourceandResonator...... 9 2.3.3 PopulationInversion...... 10 2.3.4 Round-trip Gain and Threshold ...... 11 2.4OrganicFluorescentDyes...... 13 2.4.1 BondingTheoryforOrganicDyes...... 13 2.4.2 Physical Properties of Organic Dyes ...... 14 2.4.3 TheXantheneDyes...... 18 2.4.4 DyesasLaserMedia...... 19 2.5RateEquationModelofDyeLasers...... 22 2.5.1 Time-dependent Coupled Rate Equations ...... 22 2.5.2 A Simple Stationary Solution for the Threshold Condition . . . . . 24 2.6Mirrors...... 29 2.6.1 Optical Modelling of Multi-layers ...... 29 2.6.2 DielectricMirrors...... 31 2.6.3 Metallic Mirrors ...... 31 2.7TheFabry-PerotInterferometer...... 34 2.7.1 TheAiryFunction...... 34 2.7.2 Stability and Cavity Modes ...... 35 2.7.3 Frequency Pulling of the Lasing Mode ...... 37 2.8FluidicPropertiesinMicroﬂuidicChannels...... 38 2.8.1 FluidicFlowinMicrosystems...... 38 2.8.2 BasicFluidicFlowTheory...... 38 2.9ChapterSummary...... 42 Contents v

3 Design and Fabrication 44 3.1ChapterOutline...... 44 3.2ConsiderationsAboutDeviceRequirements...... 45 3.2.1 Round-trip Gain in Micron Sized Cavities ...... 45 3.2.2 MirrorOptions...... 46 3.2.3 Excitation and Laser Light Collection ...... 46 3.2.4 DyeFlowSpeed...... 47 3.2.5 FluidicConnections...... 48 3.3DeviceRequirements...... 49 3.4 Fabrication Schemes for the Micro-Cavity and Layout ...... 50 3.4.1 KOHAnisotropicEtch...... 50 3.4.2 SOIWafers...... 51 3.4.3 SU-8Photoresist...... 51 3.4.4 DeviceLayout...... 52 3.5 Bonding and Sealing of Microﬂuidic Channels ...... 52 3.5.1 AnodicBonding...... 53 3.5.2 PolymerBonding...... 53 3.6FabricationProcessDescriptions...... 55 3.6.1 KOHEtchBasedDevice...... 55 3.6.2 SU-8PhotoresistBasedDevice...... 56 3.7 Fabrication Results for the KOH Etch Based Device ...... 58 3.8 Fabrication Results for the SU-8 Based Device ...... 61 3.9ChapterSummary...... 63

4 Optical Characterization 64 4.1ChapterOutline...... 64 4.2ExperimentalMeasurementSetups...... 65 4.2.1 ArgonIonLaserSetup...... 65 4.2.2 Nd:YAGLaserSetup...... 67 4.3ImportantDeviceParameters...... 69 4.3.1 Facts About the Optical Measurements ...... 69 4.4MeasurementResults...... 70 4.4.1 KOH Etched Chip with Argon Ion Laser Pumping ...... 70 4.4.2 SU-8 Chip with Nd:YAG Laser Pumping ...... 71 4.5 Discussion of Results Obtained from the SU-8 Dye Laser ...... 77 4.5.1 OpticalPathLength...... 77 4.5.2 PeakWavelengthandLinewidth...... 78 4.5.3 Threshold Power and Critical Inversion Concentration ...... 79 4.6PerspectivesandOutlook...... 83 4.7ChapterSummary...... 84

5 Conclusion 85

Bibliography 88

List of Figures 93 Contents vi

List of Tables 99

Appendices 101

A Fabrication Process Sequence for SU-8 Resist Based Chip 101 A.1ProcessSequence...... 101 A.2ProcessRecipes...... 103

B Fabrication Process Sequence for KOH Etch Based Chip 105 B.1ProcessSequence...... 105 B.2ProcessRecipes...... 111

C Fresnel and Power Coeﬃcients 113

D Modelling of Optical Reﬂection and Transmission 114

E Gain and Laser Threshold for cw Lasers 117

F Refractive Index of Gold (Au) 119

G Refractive Index of Aluminum (Al) 120

H Refractive Index of Chromium (Cr) 121

I Lithographic Masks for Chips Fabricated with SU-8 122

J Lithographic Masks for Chips Fabricated in KOH Etched Si 123

K Measurement Series 124

L Publications 125 Chapter 1 Introduction 1

Chapter 1

Introduction

1.1 Motivation

Micro total analysis systems (µ-TAS) or lab on chip components have shown their potential over the last years as future sensors for especially the field of bio/chemical analysis. The field is wide from DNA array chips to chemical analysis chips. As the field of microsystems is becoming a mature technology ready for production, the level of system integration is increased all the time. In the field of optical microsystems integration of optical waveguides and microfluidic channels, which are suitable for various bio/chemical analysis is looking promising [18]. Light sources for microsystem are fabricated with III-V semiconductor materials which mostly are used for telecommunication. III-V materials like gallium arsenide are not easily integrated with microsystems for which the preferred material is silicon. Growth of III-V materials on silicon substrates is no durable since the lattice constants do not match. Also the emission wavelength of semiconductor lasers is rather fixed except for more expensive semiconductor alloys or quantum well lasers. Enhancement of optical microsystems will clearly be provided by light sources with easy add-on or integration which allows for compact devices and stand alone operations. Organic dye lasers might prove to be a new generation of micron sized monochromatic light sources [28]. Organic dye lasers provide the interesting property of tunability of the emission wavelength, which will increase the flexibility of analytical chemistry or bio analysis equipment realized with microsystems.

1.1.1 Microtechnology Meets Biotech and Analytical Chemistry Microsystems fabricated by means of microtechnology have the advantage of small sizes comparable to physical objects, which are going to be measured or manipulated. For instance biological cells are in the size of microns. The reaction speed of chemical reactions is diﬀusion limited if no mixing is present. Diﬀusion can only be altered by means of the temperature as shown here:

k T D = B (1.1) 6πηR Chapter 1 Introduction 2

D is the diffusion coefficient. R is the radius of the particle considered. η is the viscosity and kBT is the thermal energy. Having defined the diffusion coefficient the time t,which it takes a particle to travel a distance d is given by:

d2 t = (1.2) 2D

Reducing the distance d or the reaction chamber volume will reduce the time before a chemical reaction can take place. This means that microsystems with microﬂuidic channels, chemical reaction chambers etc. have the advantage of fast reaction times due to their small reaction volumes, which also means low reagent consumption. Chemical substances can be analyzed by means of absorption measurements with monochromatic light sources as lasers. The concentration for a solution can be determined by Lambert-Beer’s law if the absorptivity ε is known for the solution: P A =log 0 = εcL (1.3) P

A is the absorbance or optical density (OD). c is the concentration and L is the path length. P0 is the reference power and P is the measured power after absorption by the solution. Absorbance measurements requires a monochromatic light source with a specific wavelength depending on the substance which are going to be analyzed. Due the wavelength dependence of the absorption by a chemical substance the light source should ideally be monochromatic. A monochromatic light source with a wavelength corresponding to the wavelength for maximum absorption in substance will give the highest sensitivity of the measurement. The disadvantage of most lasers is their laser medium, which allow only specific emission wavelengths and thereby reducing the flexibility. Tun- ability of light sources in analytical chemistry systems will greatly enhance the flexibility and the sensitivity of such a system.

Figure 1.1: Spectra for diﬀerent laser dyes. Supplied by Exciton Inc. 1.1 Motivation 3

1.1.2 Dye Lasers for µ-TAS Tunability of light sources used for analytical chemistry or other kinds of measurements or sensing tasks can be provided by dye lasers. The active medium of dye lasers consists of organic fluorescent molecules contained in a solvent or host matrix. Fluorescent molecules compared to other light emitting materials have a very wide fluorescent optical band from around 100 nm to 200 nm for a single dye. A series of output spectra are shown in figure 1.1 for a whole range of different commercial available laser dyes. Before use, the organic dyes need to be dissolved in a solvent for instance ethanol. As seen from figure 1.1 the whole visible spectrum is covered with different laser dyes. A long term research goal could be to build a microfluidic system, which can transport a dye solution to a laser resonator, which allows for realtime wavelength tuning and perhaps change of dye solution. The dream would be to have a micro-cavity laser with the ability to lase in the whole optical spectrum integrated into a single microsystem with pumps and dye reservoirs for stand alone operations.

1.1.3 Initial Considerations for a Microfluidic Dye Laser Device In order for a dye to emit fluorescence or laser light an external excitation or pump source is needed. Semiconductor lasers as pump sources could be a possibility. The pump source does not necessarily have to be integrated with the microfluidic laser system but coupled with a fiber to the microsystem. A solution for the integration problem between silicon based technology and III-V will probably be solved in the near future as demands will increase. This project will not deal with the concern about integration of external light sources. Evaluation of organic dyes as laser media in micron sized cavities is of greater importance at this initial stage of this new research field at MIC. Figure 1.2 shows an idea of for a sealed microfluidic channel with integrated vertical laser cavity mirrors.

Laser Pumping Beam

Dye Flow

Laser Emission

Figure 1.2: Sketch of a microﬂuidic dye laser with horizontal emission through a waveguide with external optical pumping through the glass lid which seals the microﬂuidic channel. Chapter 1 Introduction 4

A laser dye is pumped through the channel and an external laser is used for optical pumping of the dye. A laser field is build up between the vertical mirrors and emitted through the integrated waveguide for use in for instance an absorbance measurement. This setup would be ideal with regards to integration with other microfluidic systems since the laser light can be guided around the chip and be used for various measurements. Fabrication of vertical mirrors in a sealed microfluidic channel will not be an easy task, which leads to the suggestion that vertical emitting micro dye lasers could be a better choice for initial experiments. Microtechnology processes in general are better suited for control of smoothness in lateral direction than for smoothness control of vertical sidewalls. Smoothness is required for having uniform reflectance in order to realize mirrors of good quality. From these consideration it can be concluded that a fabrication of a vertical emitting laser device will be far the easiest to fabricate compared to a horizontal emitting device. A sketch of a vertical emitting device is shown in figure 1.3.

Figure 1.3: Sketch of a microﬂuidic dye laser with vertical emission through the glass lid with external optical pumping through the glass lid which seals the microﬂuidic channel.

The first important question that needs to be answered, is it feasible to produce enough laser gain in a cavity with a length of around 10 µm. If the answer is negative one does not need to go along with the next problem of consideration. Therefore it is important to have a simple device which can give interpretable answers to well defined questions. It will not make any sense to fabricate a time-consuming sophisticated device if fundamental physics tells us that it will be impossible to obtain lasing in such a little device. Besides a complex system makes interpretation of experimental results harder. For simplicity a vertical emitting device with a microfluidic channel for guiding the dye with integrated mirrors would be ideal for performing initial measurements and characterization of the optical properties of a dye contained and optical pumped in a micro laser cavity.

1.2 Goals for the Project

The discussion in section 1.1 revealed that dye laser integration into microsystems will be an interesting research ﬁeld mainly due to tuning ability of the dye laser, which could prove to enhance existing optical µ-TAS devices. It was reasoned that the initial measurements should be performed on a very simple device, which will emit vertically, orthogonal to the device surface. 1.2 Goals for the Project 5

Tuning of lasers is usually done by means of movable mirrors. However fabrication of movable mirror with microtechnology is a difficult and time consuming task. Therefore the fabricated devices in this project will have a fixed cavity length and thereby a fixed lasing wavelength. Along with experimental activities a literature study on different areas concerning microfluidic dye lasers will be needed for rough calculations in the design process and later on for understanding and interpreting experimental results. Topics which have to be investigated can be divided into two groups namely theoretical topics and experimental topics. The theoretical topics will deal with theory which will predict the operation and behavior of the micro-cavity dye laser. A list of theoretical topics which ideally should be investigated during this project are shown here:

Theoretical topics

• General properties of lasers.

• The mechanisms of absorbtion and ﬂuorescence by dyes.

• Gain properties of organic laser dyes.

• Mirror reﬂectance and transmittance.

• Cavity modes in microcavities.

• Hydrodynamics of liquids ﬂowing in microﬂuidic channels.

The experimental topics will deal with issues of fabrication and measurements. A list of the experimental topics, which will be dealt with is shown here:

Experimental topics

• Fabrication of laser cavities in microﬂuidic channels.

• Fluidic connections.

• Excitation method.

• Measurement methods and characterization parameters.

The topics in the theoretical and experimental groups lead to a number of parameters, which can be adjusted during fabrication or the measurement experiments which eventually will inﬂuence the performance and behavior of the laser. The list of adjustable parameters is shown here: Chapter 1 Introduction 6

Adjustable parameters

• Concentration of dye in various solvents.

• Cavity length.

• Reﬂectance and transmittance of mirrors in cavity.

• Excitation power.

• Pulsed or continuous wave (cw) excitation.

• Dye ﬂow speed.

The minor goals of this thesis are to give as many answers to all three lists. Some of the answers to the topics will only be investigated theoretically and not conﬁrmed by experiments. The main goal is to give a preliminary conclusion if it is possible to fabricate a micron sized dye laser and make emit laser light in the vertical direction with a ﬁxed lasing wavelength. Chapter 2 Theory 7

Chapter 2

Theory

2.1 Chapter Outline

• Section 2.2 describes the history of the laser beginning with the important discovery of stimulated emission. The section is primarily based on information from [42, 53]. • Section 2.3 gives an introduction and explanation to the basic principals of the laser concentrating on the active gain medium, the excitation/pump source and last the laser cavity, which provides optical feedback and confinement of the laser light. The round-trip gain for a laser cavity containing an active laser medium is also introduced. The section is primarily based on information from [42, 53]. • Section 2.4 gives a qualitative physical description of light absorption and emission by dye molecules and how dyes can be used as laser media with specific attention to the important Xanthene dyes widely used for dye lasers. The section is primarily based on information from [5, 48, 53]. • Section 2.5 explains how rate equations an be used for describing the physics of dye lasers and the important concept of critical inversion for lasing. Special attention is given to solving of the stationary case for dye lasers. The section is primarily based on information from [4, 13, 48]. • Section 2.6 introduces an optical model for calculating mirror reflectance and transmission of both dielectric and metallic mirrors used for laser cavities. The section is primarily based on information from [10, 36, 42]. • Section 2.7 introduces the Fabry-Perot laser cavity with respect to transfer function, Q-factor, and cavity modes. The section is primarily based on information from [42, 50, 53]. • Section 2.8 introduces fluidic theory for microsystems. Special attention is given to geometry depended counter pressure versus flow velocity in micron sized channels. The section is primarily based on information from [3, 7, 12, 17]. • Section 2.9 summarizes the chapter. Chapter 2 Theory 8

2.2 History of the Laser

The laser theory was initially developed by Albert Einstein in 1916, where he predicted optical amplification by stimulated emission. The theoretical work of Einstein was first exploited in 1954, where C. H. Towners developed a microwave amplifier device based on stimulated emission of radiation also called a maser. The first device based on light amplification by stimulated emission of radiation, laser, was developed in 1960 by T. H. Maiman. The device was a solid state Ruby laser consisting of a Ruby crystal with a Fabry-Perot optical cavity as the laser resonator. Shortly after, also in 1960, A. Java developed the first gas laser based on a mixture of helium and neon. In 1961 speculations began about using organic compounds dissolved in a liquid as laser media, however 5 years should pass before the dye laser was invented by Sorokin and Lankard in 1966. Since then a variety of different organic dyes has been developed so that all of the optical spectrum is covered. The fact that a dye has optical gain in a wide spectrum, so that tuning is possible, makes the dye laser a very useful laser.

2.3 Basic Principles of Lasers

2.3.1 The Active Laser Medium

(a) The three different light interactions with matter (b) Definition of the three Einstein coefficients for light namely absorption, spontaneous emission, and stimulated interaction the matter [42]. emission [42].

Figure 2.1: Schematics of the light interaction with matter and the appropriate Einstein coeﬃcients for light absorption, spontaneous, and stimulated emission.

The quantum mechanical behavior of light interaction with matter was ﬁrst discovered by Planck by means of understanding black body radiation. Later the photon description of light was introduced by Einstein. 2.3 Basic Principles of Lasers 9

With the discovery of the particle behavior of light and the process of stimulated emission by Einstein, the fundamental processes of the active medium of a future device, the laser, was understood. When a photon interacts with matter it can raise an electron from its ground state to a higher energetic state. The photon energy will be completely absorbed and transferred to the electron. This process is called stimulated absorption or short just absorption, see figure 2.1(a). The excited electron can now decay after a short while in the excited state to the ground state. During this process the electron will loss its additional energy and emit a photon. This process is called spontaneous emission since no external mechanism will trigger the decay. Last and most importantly regarding lasers is the process of stimulated emission. If a radiant field of photon passes an already excited electron, the photon can stimulate the electron to undergo a decay to the ground state and thereby emit a photon. The emitted photon will be exact copy of incident photon, meaning the same energy (frequency), phase, and the same direction of movement. This means that the medium has an amplifying effect on the light passing through, however only if the medium is excited. The efficiency of the three processes just explained can be described by rate equations. Figure 2.1(b) shows the rates as Einstein formulated it. The A and B coefficients are material specific coefficients which determine the efficiencies for the processes. The rate for the spontaneous emission is only proportional to the number of excited atoms or molecules since spontaneous emission is not externally trigged. The two other processes are both proportional to the incoming radiating field ρ(ν) and the number of electrons in the respective initial energy levels. There is a number of different materials and substances, which can be used as laser media such as the noble gasses or others, solid state crystal alloys and last organic molecules in liquid or solid form.

2.3.2 The Laser Pump Source and Resonator Depending on the type of active medium you use in a laser system a specific type of excitation method or pump method is needed. Typical electrical discharge is used in a gas laser. In crystal lasers flashlamp or other kind of optically pumping are used. With semiconductor laser it is electrical current. Since laser action can be obtained in many materials there are a large variety of different combinations of laser media and pumps. For the organic dyes used in this project optical pumping is used, however there is a possibility of using electrical excitation [32, 39, 43]. Optical pumping of dye molecules are far the most efficient method since the light absorption by the dye molecules is high. Any kind of optical excitation with the right emission wavelength can be used, but laser excitation is the easiest at least in research experiments due to light the guiding and the single wavelength excitation. A complete laser device as shown in figure 2.2 has an optical resonator or cavity in order to confine the light so that only light with specific wavelengths, allowed in the resonator, gets optical amplified. The optical resonator also provides optical feedback, which helps for a large field to build up. The confinement of light into discrete specific mode helps reducing the linewidth of the lasing output. The confinement of the laser light by means of high reflectance mirrors also helps reducing the input power threshold needed to obtain lasing from the active medium. Chapter 2 Theory 10

Active Laser Medium Laser Output

R < 100 % R1 = 100 % 2

Pump Energy

Figure 2.2: Ideal laser cavity with concave mirrors separated with a distance L and with a mirror re- ﬂectance R1 = 100 % and for R2 slightly belowa 100 % ideally. The active medium is pumped with an external energy source for instance light. A strong laser ﬁeld is build up in the cavity due to mirror feedback. A small fraction of the laser light is emitted through the R2 mirror as useful light output.

Designing of a cavity, one of the mirrors should ideally reflect 100 % and the other mirror should reflect a small fraction below 100 % and allowing a small fraction of the laser light escaping the cavity. The escaped light is a monochromatic light beam with high directionality and large intensity if the beam is focused. Cavity mirrors are usually fabricated by dielectric stack coatings if they are used for lasers due to two reasons: Dielectrics mirrors do not absorb very much and can have very high reflectance. Another possibility is to use metallic mirrors. These are easy to fabricate but the optical reflectance is not that high. They can not be used in high power operations due to the light absorption.

2.3.3 Population Inversion The term population inversion refers to the situation, where the number of molecules in the upper laser level is larger than the number of molecules in the lower laser level. Population inversion can only be obtained in materials posing three energy levels or more, where the upper laser level is metastable. Dye lasers can be regarded as four level lasers. In ﬁgure 2.3(a) an example of a four level laser is shown. The advantage of four level lasers is that no large depletion of the ground level is necessary in order to obtain population inversion, which is the case for three level lasers. Population inversion is not nearly enough to create an amplifying medium. Losses in the cavity have to be overcome as well. Then all losses are compensated and the light intensity with the lasing wavelength does not get ampliﬁed or reduced critical inversion or threshold is obtained. Here the active medium is transparent for light at this frequency. Increasing the population further lasing is achieved. Figure 2.3(b) shows three situations for a gain curve of a laser medium together with the corresponding light output. Below threshold the light output corresponds to the gain curve of the medium. When threshold is reached, the cavity modes get visible in the light output. Increasing the population, the cavity mode near the gain maximum gets 2.3 Basic Principles of Lasers 11

(a) Energy levels in a four level laser [42]. (1) In the (b) Schematic drawing of a gain curve for a medium with ground state electrons are pumped and excited. (2) Non superimposed cavity modes [50]. Also the emission spec- radiative decay towards the upper metastable laser level. tra is shown for light output of a laser for three diﬀer- (3) Stimulated emission and decay to the lower laser level. ent situations: (a) Below threshold. (b) Threshold. (c) (4) Non radiative decay to the ground state. Above threshold.

Figure 2.3: To the left a sketch of electron transitions and photon emission in a four level laser. To the right a sketch of the gain curve of the active medium together with the light output of the laser in the three characteristic situations. ampliﬁed and the peak linewidth shrinks due to the feedback provided by the cavity. The lasing frequency does not necessarily has to correspond to a cavity mode since the gain maximum can pull the lasing frequency away from the cavity mode.

2.3.4 Round-trip Gain and Threshold Figure 2.4 shows a round-trip for laser light in a laser cavity. A useful equation is the spatial gain expression for the laser light in a laser cavity [48, 50]:

I(x)=I(0)e(N2σe−N1σ1)x (2.1)

Here I(0) is the initial photon flux for the light with the lasing wavelength at a position x = 0 as shown in figure 2.4. N2σe is the concentration of excited molecules in the upper laser level times the stimulated emission capture cross section. N1σ1 is the concentration of molecules in the lower laser level times the capture cross section for absorption of laser light. Equation 2.1 is a rewriting of Lambert-Beer’s law used for determining chemical concentrations with absorbance measurements, see equation 1.3. The main difference is that gain has been introduced in equation 2.1. Chapter 2 Theory 12

R1 R2

I(0) Laser Light I(2L)

X 0 L

Figure 2.4: Sketch of a round-trip for laser light in a laser cavity.

When the light has passed a trip forward and back a round-trip has been completed and the photon ﬂux will be I(2L)asgivenby:

2L(N2σe−N1σ1) I(2L)=R1R2I(0)e (2.2)

The mirrors R1 and R2 provide feedback ensuring a build-up of a strong light ﬁeld. The round-trip gain can now be deﬁned as:

I(2L) G = = R R e2L(N2σe−N1σ1) (2.3) I(0) 1 2

Threshold is reached when the round-trip gain is one, which means that the gain has compensated the loss and the laser medium can be considered transparent. The threshold round-trip condition can now be simpliﬁed to: 1 N σ = N σ − ln(R R ) (2.4) 2 e 1 1 2L 1 2 − 1 The term 2L ln(R1R2) is the cavity loss term. Equation 2.4 can be extended with more loss terms if necessary. Actually an additional loss term is introduced for dye lasers which will be explained in the sections 2.4 and 2.5. 2.4 Organic Fluorescent Dyes 13

2.4 Organic Fluorescent Dyes

2.4.1 Bonding Theory for Organic Dyes Dyes or dye molecules is a general description for organic compounds containing conjugated double bonds. Conjugated double bonds can be contained in long strings with a shifting between double and single bonds. Organic compounds, which are deﬁned as hydrocarbons, and their derivatives can be divided into two subgroups namely the saturated and the unsaturated compounds. The saturated compounds contain only single bonds whereas unsaturated contain at least one double bond. This means in the quantum mechanical picture that saturated compounds will only pose σ bonds whereas unsaturated also will contain π bonds. The quantum mechanical wave function for the σ bonds have rotational symmetry which means for instance two bonded atom nucleus can in some cases be free to rotate relative to each other. This is not the case with π bonds which form parallel electron clouds on opposite sides of the molecule plane, see ﬁgure 2.5 with the electron probability functions for σ and π bonds.

Figure 2.5: Drawings of electron probability functions for two diﬀerent σ bonds namely ssσ and ppσ and a drawing of a electron probability function for a π bond namely the ppπ. The signs shown in electron clouds indicate the sign of the electron wavefunction [5].

In the π bond the nucleus can not rotate relative to each other and thereby ensures a rigid structure at the bond site. A double bond consists of both a σ bond and a π bond, which can be illustrated with the simplest organic molecule containing a double bond namely ethylene. Figure 2.6 shows the electron probability functions for ethylene and the important hexagonal molecule benzene. The hexagonal structure is common in organic dye molecules. The conjugated chain of double bonds enables the π electrons to be delocalized over the whole system and be shared by many atoms as shown with the benzene molecule in ﬁgure 2.6(b). This means that the delocalized electrons may move around in the Chapter 2 Theory 14

(a) The ethylene molecule. (a) Electron probability func- (b) The benzene molecule. (a) The overlap between the tion for the ethylene molecule. (b) Top view showing the sp2 hybrid orbitals in the carbon atoms forming the σ 2 overlap of the two sp hybrid orbitals which will form the bond. (b) The pure px orbitals in the carbon atoms, σ bonding between the two carbon atoms. (c) Sideview which will form the π bonds between the carbon atoms. of the two pure px orbitals that will form the π bond (c) The π bonds will form delocalized electron clouds, between the two carbon atoms [5]. which allow the π electrons to move almost freely and be shared by all carbon atoms [5].

Figure 2.6: Drawings of some bonds in organic molecules. conjugated chain almost freely, which makes it possible to approximate a conjugated chain of double bonds as an inﬁnite quantum well and thereby approximately predict the optical absorption and ﬂuorescence as explained later.

2.4.2 Physical Properties of Organic Dyes

Dye molecules absorb light in the range from 200 nm to 1000 nm, which is closely related to the bonding structure of the dye. An organic molecule without double bonds will only absorb light below wavelengths of 160 nm [48]. The bonding energy of most chemical bonds is lower than the energy of a photon with a wavelength of 160 nm. This means means that the absorption of light will most like end up in a chemical bond breaking and ﬂuorescence will therefore not be emitted. The situation is quite diﬀerent in compounds with double or triple bonds. The absorption of organic compounds with conjugated double bonds will be mainly above 200 nm and mainly in the visible spectrum from 400 nm to 700 nm. In the infrared end of the spectrum, the problem of having dyes absorbing infrared radiation is thermal stability. The problem can be considered by using − the Arrhenius (Boltzmann) relation for the reaction rate k [s 1]. The formula is [48]: −E k = A exp A (2.5) kBT 2.4 Organic Fluorescent Dyes 15

12 −1 A is the Arrhenius constant typically on the order of 10 s . EA is the activation energy. kB is the Boltzmann constant and T is the temperature. Assuming a minimum half lifetime of one year for the dye, one will get an energy corresponding to a wavelength of about 1µm. This means that it will be diﬃcult to ﬁnd a dye that absorbs light in the infrared area above 1µm with a reasonable lifetime.

Figure 2.7: The picture shows the chemical structure of two common dyes. (a) 3,3´ diethyl thiatricarbocyanine iodide. (b) Rhodamine 6G. The heavy lines indicate the chromophoric regions of the dyes [53].

In a chain of conjugated double bonds the electron wavefunctions ensure a very rigid structure and ensure low energy loss due to no rotation between the atoms. These kind of rigid structures in dye molecules can be the chromophoric region where interaction with light will occur. Figure 2.7 shows two dyes where the chromophoric region is indicated by the heavy lines. Figure 2.8 shows the organic molecule phenolphthalein which has a high structural mobility resulting in a very low or non ﬂuorescent quantum yield. This

Figure 2.8: Chemical structure of phenolphthalein. The arrows indicate that parts of the molecule can rotate [48]. indicates that the molecule should have a highly rigid structure in order to have a high fluorescent quantum yield like the Rhodamine 6G dye shown in figure 2.7. The chain of conjugated bonds can to a first approximation be considered as an infinite quantum well [53]. To illustrate this principle, figure 2.9 shows how the potential of a cyanine dye can approximately be described by a quantum well. The energy in an infinite quantum Chapter 2 Theory 16

Figure 2.9: The π electron cloud is shown in a) and b) for a simple cyanine dye. In c) the potential energy of the dye is shown as a function of the dye length. Beneath in d) an approximation of the potential is shown as an inﬁnite potential quantum well [48].

well is given by [48]: h2n2 E = n =1, 2, 3... (2.6) n 8mL2 h is Planck’s constant, m is the electron mass, and L is the effective length of the conjugated chain. There are N numbers of π electrons present in the chain. Substitution of 1 the quantum number n with 2 N (since two electrons with opposite spin can occupy the same quantum state) one gets an energy difference between the first excited state and the ground state as a function of π electrons in the chain [48]:

h2(N +1) hc ∆E = = (2.7) min 8mL2 λ c is the speed of light and λ is the wavelength of either the absorbed or emitted photon. The length of the quantum well corresponds to the effective length of the chromophoric region where the π electrons in the chain of conjugated double bonds can move almost freely since they are delocalized. The simple picture of the energy in a dye molecule, described as an infinite quantum well, predicts only approximately the absorption and fluorescence energy and thereby the wavelength of the photon. To get a precise description of the energy levels in a dye a complete quantum mechanic calculation has to be applied. The calculation should also include vibrational and rotational modes of the dye molecule. However in this thesis only a qualitative description will be presented, which in general can be used for all dyes. The band structure of a dye consists of two different types of states namely Singlet and Triplet states as shown in figure 2.10. The singlet states are characterized by the paired 2.4 Organic Fluorescent Dyes 17

Figure 2.10: Energy levels for a typical organic dye [53].

π electrons having opposite directions of their spin as expected in a stable molecule. According to Pauli’s exclusion principal two electrons can not occupy the same state. Therefore the spins of the electrons have to be opposite. If one of the two highest lying π electron in energy is excited by a photon to a higher singlet state for instance S1,the electrons will still have opposite spin as indicated in figure 2.10. All the main states will have a wide range of allowed energy band superimposed on the main states for instance S0. All these superimposed states are vibrational states and rotational states. This means that absorption of a photon will usually raise the electron to a higher vibrational level of the S1 state. The electron will now decay very fast to the bottom of the S1 state. The electron in the S1 state can decay to the ground state S0 or be excited to a higher singlet state (not shown in figure 2.10). If the electron decays to the ground state by emitting a photon the electron will end up in a higher lying vibrational state of the S0 state. The energy difference between the photon absorbed and the emitted photon will be in favour of the absorbed photon. This means that the emission spectrum is red-shifted towards longer wavelengths [5]. At room temperature one will not be able to resolve the vibrational and rotational states of the optical absorption and emission spectra since the states are closely spaced, which means the spectra will appear continuously and not discrete [48]. h The momentum of a photon p = λ is very small and can be neglected in radiate transition. Neglecting the photon momentum is called the dipole approximation [53]. In the dipole approximation the electron has the same momentum before and after the radiative transition [53]. This means that radiative transitions will always be vertical in a energy vs momentum (E(p)) graph [50]. The verticalness of the optical transition is shown in figure 2.10.

If a molecule in the excited S1 state collide with another molecule an intercrossing transition is possible and the spin of the excited π electron is flipped. The collision also gives rise to another vibrational mode of the dye molecule as indicated by the configu- Chapter 2 Theory 18 ration coordinate in figure 2.10. Flipping the spin is a forbidden process in an radiative transition [5] and occurs only during collisions with other molecules [53]. From the first excited triplet state T1 the molecule can either decay non fluorescent to the ground state S0 by means of interaction with the surroundings or be excited by excitation light or by the emission light to a higher triplet state. For most dyes the decay from T1 to S0 is a non- radiative process but for instance with the phenanthrene dye a radiative process is possible from the T1 to the S0 state. Such a radiative process is named phosphorescence [5]. The S0, S1 and T1 are the most relevant states since the lifetime for higher singlet states and triplet states are in picoseconds compared to for instance the lifetime of S1, which is in the nanosecond range. The inverse of the intercrossing rate is in the range of nanoseconds to microseconds. Intercrossing can be a serious problem and in the worst case prevent lasing from the dye solution. With continuous wave (cw) pumping, intercrossing is a limiting factor for obtaining lasing due to the fact that the lifetime of the triplet state can range from microseconds to milliseconds. This means that an accumulation of molecules in the first triplet state will occur and results in a large absorption of light at the lasing frequency. With pulsed pumping, laser action is obtained before an appreciable population accumulates the triplet state. Besides refractive index gradients due to thermal heating are avoided by means of pulsed pumping. Refractive index gradients produce optical distortions, which can prevent lasing [48].

2.4.3 The Xanthene Dyes

(a) Rhodamine 6G (b) Rhodamine 110 (c) Fluorescein

Figure 2.11: Chemical structure of three Xanthene dyes [48].

The chemical structures of three common Xanthene dyes are shown in figure 2.11. The chromophoric region in this class of dyes are basically the same. It is the side groups which varies the detailed optical properties due to interaction with the surroundings and molecular vibration and rotation. The Xanthene dyes are general known as stable and very efficient laser dyes with for instance fluorescent quantum yields as high as 95 % for Rhodamine 6G [4]. The Xanthene dyes covers the range of optical laser emission from 500 to 700 nm. The strong absorption from these dyes are in the wavelength range from 450 to 600 nm where the so-called transition moment is oriented parallel to the long axis of the molecule. The transition moment is a quantum mechanical vector, which describes the efficiency of light interaction with the molecule in a specific direction of the molecule. The absorption spectra for the two dyes Rhodamine 6G and Rhodamine 110 dissolved in ethanol are shown in figure 2.12. The dyes also absorb in the UV-range since a weaker transition moment is present orthogonal to the long axis of the molecules. The emission 2.4 Organic Fluorescent Dyes 19

(a) Absorbance spectrum of Rhodamine 6G dissolved in (b) Absorbance spectrum of Rhodamine 110 dissolved in ethanol. ethanol.

Figure 2.12: Absorbance spectra for the dyes Rhodamine 6G and Rhodamine 110. spectrum of the dye Rhodamine 6G is shown in ﬁgure 2.13 together with spectra for the absorption and the triplet absorption. The spectra shown in ﬁgure 2.13 reveal that the

Figure 2.13: Absorption, emission and triplet spectra deﬁned as capture cross sections for Rhodamine 6G dissolved in ethanol. [53]. triplet absorption spectrum overlaps the emission spectrum, which indicates that triplet absorption can be a problem especially in cw operations. To circumvent the problem the dyes are pumped through the area of excitation with a high speed (around 10 m/s). The spot size of excitation is maybe around 10 µm. The dye molecules will then not experience excitation long enough to accumulate the triplet state and absorb signiﬁcant amount laser light.

2.4.4 Dyes as Laser Media Organic dyes can be used as the active medium of lasers since they have a large gain just as semiconductor lasers. However the short lifetime of the electrons in the excited singlet state results in a higher threshold power needed to create inversion population compared Chapter 2 Theory 20 to other laser types [53]. The high stimulated emission cross section however compensates for the low lifetime as seen in equation 2.8 [53]. In equation 2.8 is given an expression for the threshold excitation power Pth. ln( 1 ) R1R2 hν A Pth = (2.8) 2η τ σe

R1 and R2 are reflectance of the mirrors. η is the efficiency for the pumping process. ν is the pumping frequency and τ is the lifetime of excited state. A is the area of pumping and σe is stimulated emission cross section. From the manufacture a laser dye is a solid powder, which needs to be dissolved in some fashion to be useful as laser medium. Generally dyes are dissolved in organic solvents or water depending on application. The optical and stability properties are highest in a liquid but one has the possibility to dope solid materials with dye molecules. In research an often used method is doping of polymers in a liquid solution and then dry out the solvents of the polymer in order to solidify the polymer [28]. Polymethyl methacrylate (PMMA) is often used as host matrix for dyes. Exchange of dye molecules in a solid dye laser is not possible and is therefore sensitive to chemical breakdown/bleaching of the dye molecules. Thermal heating due to continuous pumping operation will bleach the dye molecules. Therefore solid dye lasers are usually only used in a pulsed pump mode. Liquid dye lasers can be operated in two modes, pulsed or cw. In pulsed mode ethanol or other organic solvents are often used since aggregation of dye molecules at high concentrations is not a major problem. In solvents with low dielectrical constants aggregation is prevented as with ethanol. The repulsion of charged molecules is larger in low dielectrical solvents. Water is the preferred solvent in cw operation due to the high heat capacity preventing high temperature gradients however aggregation of molecules produces dimers (dimerization) which alters the optical properties of the dye to the worse [48]. Aggrega- tion in water can be prevented by adding small amounts of detergent for instance Triton X-100 or Ammonyx LO. In this project dyes will be used in their liquid form. This gives the possibility of tuning the wavelength on micro-level, since you alter the flow of dye by controlling the dye flow thickness by means of other pumped fluids and thereby changing the optical path length. An other option is to mix a highly concentrated dye solution with solvent on the chip and thereby change the optical properties of the dye and hence the wavelength. The wavelength dependency on the dye concentration will explained later on. This method is called self tuning. A third option is to change the distance between the cavity mirrors however this way of tuning will be very difficult on chip level due to fabrication process implications. Tuning in this way is called external tuning and is the method of tuning in commercial available macro scale dye lasers. The dyes used in this project are the Rhodamine 6G and Rhodamine 110. The absorption spectra for Rhodamine 6G is shown in figure 2.12(a) where the maximum absorption occurs at around 530 nm. This makes Rhodamine 6G suitable for optical pumping with our frequency doubled pulsed Nd:YAG laser at a wavelength of 532 nm. The fluorescent maximum is around 560 nm for Rhodamine 6G as shown in figure 2.13. The Rhodamine 110 has an absorption maximum at 510 nm as shown in figure 2.12(b) and the fluorescent maximum is 530 nm. The ideal pumping source would be an argon ion laser at 2.4 Organic Fluorescent Dyes 21

514 nm however only an cw argon ion laser with a wavelength of 488 nm was available for measurements performed in this project. Chapter 2 Theory 22

2.5 Rate Equation Model of Dye Lasers

2.5.1 Time-dependent Coupled Rate Equations Rate equation models are very often used to model the physics of a laser. The rate equations are bookkeeping of molecules and photons. The rate equations keep track, with respect to time, of molecules in diﬀerent states and photons with diﬀerent wavelengths. The rate equations are quite simple to construct but can only be solved numerical if one does not make any approximations. Figure 2.14 shows the energy levels for a typical

Figure 2.14: Model for the energy levels for a typical dye molecule [4]. dye molecule. In order to describe how efficient these transitions are one can assign a capture cross section for an interaction between a photon and a molecule. These capture cross sections are usually denoted with the Greek letter σ [cm2]. If a capture cross section is large then the possibility for an interaction to occur is higher. All the transitions in a general dye molecule are shown figure 2.14. The transitions are either shown with a capture cross section σ, a rate k [s−1] or a decay time τ [s]. A capture cross section is used when an interaction takes place between a photon and a molecule or another photon. The transitions marked with oscillating arrows are non-fluorescent ones. A fluorescent decay is marked with a strait vertical line and a τ. Before proceeding with the deriving of the rate equations a condition that always is obeyed is the total concentration of molecules N [cm−3] which is constant:

N = N0 + N1 + NT = constant (2.9)

N0 is the concentration of molecules in the S0 state. N1 is the concentration of molecules in the S1 state. Last NT is the concentration of molecules in the T1 state. The higher excited states are ignored due to their short lifetime. A situation where this condition is not fulfilled could be when bleaching of the dye molecules occur due to high pump intensities. Photon fluxes are denoted by I [cm−2s−1]. From figure 2.14 it is quite easy to write up the rate equations. The change in the 2.5 Rate Equation Model of Dye Lasers 23

concentration of molecules in the ground singlet state S0 cannowbewritten[4].

∂N0 N1 NT = −N0σ0pIp − N0σ0lIl + N1σeIl + + N1kIC + (2.10) ∂t τ0 τT

The term −N0σ0pIp accounts for molecules leaving the S0 state due to the optical pumping and entering the S1 state. These kinds of interaction terms between molecules and photons are always determined by the concentration of molecules in the initial state times the capture cross section and times the photon rate. −N0σ0lIl determines the concentration of molecules which are excited from the S0 to the S1 due to absorption of fluorescent or laser light. This term N1σeIl is responsible for the stimulated emission where lasing photons stimulate molecules to decay and emit a lasing photon. N1 is the fluorescent τ0 decay. N1kIC is the rate, which the molecules in the S1 state makes a non-fluorescent decay. The last term NT is the rate, which the molecules decay from the T triplet state. τT 1 The molecules in S1 state can be accounted for with an equation that is quite similar to the negated of 2.10 except for the involvement of the triplet state [4]:

∂N1 N1 = N0σ0pIp + N0σ0lIl − N1σeIl − − N1kIC − N1kST (2.11) ∂t τ0

The term −N1kST accounts for the rate at which the molecules makes the intercrossing conversion from the excited singlet state S1 to the triplet state T1. The change in concentration of molecules for the triplet state is easy to account for and is given by [4]:

∂NT NT = N1kST − (2.12) ∂t τT

The molecules in the three states S0, S1,andT1 are now accounted for assuming that the higher excited states are ignored. This can be done for most practical uses because these transitions are much faster and also the lifetime of these states are very short compared to the ﬁrst exited states S1 and T1. To complete the rate equations one also has to count the photons for both the pumping photons and the lasing photons. The photons will also be counted according to the spatial distribution due to for instance absorption of the pumping photons. The pumping photons can be counted according to this equation [4]:

n ∂Ip ∂Ip nIp + = −(N0σ0p + N1σ1p + NT σTp)Ip − (2.13) c ∂t ∂z ctc,p

Here n means the refractive index of the active medium and c is the speed of light in vacuum. The ﬁrst term −N0σ0pIp is due to absorption of pumping photons to excite the molecules from the S0 state to the S1 state. This is the absorption that one would like to be as high as possible. The next term −N1σ1pIp determines the absorption of pumping photons due to excitation from the ﬁrst excited singlet state S1 to a higher singlet state Sn. These transitions has been ignored in the rate equations concerning molecules. The time scale for the photons is much lower and thereby these transitions becomes important for the photon concentration at least in a time depended situation. They are not important in a stationary situation. The −NT σTpIp term is due to pump light absorption from the Chapter 2 Theory 24

− nIp triplet state T1 to a higher excited triplet state Tn. The last term ctc,p is the cavity loss which is inverse proportional to the photon lifetime in the cavity. In a similar manner one can count the photons at the lasing frequency [4]:

n ∂Il ∂Il nIl N1 + = −(N0σ0l + N1σ1l + NT σTl − N1σe)Il − + (2.14) c ∂t ∂z ctc,l plτ0 The term is quite similar to what has been shown in equation 2.13. Two additional terms have been included for the lasing photons. One term accounts for the gain in the material due to stimulated emission namely the term N σ I . The other term N1 is a term which 1 e l plτ0 accounts for the part of the fluorescent photons which goes into the lasing mode. pl is the cavity mode number given by [4]: 8πν2n3V ∆ν p = (2.15) l c3 Here V is the volume of the cavity which is irradiated with the pump light. The term N1 is insignificant in large scale lasers with a cavity length of for instance one meter or plτ0 so. However in a micro cavity this term might have a great influence. The photon lifetime in the cavity is inversely proportional to the loss coefficient, which is due to the cavity length and the reflectance of the mirrors. The inverse of the product between the speed of light in cavity and the cavity loss coefficient gives the photon lifetime in the cavity [4]: n t = (2.16) c cα The cavity loss coefficient is given by [4]: 1 α = − ln(R R ) (2.17) 2L 1 2 The cavity loss coefficient can also be derived from the equations E.6 and E.7 in the appendix E. This loss coefficient can be very dominant in micro cavities however the fluorescent photons that contribute to the lasing mode can overcome the cavity loss coefficient. Threshold-less micro cavity lasers have been realized where the optical cavity path length was comparable to the wavelength since all the fluorescence went into the lasing mode [11, 34, 57, 62, 63, 64].

2.5.2 A Simple Stationary Solution for the Threshold Condition The dynamic rate equations can only be solved by numerical calculations. To make a quick analytical calculation one needs to look at the problem in a stationary situation. However this can only be done in cw laser operation. Since we are also using a pulsed pumping laser the situation is far from stationary. If one still would like to use a stationary solution one should compare the photon lifetime in the cavity to the pump plus length. Gold mirrors for instance will under these conditions have a reﬂection of around R =0.8. The cavity length is L =10µm. The photon lifetime in the cavity can then be calculated to around: n t = =2.0 · 10−13 s (2.18) c − 1 2L ln(R1R2)c 2.5 Rate Equation Model of Dye Lasers 25

If one compare this to the pulse length of 5 ns which is pulse length of our Nd:YAG laser one will get a factor of 25000 times in favour of the pulse length. This means that during the pulse, the photons will probably reach an equilibrium. This approximation will not work with high Q-factor (explained later) lasers. We assume that a photon equilibrium will be reached during the pulse. We can now use derive a stationary solution for the threshold problem. Assuming a stationary solution the ﬁve rate equations can be decoupled and analyzed individually. From section 2.5.1 we know that the rate equation which counts the lasing photon ﬂux is given by:

n ∂Il ∂Il nIl N1 + = −(N0σ0l + N1σ1l + NT σTl − N1σe)Il − + (2.19) c ∂t ∂z ctc,l plτ0 We assume that the situation is stationary which means that the time derivative is zero and the molecule concentrations in the diﬀerent states will be constant. At threshold there will be no net gain or loss throughout the cavity and the spatial derivative can therefore also be set to zero. This can be seen as the medium being transparent for the lasing photons. The triplet population will also constant and therefore proportional to the concentration of excited singlet molecules N1:

NT = kST τT N1 (2.20)

This equation is called the ”equilibrium triplet approximation” [48]. To simplify things the contribution of spontaneous emission ( N1 ) into the cavity lasing mode in equation plτ0 2.19 is ignored. The term N1σ1lIl is also ignored since it will have no greater inﬂuence in a stationary case [44, 48]: 1 N σ = N σ + N σ − ln(R R ) (2.21) 1 e 0 0l T Tl 2L 1 2 This is a threshold round-trip condition for a dye laser in a stationary situation which can be compared with equation 2.4. The additional loss term is absorption of laser light due to the triplet state. The total concentration of molecules will be shared between these three states:

N = N0 + N1 + NT (2.22)

Using the equations 2.20, 2.21, and 2.22 an approximate formula can be made for calculating the concentration of molecules needed in the excited state N1 compared to the total concentration N in order to reach lasing threshold [44, 48]:

N σ − 1 ln(R R ) 1 = 0l 2LN 1 2 (2.23) N σ0l + σe + kST τT (σ0l − σTl)

Knowing the concentration of molecules in the N1 state the threshold pump power can be calculated [48]: N1hcLA Pinv = (2.24) λpτ0 Chapter 2 Theory 26

Here h is Planck’s constant, c is speed of light, and λp is the wavelength of the pump source. L is the dye cell length and A is the area of excitation. τ0 is the radiative lifetime from state S1 to state S0. This derivation of the critical inversion concentration is somewhat similar to what has been done in the book ”Dye Laser” for cw lasers [48]. That formulation is shown in appendix E. Equation 2.23 is useful for determining the minimum concentration needed for obtaining lasing. Knowing the wavelength dependencies of the capture cross sections σe,σ0l, and σTl one can generate a three dimensional graph with the total concentration and the cavity loss as a united independent variable and then predict the self-tuning of the dye [13, 44, 48]. The dependent variable will be the ratio between the concentration of excited molecules and the total concentration of dye molecules. An example of such a curve is shown in ﬁgure 2.15 [48]. The heavy dashed line in ﬁgure 2.15 indicates the self-tuned

Figure 2.15: Calculated surface for the critical inversion concentration. The minimum on the surface represents the ratio between the critical inversion population and the dye concentration for a given cavity 1 loss and wavelength. The parameter r is the cavity loss − 2L ln(R1R2). This graph is calculated for Rhodamine 6G in water plus Ammonyx LO and is copied from ”Dye Lasers” [48]. lasing wavelength of the dye solution if no external tuning are performed by means of a very short cavity or an intra cavity grating. The tendency is that at high concentrations 2.5 Rate Equation Model of Dye Lasers 27 of dye, the lasing wavelength will go towards longer wavelengths. The phenomenon is called self-tuning and originates from the fact that the diﬀerent capture cross sections are wavelength depended. Another eﬀect that arises from equation 2.23 together with equation 2.24 is that the threshold power needed rises with dye concentration.

0.8

0.6 N1/N 0.4

0.2

0 0

0.2 0 0.4 0.002 0.6 0.004 0.006 0.8 0.008 1 0.01 R² N [mol/L]

Figure 2.16: Calculation of the critical inversion surface depending on the product of reﬂectance of the cavity mirrors and the dye concentration. The ratio between the critical concentration of excited molecules N1 and the total concentration N of dye molecules is plotted. The dye simulated is Rhodamine 6G dissolved in ethanol and the cavity length is 10 µm. The data for the calculation of rate equation 2.23 are from Barroso et al. [4]. The values used correspond to a wavelength of λ = 580 nm for lasing and thereby ignoring self-tuning of the dye. Every value above 1 for the ratio N1/N is ignored and assigned the value of 1. Critical inversion can in theory of cause only happen when the ratio is below 1. Dimerization at higher concentrations are ignored in this calculation.

Having considered the lasing wavelength dependency on the concentration we can use the rate equation 2.23 to predict the critical inversion surface for dye solutions with different concentrations and for different reflectivities of the cavity mirrors. Figure 2.16 shows a calculated critical inversion surface for a Rhodamine 6G dye dissolved in ethanol. Thecavitylengthisfixedon10µm, meanwhile the concentration and the product of the mirror reflectance are varied. The calculation is done for dye data values corresponding to a wavelength of 580 nm [4]. The calculated inversion surface for lasing in a 10 µm cavity shows that lasing is possible for a large range of concentrations provided the reflectance is high enough. The flat horizontal brownish surface with a threshold ratio N1/N value of 1, illustrates that lasing is not physically possible. The actually calculation gave threshold ratios larger than 1, for the flat brown area, which is not physical possible and was assigned a value of 1. The threshold ratio N1/N can not be larger than 1. If the length of the cavity containing dye is enlarged the threshold ratio will drop due to a larger round-trip gain in the cavity. Self-tuning of dyes is also dye cavity/cell length depended as for the dye − 1 concentration. In the term 2LN ln(R1R2) from equation 2.23 the length L has the same influence on lasing wavelength as the concentration N meaning longer dye cell longer Chapter 2 Theory 28 lasing wavelength. This was theoretical and experimental verified by Farmer et al. [13]. The length L will naturally change the optical path length and thereby the allowed modes in the optical cavity. This can be regarded as an extrinsic effect whereas the self-tuning due to the dye cell length is an intrinsic effect rising from the wavelength properties of the dye itself. The concentration will most likely will change the refractive index of the dye solution and thereby contribute to both the intrinsic and extrinsic tuning of the lasing wavelength. AccordingtoSchäfer [48] a value of around 1/10 is the maximum practical limit of the critical inversion ratio as a rule of thumb. 2.6 Mirrors 29

2.6 Mirrors

Reflection and transmission of materials are determined by three mechanisms namely the Fresnel part due to the refractive index, the smoothness of the surface, and finally the absorption of light in the material. The effect of surface smoothness on reflection will be hard to model, but one should make sure that the surface is as flat as possible. If not, then diffuse reflection will dominate. If an interface between two materials is completely flat, the optical properties of the materials can be determined with the Fresnel coefficients, which can be calculated if the refractive indices of the materials are known. The Fresnel coefficients are stated in the appendix C. Calculation of the reflectance of a structure consisting of two layers at normal incidence is a fairly simple calculation. The expression is given in equation 2.25. 2 n − n R = 2 1 (2.25) n2 + n1 Modelling of layers consisting of more than 2 layers requires a more sophisticated model.

2.6.1 Optical Modelling of Multi-layers Using structures with three or more layers a systematical method should be applied in order to take the thickness of the layers into account. The thickness is very important when considering absorbing materials and structures where interference of light is applied. A systematical model has been derived by Cory el al. [10]. The detailed derivation is shown in the appendix D. The model can be derived by investigating ﬁgure 2.17. The

δ δ −i 2 −i3 2 2 2 t32t21e t32r21r23t21e θ 1 T 1 2 1 r21 ,t21 θ 2 2 d2 3 r32 ,t32 3 θ θ 3 3 = E 1 R2

−iδ E = 1 r32 2 t32r21t23e (a) (b)

Figure 2.17: Sketch for optical transmission and reflection for multilayer films both dielectric and metallic films. infinite number of reflection and transmission terms are summed up to a finite expression. The results for the reflection, the transmission, and the phase argument are list here [10]:

−iδj r + R − e R = (j+1,j) (j 1) (2.26) j −iδj 1+r(j+1,j)R(j−1)e Chapter 2 Theory 30

−iδj /2 t T − e T = (j+1,j) (j 1) (2.27) j −iδj 1+r(j+1,j)R(j−1)e

4πnjdj cos(θj) δj = (2.28) λ0 j is the index number for the layer of current consideration. One starts calculating the reflectance and the transmission of the lowest layer and then calculate in an iterative way through the layers. The term R(j−1) is the total E-field reflection for all the layers calculated before the current layer. The term r(j+1,j) is the Fresnel coefficient for the interface between the current layer and the next layer. The same applies for the transmission terms. The calculated reflection and transmission using the above equations is the Fres- nel coefficients for the whole structure. The power reflectance and transmittance can be calculated knowing the relationship the between power and the E-field of electromagnetic waves. The power per unit area for an electromagnetic wave can be calculated using the Poynting vector given by [42]: 2 S = +0c E × B (2.29) The Poynting vector can not be measured due to the fact that the frequencies in the visible range is 1014 to 1015 Hz. The average of the power per unit area also called the irradiance Ee is a measurable quantity and is given by: 1 E = |S| = + cE2 (2.30) e 2 0 E is the magnitude of the E-field in the electromagnetic wave. The power reflectance can now be calculated knowing that the intensity is proportional to the square of E-field [42]: 2 Pr Er 2 R = = = RN (2.31) Pi Ei

RN is the calculated Fresnel coefficient for reflection from equation 2.26 for complete sandwich structure with N layers. The power transmission compared to the power reflection is further more dependent on the angle of incidence and the angle of transmission since these are not the same according to Snell’s law. The power transmission is also proportional to the ratio between the refractive indices of the surroundings [42]: 2 Pt nt cos(θt) Et nt cos(θt) 2 T = = = TN (2.32) Pi ni cos(θi) Ei ni cos(θi)

TN is the calculated Fresnel coeﬃcient of transmission from equation 2.27 for complete sandwich structure with N layers. According to conservation of energy for non-absorbing media the following condition should be fulﬁlled: R + T = 1 (2.33) In absorbing materials like metals the condition does not hold. A detailed derivation of the optical multi-layer model is given in the appendix D. 2.6 Mirrors 31

2.6.2 Dielectric Mirrors

High reflectance dielectric mirrors consist of a substrate coated with transparent dielectric materials. Two dielectric materials are used: One with a high refractive index and one with a low refractive index. The coatings are shifted between the two materials and the thickness of each layer is exactly matched to be a quarter of a wavelength with respect to the optical path length. Such a structure is shown in figure 2.18(a) and a corresponding optical reflectance spectrum is shown in figure 2.18(b) with nitride as high index material and oxide as the low index material, materials common used in cleanrooms. The calculation of the reflectance spectrum in figure 2.18(b) is based on the model from section 2.6.1 and the refractive indices are held constant over the spectrum, which is a reasonable approximation. The multiply reflections from every interface ad up in phase meaning

Reflectance of a QW−mirror stack on Pyrex 1 N=11 0.9 N=4 N=1 0.8

0.7

0.6

0.5

Reflectance 0.4

0.3

0.2

0.1

0 350 400 450 500 550 600 650 700 750 800 850 Wavelength [nm]

(a) Layer structure of a dielectric mirror with alternat- (b) Plot of the calculated reﬂection in the range from ing periods of dielectric materials with a high refractive 350 nm to 850 nm of a quarter wave mirror stack with index and then a low one. The thicknesses of all layers a center wavelength of 560 nm for diﬀerent numbers N except the substrate are exactly a quarter of the center of double layer periods. The stack materials are nitride wavelength in optical path length [42]. (n=2.1) and oxide (n=1.46).

Figure 2.18: Dielectric stack mirror with calculated reflectance spectra. that the mirror is based on constructive interference of light. The advantage of dielectric mirrors is the nearly non-absorbing properties, which allows for use as cavity mirrors in high power lasers. Another advantage is the design aspect, which allows for design of very high reflectance in a specific optical band. The disadvantage is the fabrication due to strict process control of the layer thickness, which in most cases means on-line thickness measurement during growth or deposition.

2.6.3 Metallic Mirrors

While the refractive index for glass and other transparent media in the visible range can be considered non-absorbing with a fairly constant real refractive index, the same approximation for metals do not apply. Besides a varying real refractive index metals are also strongly absorbing. The refractive index in the model from section 2.6.1 will consist Chapter 2 Theory 32

Refractive index of Au, Al, and Cr 16 n: Au k: Au 14 n: Al k: Al 12 n: Cr k: Cr 10

6 Refractive index n,k 4

0 200 400 600 800 1000 1200 1400 1600 Wavelength [nm]

Figure 2.19: Plot of the complex refractive indices in the range from 200 nm to 1550 nm of gold, aluminum, and chromium. The values for the refractive indices for the various metals are listed in the appendices F,G, and H [36].

of a positive real part and a negative imaginary part:

n(λ)=n(λ)Re − ik(λ)Im (2.34)

Optical constants for solids are given in the series of books ”Handbook of Optical Con- stants of Solids” [36]. The real part and the imaginary part for three metals commonly used in the cleanroom are shown in figure 2.19. Using the optical model presented in section 2.6.1 together with the refractive indices for gold, aluminum, and chromium the reflectance is calculated and plotted in figure 2.20(a) for 150 nm thick layers.

Reflectance of 150 nm thick Au, Al and Cr on Pyrex Reflectance and Transmittance at 560 nm 1 1

0.9 0.9

0.8 0.8

0.7 0.7

0.6 0.6

0.5 0.5

Reflectance 0.4 0.4

0.3 0.3 R: Au T: Au

0.2 Reflectance and Transmittance 0.2 R: Al Au T: Al 0.1 Al 0.1 R: Cr Cr T: Cr 0 0 200 400 600 800 1000 1200 1400 1600 0 25 50 75 100 125 150 Wavelength [nm] Thickness [nm]

(a) Plot of the calculated reflectance in the range from (b) Plot of the calculated reflectance for thicknesses rang- 200 nm to 1550 nm for gold, aluminum, and chromium. ing from 0-150 nm for gold aluminum, and chromium. The thickness of the metals is 150 nm. Solid curves represent the reflectance and dashed cureves represent the transmittance.

Figure 2.20: Reflectance dependencies of the wavelength for tree different metallic materials and also reflectance and transmittance dependencies of the layer thickness for the metals. 2.6 Mirrors 33

The plot in figure 2.20(a) reveals that aluminum is a very broad band reflecting metal with a fairly high reflectance. Gold reflects light efficiently in the infrared area of the spectrum but the reflectance drops off below 600 nm. In the blue area of the spectrum the reflectance for gold is reduced to a value below 40 %. This is in agreement with the yellow appearance of gold since yellow is the complementary color to blue, which is not reflected to that great extent. Non-coated aluminum is rarely used in optics since oxidation will degrade the reflectance. Chromium is not used for mirrors, but acts as a adhesion layer for gold deposited on dielectric substrates. Figure 2.20(b) show the reflectance and transmittance dependencies on the layer thickness and reveals that only very thin layers of aluminum and chromium allows for transmission of light at around 560 nm. Gold has a larger transmission at this wavelength. Figure 2.20(b) reveals that the reflectance and the transmittance of metals can be adjusted by the layer thickness. Metals can be deposited in various ways but the most obvious is electron-beam (e- beam) deposition. Very thin layers can be deposited with on-line thickness control by means of an oscillating quartz crystal. E-beam deposition is available in most cleanrooms for deposition of conducting metal wires for micro system or micro electronics. E-beam deposited films are not like bulk crystalline material. However the smoothness of a e-beam deposition is very high and will most likely be comparable with a surface of a polished bulk material. Chapter 2 Theory 34

2.7 The Fabry-Perot Interferometer

R1 R2

Figure 2.21: A Fabry-Perot cavity with some of the allowed cavity modes.

The Fabry-Perot interferometer is basically an optical filter, which consist of two mirrors separated a with certain distance. The two mirrors will confine the light passing through or generated inside the cavity, and a standing wave will be build up. Due to the interference of light only the wavelengths of light that interferes constructive can exist between the mirrors. A schematic drawing of a Fabry-Perot etalon, where the two mirrors R1 and R2 are separated with a distance L is shown in figure 2.21. The Fabry-Perot interferometer has many applications. Filter applications, precision wavelength measurements, analysis of hyperfine spectral line structures, determination of refractive indices of gases etc. Another application is laser cavities. The Fabry-Perot cavity is used for cheap horizontal emitting semiconductor lasers. Vertical cavity surface emitting lasers (VCSEL) fabricated with semiconductor materials are also using the Fabry-Perot resonator for instance by means of metal organic chemical vapor deposition (MOCVD) deposited dielectric mirrors [50]. Other applications are tunable micro optical filters with silicon technology for future compact optical sensors [22, 45].

2.7.1 The Airy Function The transfer function for a Fabry-Perot etalon is an Airy function. The Airy function is derived in the book ”Introduction to Optics” [42]. The result for the transmitted light intensity through the Fabry-Perot etalon is given by: (1 − R )(1 − R ) 4πnL cos(θ) T = √ 1 √ 2 where φ = (2.35) − 2 1 2 λ 1 R1R2 +4 R1R2 sin 2 φ Here θ is the angle by which the light is transmitted into the Fabry-Perot etalon. The Airy function is dependent on the reﬂectance of the mirrors and the separation L between. The separation L will determine the distance between the transmission peaks in the spectrum and together with the mirror reﬂection the spectral linewidth can be determined. 2.7 The Fabry-Perot Interferometer 35

The number which gives a description of the performance of an optical resonator is the quality factor or short Q-factor. The Q-factor is also used to describe the performance or quality of resonating electronic circuit and mechanical vibrating structures. 1 2πnL cos(θ)(R R ) 4 ν Q = √ 1 2 ∼ (2.36) λ 1 − R1R2 ∆ν The Q-factor is generally defined as the ratio between the peak wavelength ν and the full width half maximum (FWHM) linewidth ∆ν, however this relation does not correspond exactly to the analytical Q-factor definition for a Fabry-Perot resonator. The light output from an optical cavity is often approximated with a lorentzian lineshape [53] which leads to an agreement between the Q-factor definition and the output lineshape. Neverthe- less equation 2.36 gives an insight into how the cavity length and the mirror reflectance influence the Q-factor.

Airy fumction for a Fabry−Perot laser cavity 1

0.9

0.8

0.7

0.6

0.5

0.4 Transmittance 0.3

0.2

0.1 L=3.5 µm L=10 µm 0 540 550 560 570 580 590 600 610 620 630 640 650 Wavelength [nm]

Figure 2.22: Plot of the transmittance through a Fabry-Perot with the reﬂectance R1 =0.8andR2 =0.8. The transmittance curves are called Airy functions. The medium between the mirror is transparent with a refractive index of n =1.36.

Figure 2.35 shows two curves representing the light transmission through a Fabry- Perot cavity for two different cavity lengths. It can be seen that the sharpness of the peaks gets better for longer cavities while the numbers of modes increases. Higher reflectance (dependency not shown) will also improve the linewidth of the cavity modes. Reducing the cavity size to micron size will reduce the number of cavity modes at the expense of less defined modes.

2.7.2 Stability and Cavity Modes The Fabry-Perot resonator can be used to conﬁne light and provide optical feedback for lasers. The advantage of a Fabry-Perot cavity is the easy fabrication with micro technology. The stability criterium for a laser cavity is given by [53]: L L 0 < 1 − 1 − < 1 (2.37) r1 r2 Chapter 2 Theory 36

Number of Cavity Modes vs Cavity Length Cavity Mode Wavelengths with Mode Numbers 20 50

18 49

16 48

14 47

12 46

10 45

8 44 Mode number: q 6 43 Number of Cavity Modes: m 4 42

2 41

0 40 0 2 4 6 8 10 12 14 16 18 20 540 550 560 570 580 590 600 610 620 630 640 650 Cavity Length [µm] Wavelength [nm]

(a) Plot of the calculated number of cavity modes in (b) Plot of the calculated cavity modes in a 10 µmlong a cavity containing Rhodamine 6G dissolved in ethanol cavity containing Rhodamine 6G dissolved in ethanol. which is 10 µm long.

Figure 2.23: Calculated cavity modes for a cavity containing Rhodamine 6G.

Here r1 and r2 are the radius of curvature for the mirrors and L is the cavity length. It can be reasoned that a flat mirrored Fabry-Perot cavity is not stable since the product in equation 2.37 gives 1 since r1 and r2 are infinite. At least one of the mirrors has to be a concave in order to satisfy the the stability criterium. In macro scale lasers usually both mirrors are concave which provides a stable gaussian laser beam output. Three dimensional concave mirrors will be very difficult to fabricate with micro technology. In a Fabry-Perot cavity a infinite number of lasing modes can exist theoretically at least. It is the light emission spectrum from the active medium, which puts a limit to the number of modes that will exist in the cavity. For each cavity mode the following condition for the length of the cavity should be fulfilled [50]: λ L = q (q +1) q =0, 1, 2, 3... (2.38) 2n Here n is the refractive index of the medium in the cavity and q is the mode number and λq is vacuum wavelength. L is the length of the cavity. To predict the cavity mode spacing a relation for the frequency spacing ∆νc between the modes in the cavity can be used [50]: c ∆ν = (2.39) c 2nL Here c is the speed of light in vacuum. It can be seen that in a large scale cavity there will be a lot of modes due to the low frequency spacing ∆νc. The fluorescent frequency range ∆νs for the active medium, expressed by the short wavelength λ1 where the fluorescent spectrum starts and the long wavelength λ2 where the fluorescent spectrum ends, is given by: 1 1 ∆νs = c − (2.40) λ1 λ2 Using equation 2.39 and 2.40 one gets an expression for the number of modes m that can exist in cavity with a certain length L. The spectral range is expressed as the wavelengths 2.7 The Fabry-Perot Interferometer 37

λ1 and λ2 as starting and ending points for the fluorescent spectrum: ∆ν 1 1 m = s +1=2nL − + 1 (2.41) ∆νc λ1 λ2 The result of the term 2nL 1 − 1 has to be truncated to nearest lowest integer. The λ1 λ2 number of modes has on average a linear relationship to the cavity length L as seen in figure 2.23(a) expect that the curve rises in discrete jumps like a stairway. Figure 2.23(b) shows the possible wavelengths with the respective mode numbers in a 10 µm long cavity filled with Rhodamine 6G dissolved in ethanol. With a fluorescent spectrum from 540 nm to 650 nm and a refractive index of 1.36 of the Rhodamine 6G/ethanol solution one gets 9 allowed cavity modes. The mode which will dominate and become lasing mode is the one which is closest to the maximum gain wavelength for the system.

2.7.3 Frequency Pulling of the Lasing Mode

Figure 2.24: Sketch of frequency pulling of the lasing mode away from the cavity mode towards the gain maximum [53].

Frequency pulling is a competing process between the gain maximum of the dye and the cavity mode nearest the gain maximum. Below lasing only the cavity modes will be visible. When threshold is reached the gain maximum will start to pull the lasing mode away from the cavity mode towards the gain maximum. Depending on the size of the Q-factor for the cavity and the gain curve the pulling will be more or less dominant. Remembering that the Q-factor can be written as the lasing frequency divided by the full width at half maximum one can write an expression for the lasing frequency [53]: ν0 + νc ∆ν0 ∆νc Q0 + Qc νL = = (2.42) 1 + 1 1 + 1 ∆ν0 ∆νc ∆ν0 ∆νc

The variables in the equation 2.42 are explained in ﬁgure 2.24. Chapter 2 Theory 38

2.8 Fluidic Properties in Microﬂuidic Channels

2.8.1 Fluidic Flow in Microsystems The flow in microsystems by means of microchannels is normally dominated by laminar flow. which ensures minimum mixing of chemical agents. This leaves diffusion as the only mixing mechanism. Two types of liquid driving methods are generally applied. Pressure driven flow by means of external pump systems or by for instance integrated movable membranes. Pressure driven flow has a characteristic parabolic velocity profile of the fluid in the channel with maximum velocity in the middle and zero velocity at the channel boundaries. The other widely used method is electroosmotic flow (EOF) where an electrical field is applied over the flow channel. The net flow of uncharged molecules is directed towards the cathode since the smaller positive ions gain a higher velocity than the heavier negative ions. EOF is applied in glass channels and can be used for separation of chemicals due to the very flat velocity profile across the channel cross section. The only suitable driving force for dye flow through a microchannel is pressure driven flow. This method is reliable, fast, and is not setting any limits for the choice of material for the microchannel. Driving a fluid through a channel at a constant velocity will create a counter pressure in the channel, which needs to be overcome by the pump, which is topic for section 2.8.2. There will also be a discussion about the evaluation of laminar flow in a channel by means of the Reynolds’ number.

2.8.2 Basic Fluidic Flow Theory The general equation for fluid dynamics called the Navier-Stokes equation can be used to fully predict the flow of a liquid or a gas in an arbitrary shaped duct. The general Navier-Stokes equation is a nonlinear differential equation, which can take temperature, gravitational, and compressional into account [12]:

DV ∂V = +(V ·∇)V (2.43) Dt ∂t In most cases it is sufficient to consider incompressible fluids (newtonian fluids) with constant viscosity η under constant temperature and pressure P and neglecting gravitational effects. The velocity v of the fluid will thereby be constant in the flow direction x and only vary in the transversal directions. This kind of flow is referred to as Poiseuille Flow and a simpler differential equation can be applied [12]:

∂2v ∂2v 1 ∂P x + x = = constant (2.44) ∂y2 ∂z2 η ∂x

With the boundary condition vx = 0 at the channel surface one can solve the equation and find the relation between pressure drop ∆P and flowrate Φ for a stationary fully developed laminar flow in a channel [12]:

ηL ∆P = C Φ (2.45) A2 2.8 Fluidic Properties in Microﬂuidic Channels 39

Here L is the length of the channel and C is a constant depending on the profile of the channel cross sectional area A.WithC =8π for a circular tube one gets a pressure drop given by [12]: 8ηL 8ηL ∆P = Φ= v (2.46) πR4 R2 Where Φ = Av and v is the average velocity of the liquid. Equations 2.45 together 2.46 can be extended for any geometry if and effective an effective radius can be found. Such parameters exist for the most common geometries and is called the hydraulic diameter Dh. For a circular channel this is simply the diameter of the channel. For a rectangular channel the hydraulic diameter is given by [3]: 2HW D = (2.47) h W + H Here W is the width and H is the height of the channel. An expression for the pressure drop in a rectangular channel can now be expressed with the appropriate value for the constant C and the hydraulic diameter Dh for the rectangular shape [17]: η(W + H)2L η(W + H)2L ∆P =8 Φ=8 v (2.48) (WH)3 (WH)2 Figure 2.25 shows calculated pressure drops for different sizes of rectangular and circular shaped channels.

Pressure Drops in Micro−channels 4 10

3 10

2 10

1 10

0 10 Pressure [atm]

−1 10 H=1µm H=10µm −2 10 H=100µm H=1000µm

−3 Circular 10 0 1 2 3 10 10 10 10 Channel Width or Diameter [µm]

Figure 2.25: Pressure drops across 1 mm long micro-channels with ethanol ﬂowing at a velocity of 10 m/s. Solid curves represent channels with rectangular shape while the dashed curve represents a circular shape.

The important Reynolds’ number Rn used in hydrodynamics to predict laminar or turbulent behavior of a ﬂow. Reynolds discovered the relation between inertia (kinetic) forces and the viscous forces in a circular pipe. The Reynolds’ number can be extended to other geometries by means of the hydraulic diameter and is given by [3, 7]:

inertia forces ∝ ρv2 ρvD R = = h (2.49) n viscous forces ∝ η v η Dh Chapter 2 Theory 40

Reynolds´ number for Micro−channels 4 10 H=1µm H=10µm H=100µm H=1000µm 3 10 Circular

number 2 ´ 10 Reynolds 1 10

0 10 0 1 2 3 10 10 10 10 Channel Width or Diameter [µm]

Figure 2.26: Reynolds’ number for 1 mm long micro-channels with ethanol ﬂowing at a velocity of 10 m/s. Solid curves represent channels with rectangular shape while the dashed curve represents a circular shape.

Figure 2.26 shows the calculated Reynolds’ numbers for different sizes of rectangular and circular shaped channels. Reynolds reasoned that at low flow velocities the viscous forces were dominating and the flow would be laminar. At high flow velocities the inertia forces will dominate and turbulent flow will be more likely. Fluidic flows with a Reynolds’ number lower than 2300 can be considered fully laminar in a channel after a certain entrance length Le [7]. If a channel is abruptly narrowed the fluidic flow will be turbulent around the narrowing. One can define the entrance length where the flow will be laminar again. With the hydraulic diameter and the Reynolds’ number, one can estimate the entrance length for development of laminar flow [12]:

Le ≈ (0.5+0.05Rn)Dh (2.50)

Figure 2.27 shows calculated Reynolds’ numbers for different sizes of rectangular and circular shaped channels. The average velocity for the calculated curves, for the pressure, the Reynolds’ number, and the entrance length, is assumed to be 10 m/s. This is a very high flow velocity and is chosen since this is the speed used in traditional macro-sized cw dye lasers. High velocities of around 10 m/s can prevent triplet absorption, if the spot size of excitation is around 10 µm as a rule of thumb. 10 m/s is hardly durable in a microsystem since it puts high demands on the quality of the fluidic connections. It can be seen from figure 2.25 for a constant average fluid speed and for a large width W meaning W H the pressure is only depended on the height H. This can also be reasoned from equation 2.48 where it can be seen that the pressure is inverse proportional to the height to the power of two. This will put a restriction on how small the channel hight can be since additional increase of the width will not help reducing the pressure. The length of channel could be reduced in order to reduce pressure but bulky fluidic connections will put a limit on how short the channel length can be. 2.8 Fluidic Properties in Microfluidic Channels 41

Entrance Length for Micro−channels 6 10 H=1µm H=10µm 5 10 H=100µm H=1000µm 4 Circular 10

3 10

2 10

1 Entrance Length [µm] 10

0 10

−1 10 0 1 2 3 10 10 10 10 Channel Width or Diameter [µm]

Figure 2.27: Entrance length for developing laminar flowin 1 mm long micro-channels withethanol flowing at a flow velocity of 10 m/s. Solid curves represent channels with rectangular shape while the dashed curve represents a circular shape. Chapter 2 Theory 42

2.9Chapter Summary

The important principal of stimulated emission, discovered by Einstein, was presented. Stimulated emission is the fundamental process for laser operation. The important concept of the round-trip gain for lasers was introduced and verified the importance of cavity mirrors which provide the optical feedback. The dye molecules consist of conjugated double bonds where the bonding π electrons are delocalizes and can move freely in the so called chromophoric region. The absorption and fluorescence of organic dye molecules were explained by considering the chromophoric region of the dye molecule as an infinite quantum well. The important Rhodamine laser dyes belonging to the group of Xanthene dyes was introduced. Rhodamine dyes are especially interesting due to their high fluorescent quantum yield. The three important energy levels in a dye was introduced. The two lowest energetic singlet states are responsible for the light absorption and emission. Excited π electrons can make an intercrossing by means of flipping the electron spin and then end up in the triplet state. The triplet state will absorb laser light and is considered as a major obstacle for obtaining lasing in continuous wave mode. Time-dependent coupled rate equations were presented for keeping track of electrons in the different states and also for keeping track of photons flux having the excitation wavelength and the lasing wavelength. A stationary approximated solution for the lasing photons lead to the condition for the critical inversion population for lasing threshold. Previously studies in the late 1960’s revealed that a dye has a self-tuning mechanism of the lasing wavelength which depends on both dye concentration and the cavity/dye cell length. The lasing wavelength goes towards longer wavelength for both increasing concentration and cavity/dye cell length. Besides the self-tuning a calculation revealed not surprisingly that the threshold molecule concentration is reduced then mirror reflectance is increased and threshold concentration is increased when the dye concentration is increased. Theoretical studies of mirror reflectance and transmission were employed and both dielectric stacks and metallic mirrors were considered. Both types of mirrors were studied with the same model which can be used for multi-layers and by introducing the imaginary refractive index all kinds of materials can be considered. The advantage of dielectric mirrors is the design freedom to control the desired reflectance and very high reflections can be obtained. The dielectric mirror is also in simple optical theory considered non-absorbing. Fabrication of such mirrors is clearly a disadvantage since it has high requirements on the thickness control. Metallic mirrors have the advantage of fairly easy deposition and thickness control by means of e-beam deposition. The clear disadvantage is the absorption of light which prevents high power operation. Laser cavities consist of two mirrors separated with a certain distance which contains the active gain medium of the lasers. Cavities provide optical feedback and confinement of the laser light into discreet modes. Changing the distance between the mirrors will tune the laser light output. Special attention was payed to the flat mirrored Fabry-Perot cavity, which can easily be integrated into microsystems compared to concave mirrors normally used in lasers due to stability reasons. The transfer function (Airy function) for the Fabry-Perot cavity was exploited. It was reasoned what small cavities had the advantage of fewer cavity modes, but lacked the very small linewidth which large scale 2.9Chapter Summary 43 cavities provide. Last the fluidic properties of newtonian fluids in microsystems were investigated by considering the fluidic flow as Poiseuille flow. Flowing a liquid through a channel creates a counter pressure which has to be overcome by the fluid pump. Rectangular and circular channel cross sectional shapes was compared. The rectangular shape with a width much larger than the height showed independency of the width on the pressure, if the flow velocity was kept constant. The pressure will then be proportional to the inverse of the height to the power of two. Reynolds’ number was introduced and it was reasoned what Reynolds’ number can distinguish between laminar and turbulent flow. Flow in micron sized channels are dominated by laminar flow. The Reynolds’ number could also be used for determining the entrance length into a channel before laminar flow is developed. Most of the topics considered here in this chapter were in the extend possible used for designing the micro-cavity dye laser, which will be presented in chapter 3. In chapter 4 measurements and characterization will be presented. The obtained data will be compared with the theory presented in this chapter. Chapter 3 Design and Fabrication 44

Chapter 3

Design and Fabrication

3.1 Chapter Outline

• Section 3.2 deals with the considerations for the design of the micro-cavity fluidic dye laser. The device dimensions are based on rough calculations of the round trip gain and the fluidic counter pressure. Mirror options are discussed together with excitation and light collection methods. The possibilities for fluidic connections are also considered.

• Section 3.3 deﬁnes and lists the device requirements considered in section 3.2.

• Section 3.4 discusses three diﬀerent fabrication schemes and their possible realiza- tion. The fabrication methods and device layout are presented for the diﬀerent fabrication schemes.

• Section 3.5 deals with two bonding methods namely anodic bonding and polymer bonding with SU-8 photoresist.

• Section 3.6 describes the two applied fabrication processes namely fabrication of microﬂuidic channel by means of KOH etch of silicon and the other by means of UV-lithography applied with SU-8 photoresist.

• Section 3.7 describes and discusses the fabrication results of the KOH etch based device by examination of photos of the devices.

• Section 3.8 describes and discusses the fabrication results SU-8 photoresist based device by examination of photos of the devices.

• Section 3.9 summarizes the chapter. 3.2 Considerations About Device Requirements 45

3.2 Considerations About Device Requirements

3.2.1 Round-trip Gain in Micron Sized Cavities

0.1

0.08

0.06 N1/N 0.04

0.02

0.2 0 0.4 0.02 0.6 0.04 0.06 0.8 0.08 1 0.1 R² N [mol/L]

Figure 3.1: Calculated critical inversion surface with a maximum threshold ratio value of 1/10. See ﬁgure 2.16 for further information.

The main concern about scaling down a laser to micro-level is whether the round-trip gain will be high enough to reach threshold, see equations 2.4, 2.21 and 2.23. The round- trip gain in a micro level laser will be small since the cavity/dye cell is very short as seen here: 2L(N1σe−N0σ0−NT σT ) G = R1R2e (3.1) Increasing the concentration will compensate the small length of the cavity since the potential gain per unit length will be larger. Increasing the concentration too much, dimerization will become a problem [9, 48]. Equation 3.1 can be rewritten to a form where the overall gain is expressed as gain per unit length for instance the unit [cm−1]: ln(G) 1 g = = N σ − N σ − N σ + ln(R R ) (3.2) 2L 1 e 0 0 T T 2L 1 2 Designing a laser cavity the cavity loss term is the parameter which can be adjusted: 1 2L ln(R1R2). To circumvent the problem of having a small cavity length is to ensure high reflectance of the cavity mirrors and to increase the concentration. As seen from figure 2.16 it should be feasible to have a cavity with a length in the micron range for instance 10 µmand still be able to obtain lasing. It requires though that the concentration is fairly high. The graph in figure 2.16 might be somewhat optimistic as argued before. According to Schäfer [48] a realistic threshold ratio should be around 1/10 for most practical systems rather than 1. Figure 3.1 shows a calculated critical inversion surface with a critical inversion ratio of 1/10. This gives a somewhat more pessimistic view and decreases the usable range of Chapter 3 Design and Fabrication 46 the concentration for which lasing is possible. However lasing should still be possible for high concentrations and fairly high mirror reflectance in a 10 µmlongcavity.

3.2.2 Mirror Options

Two types of high reﬂectance mirrors basically exist, namely dielectric and metallic mirrors. Common for both of them is that they need to be deposited in some fashion. As mentioned in chapter 1 only vertical emitting device is considered in this thesis. Fabrication of dielectric mirror as explained in section 2.6 will put high demands on the thickness control. The cleanroom at MIC does not have a tool for depositing of dielectric materials, where the thickness can be measured under deposition. Thickness measurements under deposition is required for making good quality optical interference layers, which depends highly on the layer thickness. Therefore metallic mirrors deposited by means of e-beam evaporation is the only solution if one considers the tool set available in MIC’s cleanroom. The sum up of this discussion is that vertical emitting devices with e-beam evaporated metallic mirrors would be the simplest solution and therefore best suited for the initial devices. Gold mirrors at wavelengths in the yellow area will have a maximum reﬂectance of around 80 %.

3.2.3 Excitation and Laser Light Collection

Two obvious excitation schemes could be applied for external laser excitation. One method is to make use of integrated waveguides to guide the pump laser light to the laser cavity. The other approach is just to excite through the glass lid on the chip. The waveguiding approach would be nice for compact devices. For instance a semiconductor laser could be coupled to an optical fiber, which was attached to the waveguide on the chip. Alignment of optical fibers to waveguides is a very time consuming process and is therefore not a very flexible solution in a laboratory measurement setup. The other approach is very suited for a lab setup since no fiber/waveguide alignment is required. The optical pump laser beam only needs to be guided by external mirrors down onto the surface of dye laser cavity. Using this optical pumping scheme, the fabrication process will also be simplified since integrated waveguides are not needed. Dye laser light collection for measurements of the optical spectrum will be rather easy. An optical fiber placed above the laser cavity can be used for collection of laser light. A better choice would be to have a microscope for collection of the emitted light with an output for measurements. In this way the distance between the microscope lens and chip will be the same when the microscope is in focus. Alignment of the microscope lens to the point of light collection will be easy due to the magnification of the microscope. Placing an optical fiber vertical precisely above the laser spot of the micro-cavity dye laser will be difficult. However, this is the only choice if the microscope option is lacking. Precise alignment will be necessary if the amplitude of optical spectra from different measurement series are being compared. 3.2 Considerations About Device Requirements 47

3.2.4 Dye Flow Speed Dye ﬂow speed is important when considering dye lasers with continuous wave optical pumping due to three things: • Avoiding triplet absorption of laser light.

• Avoiding thermal heating leading to thermal induced gradients of the refractive index of the laser dye.

• Avoiding bleaching of dye molecules due to high pump intensities since old dye is replaced with new. Continuous wave optical pumped dye lasers often use water and a detergent as solvent. Water has a high heat-capacity but dyes easily forms aggregates or dimers/trimers etc. in water at high concentrations. Therefore a detergent is often added to lower the surface tension between the molecules. The rule of thumb for the dye speed is 10 m/s for an excitation spot of 10 µm. This results in a transition time of 1 µs through the excited area. This should be suﬃcient to avoid a large part of the triplet absorbtion of the stimulated emission laser light. Triplet absorbtion is a limiting factor, which can prevent lasing in cw mode [48, 55]. These fast velocities can also be employed in large scale dye jet steams. Ethylene glycol is then used as a solvent due to a high viscosity ensuring a laminar ﬂow of the jet stream. Rhodamine 6G dissolved in ethylene glycol has a high triplet state lifetime of around 5 µs[55,56]. Fast transition times of the dye solution through the optical pump region is required in order to a large absorption by the triplet state.

Pressure Drop Pressure Drop 3 3 10 10

2 10 2 10

1 10

0 10 Pressure [atm] Pressure [atm]

0 10 −1 10

−1 −2 10 10 −2 −1 0 1 0 1 2 3 4 10 10 10 10 10 10 10 10 10 Water Flow Speed [m/s] Water Flowrate [µL/min]

(a) Pressure drop due to water ﬂowing in a channel which (b) Pressure drop due to water ﬂowing in a channel which is 10 µm high, 1 mm wide, and 2.5 cm long. is 10 µm high, 1 mm wide, and 2.5 cm long.

Figure 3.2: Counter pressure in a channel calculated for different water flow speeds or flowrates.

Thiel et al. [56] was able to produce cw lasing from solution of Rhodamine 6G dissolved in water plus 2 % Ammonyx LO only flowing at a speed of 0.03 m/s corresponding to transition time of the dye through the excited area of 0.5 ms. This suggests that the lifetime of the triplet state is reduced with water as a solvent. It should however be noted Chapter 3 Design and Fabrication 48 that mirrors with reflectance of above 99.8 % were applied and the dye cell depth was 1 mm. The concentration was 10−4 mol/L. The high reflectance and the relative large dye cell gives a much higher round-trip gain than one would be able to obtain in a micron size dye cavity. If dielectric mirrors with the same reflectance were used as cavity mirrors for a10µm long cavity, it would roughly require a 100 times higher concentration to obtain cw lasing with a dye flow speed of 0.03 m/s. Increased the dye concentration a 100 times dye aggregation in water will become a problem. In a pulsed optical pump mode the dye flow speed is not of any greater concern since thermal problems is smaller and the triplet excitation would be negligible at least for pump pulses in nanosecond range [44]. In section 2.8 the pressure versus velocity dependency was derived and a graph was shown in figure 2.25. For a 10 µm high channel it was seen that the pressure was not reduced further by increasing the width beyond 1000 µm for a constant velocity. Con- sidering a channel, which is 10 µm high, 1 mm wide, and 2.5 cm long with water flowing through, a pressure versus flow speed curve can be calculated as shown in figure 3.2(a). As seen, a flow speed of 10 m/s is unrealistic in a channel of this size due to a counter pressure of 200 atm. A realistic maximum counter pressure is around 10 atm according to Larsen [26] who studied micro liquid handling for his Ph.D. He tested different fluidic connections schemes and got a maximum allowed counter pressure of around 10 atm for his best system. The system was based on a O-ring principal for tightening between the microfluidic chip and the chip mounting system. A pressure of 10 atm in a channel with the dimensions described above corresponds to a flow speed of 0.5 m/s.

3.2.5 Fluidic Connections

The important, and sometimes ignored, concept in microtechnology research is packaging. Fluidic connections is one of many packaging subjects and perhaps one of the more important ones. Having fluids transported to a microfluidic system may not be a trivial thing and should be considered very carefully. Fancy and complicated chips fabricated in the cleanroom might suffer from bad design with respect to connection of fluidic hoses etc. In some cases it ends up with gluing of hoses onto the chip. This is a very cumbersome process with respect to get a tight and strong fluidic connecting. The flexibility of a glued system is zero if change of the chip is required.

Inlet Outlet O-Ring

Chip

Figure 3.3: Sketch of the fabricated ﬂexible chip mount system, which allows for fast change of the chip. O-rings have been applied for ensuring a tight interface between the chip and the chip mount system. 3.3 Device Requirements 49

Therefore a mounting system is preferred with hoses for fluidic transport and O-rings for tightening which are permanently connected and attached. The mounting system would consist of a base and a lid, which can be mounted together by means of screws. The microfluidic chip can then be placed on the base and a lid can be mounted on the top. Such a system is flexible and will always be tight. Change of the microfluidic chip could be done in order of minutes and experiments could be carried on without much of delay. Figure 3.3 shows a sketch of the fabricated flexible chip mount system. Inlet and outlet can be placed on either the top side or the bottom side. The top side for inlet and outlet is chosen since mounting of the chip holder to an xyz stage will be easiest without any inlet and outlet on the bottom side.

3.3 Device Requirements

In section 3.2 there were considerations about the size of the laser cavity and whether if it would be possible to obtain lasing in a 10 µm long cavity? The answer is yes for high dye concentration and with fairly high mirror reflectance. A 10 µm long cavity will provide few cavity modes and a comparable size with other microfluidic components which will ease up future integration. It was argued in section 3.2 that vertical emitting devices would be the best choice and the mirrors should be fabricated be means e-beam evaporation of gold. Gold is a better choice than aluminum since gold does not corrode as aluminum, which in return has a better reflectance. A cavity length of 10 µm puts limits on how large a flow that can be pressed through the channel, and realistically a maximum obtainable velocity is around 0.5 m/s. This should be compared with the 10 m/s usually applied with jet stream dye lasers. The channel length should be in the centimeter range since bulky fluid connections take up some space. The width of the channel should be around 1 mm since additional increase will not significantly decrease the counter pressure developing in the channel when a fluid is flowing through with constant velocity. The design of the microfluidic chip should allow for fluidic connections which allows for fast and flexible change of the microfluidic chip. The main requirements for the chip design is listed here: • Cavity length of around 10 µm. • Channel width of around 1 mm. • Channel length of around 2 cm. • Metallic gold cavity mirrors. • Vertical emitting device. • Excitation through the glass top lid of the chip. • Collection of laser light through the glass top lid of the chip. • Allow for chip mount system. • Inlet/Outlet through the top glass lid of the chip. Chapter 3 Design and Fabrication 50

Having established the the device requirements we are ready to move on to design of the micro-cavity ﬂuidic dye.

3.4 Fabrication Schemes for the Micro-Cavity and Layout

With the design requirements defined the design of the micro-cavity dye laser will be fairly simple process. The actual design will depend on which kind of fabrication process one applies. The fabrication processes that will be used are cleanroom standard silicon microtechnology processes or silicon technology compatible processes. The microfluidic channel can be fabricated by different schemes. Some of the possibilities are considered in the next sections.

3.4.1 KOH Anisotropic Etch

Figure 3.4: Sideviewof a laser cavity. Sketch of a KOH etched micro-cavity withgold/chromium mirrors as top and bottom. The Pyrex glass lid is anodic bonded onto the silicon wafer. The dye ﬂow is in the y-direction.

The bottom of the channel should act as a maximum reflecting mirror for the cavity. One would like a smooth flat bottom which can be used for deposition of the gold mirror. KOH-etching of silicon is anisotropic and the etch rate is depended on the crystal planes [30, 54]. KOH dissolved in water has the highest etch rates at a concentration of around 7 mol/L [66] where the etch rate of the (100) plane is highest and the (111) plane has the lowest etch rate. A (100) silicon wafer will be etched in a trapezoidal form, where the (111) plane will appear as the oblique surfaces. The (100) plane will appear at the bottom of the etched channel. If the bottom (100) plane is flat enough it will be suitable as a deposition surface for gold. Figure 3.4 shows a micro-cavity constructed in a KOH etched channel. The mirrors are e-beam deposited gold with chromium as adhesion layer. The glass lid is anodic bonded to the silicon wafer. The obvious drawback of this etch technique is that the layout of the microfluidic channel is restricted to rectangular shapes aligned to the crystal planes of silicon. 3.4 Fabrication Schemes for the Micro-Cavity and Layout 51

3.4.2 SOI Wafers

Figure 3.5: Sideviewof a laser cavity. Sketch of a micro-cavity defined in a SOI wafer. The micro-cavity is defined by a deep dry etch of the top silicon layer and then a wet oxide etch. The mirrors are e-beam deposit gold/chromium as the top and bottom. The Pyrex glass lid is anodic bonded onto the silicon wafer. The dye flow is in the y-direction.

Silicon on insulator (SOI) wafers are silicon wafers with a buried oxide. The idea is to etch down vertically to the oxide, which acts as an etch stop. The etch could be performed by a deep reactive ion etch. The oxide could then be removed by a wet HF etch and the smooth surface of silicon would then appear in the bottom of the channel. Figure 3.5 shows a sketch of a fabricated micro-cavity with SOI technology. The glass lid is anodic bonded to the SOI wafer and the cavity mirrors are e-beam deposited gold and chromium. The shape of the microﬂuidic channel is not restricted as for instance with fabrication by means of KOH etch.

3.4.3 SU-8 Photoresist

Figure 3.6: Sideviewof a laser cacity. Sketch of a micro-cavity defined withSU-8 photoresist. The mirrors are e-beam deposit gold/chromium as the top and bottom. The substrate wafer is bonded onto the SU-8 defined micro-cavity by means of a SU-8 bonding layer. The dye flowis in the y-direction. Chapter 3 Design and Fabrication 52

SU-8 is a photo-definable UV-epoxy photoresist, which can be spin-coated in thick layers on smooth substrates. SU-8 can be spin-coated in layers ranging from 1 to 500 µm, which makes the resist very useful for many purposes. SU-8 photoresist is not restricted to silicon fabrication techniques but uses only UV lithography for definition [29]. Besides being a useful thick photoresist SU-8 has the advantage of having nearly vertical sidewalls, which makes SU-8 useful for microfluidic channels [21, 27]. Sealing of a SU-8 channel can be performed by polymer bonding for instance with a thin SU-8 layer as indicated in figure 3.6. There is no restriction on the microfluidic channel shape as with the KOH etch. Fabrication with the SU-8 photoresist starts with a spin-coat of a thin film of liquid SU-8. The solvents in the SU-8 solution is baked out. An UV-exposure through a mask crosslinks the SU-8 and is followed by a bake for further crosslinking. Last the SU-8 is developed by propylene glycol monomethyl ether acetate (PGMEA) which removes the non-exposed SU-8 [29].

3.4.4 Device Layout

Fluidic Transport Part Fluidic Transport Part

InletCavity Area Outlet Inlet Cavity Area Outlet Y Y

X X 1 mm 2 mm 1 mm 2 mm

2.5 cm 2.5 cm (a) Topview. Layout of the microfluidic channel for the (b) Topview. Layout of the microfluidic channel for the KOH etched microchannel. The two wide parts of the SU-8 photoresist based channel. channel are called the fluidic transport part of the channel.

Figure 3.7: Layouts for the KOH etched and SU-8 based microﬂuidic channels.

Based on the device requirements defined in section 3.3 and the fabrication schemes discussed in the last three sections a device design can now be suggested. Two different channel layouts are shown in 3.7. The option with the SOI wafer was only considered if non of the other two approaches for fabrication would work. The inlet and outlet are also indicated on the drawings. The inlet and outlet will be around 1 mm in diameter due to drill size. Both types of channels are expanded to 2 mm in size due to the inlet/outlet hole of around 1 mm. In both ends of the SU-8 based layout the round expansion is even larger. In the thin part in the center of the channel, the channel is 1 mm wide, which should be sufficient in order to reduce the fluidic pressure. The pressure will be high since the channel height will be around 10 µm. The rectangular shapes of the KOH etched channel is simple due to the anisotropic properties of the KOH etch rates in the different crystal directions of silicon.

3.5 Bonding and Sealing of Microﬂuidic Channels

Sealing of microsystems by means of bonding is a very important subject for fabrication of microsystems. Microsystems especially ﬂuidic microsystems will require sealing or 3.5 Bonding and Sealing of Microﬂuidic Channels 53 packaging of some kind. Problems with fabrication of microsystems are often due to bonding of lids etc.

3.5.1 Anodic Bonding One of the most applied bonding types is anodic bonding. Anodic bonding involves an electrical field as the name indicates. Anodic bonding involves a silicon wafer and sodium doped glass wafer. Bonding a Pyrex sodium doped glass wafer onto a silicon wafer with microfluidic channels is a common task for fabrication of µ-TAS components. The bonding process is sketched in figure 3.8. The process of anodic bonding relies on

O2- O2- O2- O2- O2- Glass Na+ Na+ Na+ Na+ Na+

Figure 3.8: The process of anodic bonding. The anode is placed on the silicon substrate and the cathode is placed on the sodium doped glass. Thermal heating is applied in the temperature range 200 ◦Cto 450 ◦C. The applied field can be ranging from 100 V to 1000 V depending on the wafer or film thicknesses. The illustrated principal is proposed by Berthold et al. [6]. the applied electric field and the thermal applied heat. The thermal energy will make the ions in the glass mobile. The electrical field pulls the positive sodium ions to the cathode and the negative oxygen ions towards the anode. It has been proposed by Berthold et al. [6] that the following reaction takes place at the interface between the silicon and the glass: 2− − Si + 2O → SiO2 +4e (3.3) The electrical field will deplete the region near the interface and a large part of the electrical field will be located here. Anodic bonding gives a very strong bonding which are suitable for sealing of fluidic channels. The drawbacks of anodic bonding is the high electrical field which might be a problem with electrical circuits. The relative high temperature is generally a problem for thin film deposit metals.

3.5.2 Polymer Bonding Polymer bonding is a low temperature bonding method, which beneﬁts from the sticki- ness of polymers above their glass transition temperature. Pan et al. [37] has developed Chapter 3 Design and Fabrication 54 a polymer bonding method for low temperature, which requires a bonding tool providing vacuum, thermal heating, and sample pressure. The photoresist for polymer bonding is spin-coated onto both wafers surfaces. The photoresist on both wafers are patterned by means of UV-exposure and development. This allows for localized bonding and encap- sulation of microstructure or systems in airtight microcavities. The localized polymer bonding pads on both wafers are aligned together and vacuum is established. The wafers are heated up and pressed together. Bonding with SU-8 at a temperature of 90 ◦Canda applied pressure of 50 N gave the highest bonding strength. Pan et al. tested four types of photoresist and found that SU-8 had the highest bonding strength and reasoned that this could primarily be explained by the epoxy features of SU- 8.

Development Hole Lid Glass Wafer Lid Glass Wafer SU-8 Bonding Layer SU-8 Channel Layer SU-8 Layer

Substrate Wafer Substrate Wafer

(a) Sideview. Sealed and bonded SU-8 structure by (b) Sideview. Sealed and bonded SU-8 structure by means of SU-8 bonding layer. means of SU-8 development through inlet and outlet holes.

Figure 3.9: Sideviewsketch of twoSU-8 bonded structures bonded withdiﬀerent procedures proposed by Jackman et al. [21].

Jackman et al. [21] have presented a bonding procedure with SU-8 which does not require expensive bonding equipment. Jackman et al. have investigated sealing of microfluidic channels fabricated with SU-8. The sealing of the SU-8 structures can be done in two ways. Spin-coat a thin SU-8 layer on the lid wafer of glass and bake out the solvents. Heat up both wafers up to around 75 ◦C and press them manually together and then UV-exposure plus a post exposure bake for crosslinking. A sketch of the bonded structure is shown in figure 3.9(a). The other approach is to fabricate the SU-8 structures within the bonding layer of SU-8. A SU-8 layer is spin-coated onto the substrate wafer and solvents is evaporated of by a pre-exposure bake. The top wafers has pre-drilled or etched holes for inlet and outlet. The wafers are heated up to around 75 ◦C and pressed together. The SU-8 layer is UV-exposed through the glass lid and the SU-8 channels are defined. The bonded wafers are post exposure baked and then developed through the inlet and outlet holes. Figure 3.9(b) shows a sketch of the bonded structure. 3.6 Fabrication Process Descriptions 55

3.6 Fabrication Process Descriptions

The fabrication sequences and the process recipes for both the the SU-8 based device and KOH etch based device are shown in appendices A and B respectively. The appendices A and B contain a short numbered stepwise comment on each process step. The number corresponds to the number shown at the corresponding drawing of the fabrication process sequence also shown in the appendices. The photolithographic masks are shown in the appendices I and J for the SU-8 based device and the KOH etch based device respectively.

3.6.1 KOH Etch Based Device The idea of the KOH etch based laser was to make use of the flat (100) plane, appearing in KOH etched silicon in the bottom of the etched channel, as the bottom mirror. It is therefore crucial that the bottom (100) plane is really flat and smooth without any structures on surface. A rough surface will result in diffuse light reflection, which means that light will be reflected in all direction instead of normal to the surface as desired for a laser cavity mirror. Strong reflectance will increase the optical feedback in the laser cavity. The microfluidic channel will be etched in two steps. The first step will etch the wide part of the channel, which are called the fluidic transport part of the channel, see figure 3.7(a). The last step will etch both the transport parts and the cavity part. This will leave us with some long deep transports channel parts, which will reduce the fluidic counter pressure. The KOH etch can be divided into two steps by using three materials as temporarily masking layers: Photoresist, LPCVD nitride (Si3N4), and thermal oxide (SiO2). The whole microfluidic channel is defined by a photolithographic step with a photoresist. The pattern is transferred to the nitride by a RIE etch of the blotted nitride. Another photolithographic step is employed and will only define the wide fluidic transport channels. The pattern is transferred to the oxide by means of a buffered hydrofluoric acid (BHF) etch of the blotted oxide. The first KOH etch can now be performed. KOH will also etch some of the oxide at the laser cavity site and therefore the oxide is fairly thick (see appendix B for illustrations). The remaining oxide at the laser cavity site is removed and the last KOH etch can be performed. The existing nitride is removed in a hot phosphoric acid (H3PO4) and the underlying oxide is removed during the RCA cleaning in HF. Next a photolithographic step is employed for defining the bottom gold mirror on the (100) plane of the etched silicon in the laser cavity site. Chromium and gold is e-beam evaporated and a lift-off metal process is employed in acetone with ultra sonic. The lift of process will remove the unwanted gold and chromium together with the photoresist. A Pyrex glass wafer for the lid is cleaned according to the procedure explained in appendix B. The glass wafer is covered with a blue plastic film, which is chemical resistant towards HF. The blue plastic film is patterned for inlet and outlet holes by means of burning holes in the plastic film with a laser writing system. Afterwards the Pyrex wafer is etched in 40 % HF for defining the inlet and outlet holes. The HF etch gives very large holes since it is an isotropic etch. The structured silicon wafer and the structured pyrex are then bonded together with anodic bonding. The holes for inlet and outlet are hard to align to the fluidic channels Chapter 3 Design and Fabrication 56 due to alignment by hand. Last the wafer is diced into individual chips. The width of the cavity area are fabricated in three sizes: 100 µm, 500 µmand 1000 µm in order to test the fabrication and optionally the performance of the laser cavity for different widths.

3.6.2 SU-8 Photoresist Based Device The fabrication sequence is shown in ﬁgure 3.10 in a compact version. The fabrication

1: Top: 1.5 µm photoresist 5: Top: 10 µm SU-8

2: Top: Expose and develop 6: Top: Expose, bake, develop

6: Bottom: 5µmSU-8 3: Top: Cr/Au5/40nm 7: Bonding, expose, bake

4: Top: Lift off metal 8: Dicing. Drill inlet and outlet

4: Bottom: Cr/Au 10/150 nm

Figure 3.10: Fabrication process steps. 500 µm thick Pyrex glass wafers are used as top and bottom substrates. 1-4: Metallic mirrors are deposited on the top- and bottom wafers by standard UV-lithography, electron-beam evaporation and lift-off. 5-6: The micro-flowchannels are defined in SU-8 photoresist on the top wafer by spin-coating, soft-baking baking, UV-exposure, post exposure baking and development. A5µm thick SU-8 bonding layer is deposited on the bottom wafer by spin-coating and soft-baking. 7: The wafers are bonded by a manually applied pressure at 75 ◦C. The wafers are further sealed by a bake at 90 ◦C for an hour and then cooled down before cross-linking of the SU-8 by an UV flood-exposure, followed by a post exposure bake. 8: The chips are diced and inlet/outlet holes are drilled. starts by cleaning of the both top and bottom Pyrex glass wafers by means of Triton X-100 soap and a 7-Up etch (hot sulphuric acid + ammoniumperoxodisulphate). The top mirror is defined by a standard lithographic step followed by e-beam evaporation of 5 nm chromium and 40 nm gold. The excess chromium and gold is removed by a standard lift-off process by means of hot acetone and ultra sonic. The top mirror is designed for allowing light to be transmitted. The calculated reflectance for the top mirror is 72 % and the transmittance is 6 % in the yellow part of the optical spectrum. The calculation are performed with the optical model presented in section 2.6.1. The bottom mirror is fabricated only by means of e-beam evaporation. The chromium thickness is 10 nm and the gold thickness is 150 nm resulting in the maximum reflectance of 83 %. The transmittance is 0 % and the rest is absorbance. The calculated curves for the optical properties of the top mirror are shown in figure 3.11. The top wafer is spin-coated with a10µm thick layer of SU-8 photoresist and is then soft-baked on the spinner hotplate. Prior to spin-coating the wafer has been dehydrated by means of a bake at 120◦C for 3.6 Fabrication Process Descriptions 57

12-18 hours . The SU-8 photo resist is exposed by UV-light through a photolithographic mask defining the microfluidic channel. The mask also contains the option for fabrication of waveguides in SU-8. The waveguides is placed perpendicular to the laser cavity in the center of the microfluidic channel. The SU-8 polymer waveguides could be used for excitation or for absorption measurements. The mask is shown in appendix I. After the UV-exposure the wafer is post exposure baked for crosslinking. Now the wafer can be developed in PGMEA followed by a rinse in isopropyl alcohol (IPA). The bottom wafer is dehydrated by a bake at 120◦C for 12-18 hours before spin-coating a 5 µmthicklayer of SU-8. The SU-8 photoresist is soft-baked on the spinner hotplate. The procedure in section 3.5.2 presented by Jackman et al. [21] is the basis of the polymer bonding, which will be presented here. Both wafers top and bottom are heated to a temperature of 75 ◦C on a hotplate. The wafers are manually pressed together until nearly all Newton rings have disappears. A bake at 90 ◦C for one hour is an extra process step compared to method of Jackman et al. This step has proven to increase the hermetic quality and the bonding strength and was inspired by Pan et al. [37]. Pan et al. used the temperature of 90 ◦C during bonding, which gave the highest bonding strength. The bonded wafers are then UV-exposed and post exposure baked for crosslinking the bonding layer. Last the bonded wafers are diced into chips and inlet and outlet holes are drilled. The width of the cavity area is fabricated in three sizes: 100 µm, 500 µm and 1000 µm in order to test the fabrication and optionally the performance of the laser cavity for different widths.

Au/Cr top−mirror at a wavelength of 570 nm Au/Cr top−mirror at a wavelength of 532 nm 1 1 0 nm Cr 0.9 R 0.9 5 nm Cr 10 nm Cr 0.8 0.8 R 0.7 0.7

0.6 0.6 0 nm Cr 0.5 5 nm Cr 0.5 10 nm Cr 0.4 0.4 A 0.3 0.3

0.2 A 0.2

0.1 0.1

Reflectance, Transmittance, and Absorbance T Reflectance, Transmittance, and Absorbance T 0 0 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 Au thickness [nm] Au thickness [nm]

(a) The optical properties for the top mirror at a wave- (b) The optical properties for the top mirror at a wavelength of 570 nm. length of 532 nm.

Figure 3.11: Comparison of the optical properties at wavelength of 570 nm (lasing) and a of wavelength 532 nm (pumping) of the top mirror. Chapter 3 Design and Fabrication 58

3.7 Fabrication Results for the KOH Etch Based De- vice

The KOH etch based micro-cavity devices were the ﬁrst type of devices to be fabricated. A fabricated device is shown in ﬁgure 3.12(a). The narrow part of the channel is the

(a) Topview. Photo of a KOH anodic bonded and diced (b) Topview. Zoom in on a laser cavity with evaporated device. The chip is 3 cm long and 1 cm wide. The cavity gold and chromium in the cavity area. The photo is taken in the center of ﬂuidic channels is 1.5 mm long and 1 mm before the anodic bonding. Small bumps due to the KOH wide. etch are visible on the gold surface. The width of the this cavity channel is 0.5 mm.

Figure 3.12: Photos of fabricated KOH-etched based devices. See the layout sketch in figure 3.7(a) for comparison. cavity area where the bottom is covered with e-beam evaporated gold and chromium. In the ends of the channel are located the HF etched inlet and outlet in the glass wafer. The etch gives very wide holes, which has been accounted for in the design of the chip mount by means of large O-rings. As seen from figure 3.12(a) the holes is around 2.5-3 mm in diameter. The principal of etching the microfluidic channel worked well but the KOH etch itself gave problems. It turned out that the theoretical ideal KOH etch is not that ideal on micro level. The KOH etch in silicon releases hydrogen, which sticks to the surface in small bubbles. The hydrogen bubbles will introduce the effect of micro-masking [33]. Micro-masking will eventually give the KOH etched surface a rough character, which will increase as the etch progresses. The effect of micro-masking is shown at figure 3.12(b), where a zoom in on the laser cavity is shown. A layer of gold and chromium is deposited on the bottom of the cavity. Clearly visible bumps are seen. These bumps were present in all fabricated devices and seemed to be increasing as the etch depth was increased. It became clear with the initial fabrication run that the roughness would become a problem and a quick etch test was conducted. It showed that the roughness was increased with etch depth. Figure 3.13 shows the roughness of the (100) surface plane in the bottom of a channel as a function of etch depth. Figure 3.13 clearly shows a steady increase in peak to peak roughness as the etch depth is increased. In this particulary case the peak to peak roughness was 40 nm at an etch depth of 10 µm. The problem with the bumps could 3.7 Fabrication Results for the KOH Etch Based Device 59

Roughness of the [100] plane in KOH etched channels 80

30 Roughness peak to [nm] 20

10 3 4 5 6 7 8 9 10 11 12 13 14 Channel depth [µm]

Figure 3.13: The measured peak to peak roughness on the (100) plane in a KOH etched channel for different etch depth. prove to be a problem, when using the etched surface as a cavity mirror. The bumps will produce diffuse reflection. The next crucial step after the KOH etch and the metal evaporation is the anodic bonding. The high temperature required by the anodic bonding might prove to be a problem for the gold/chromium layer deposit on the silicon surface. The standard anodic bonding process at MIC requires a temperature of 350 ◦C. The high temperature will enable the ions in the glass wafer to become mobile in the electrical field which is put across the structure. The fairly high temperature did prove to be a major problem as shown on the photo in figure 3.14. The photo shown in figure 3.14 reveals a very rough

Figure 3.14: Photo of a the bottom mirror surface in the cavity after the anodic bonding. The wide black vertical line is the oblique (111) plane leading down to the deep transport channels. The visual appearance, seen with the naked eye, of the mirror surface seemed dull and the yellow appearance of gold is gone. surface which is far from smooth as required for a mirror to be efficient. The appearance Chapter 3 Design and Fabrication 60 seen with the naked eye reveals a very dull surface indicating a very bad reflectance. The high temperature have enabled the chromium and gold to diffuse into the silicon an thereby ruined the fine optical properties of gold. This approach for making optical cavities was then rejected due to the two problems discussed above. Optimizing the anodic bonding with respect to temperature might be help. The KOH etch can be modified some by adding isopropyl alcohol (IPA) this should according to Zubel et al. [66] give smoother (100) planes. It was reasoned that optimizing of the KOH etch process and the anodic bonding process would be very time-consuming. Therefore other fabrication methods had to be considered. 3.8 Fabrication Results for the SU-8 Based Device 61

3.8 Fabrication Results for the SU-8 Based Device

(a) Topview. Photo of a fabricated SU-8 bonded device. (b) Topview. Close in on the transparent mirror in ﬁg- See ﬁgure 3.7(b) for comparison. ure 3.15(a).

Figure 3.15: Photos of a fabricated SU-8 bonded device.

SU-8 has been used a lot at MIC for some years for creating microfluidic channels, but not so extensively for complete wafer bonding. Bonding by means of SU-8 at MIC has to my knowledge been done on chip scale and not on wafer scale. A fabricated SU-8 based device is shown in figure 3.15(a). A zoom in on the cavity top mirror is shown in figure 3.15(b), where the microfluidic channel can be seen underneath since the mirror is semi-transparent.

(a) Topview. Drilled hole in the bonded SU-8/glass (b) Topview. Drilled hole in the bonded SU-8/glass structure showing Newton rings due to weak bonding structure showing no Newton rings indicated large bond- strength. Bonded without additional baking at 90 ◦C ing strength. Bonded with additional baking at 90 ◦C for 1 hour. for 1 hour.

Figure 3.16: Comparison of two drilled holes indicating the strength of two diﬀerent polymer bonding methods.

As discussed earlier the bonding method is primarily based on the method, which Jackman et al. [21] developed. The additional step with a bake at 90 ◦C for an hour was introduced since the bonding did not seem that strong. It can be seen from the photo in Chapter 3 Design and Fabrication 62

ﬁgure 3.16(a). The drilled hole for inlet/outlet has some Newton rings around as seen on photo. These rings appeared when the drill reached through the SU-8 layer. This indicates that the bonding is not that strong. The long baking step at 90 ◦C was introduced and the problems with Newton rings around the holes were eliminated as shown at ﬁgure 3.16(b). During the long bake the bonding became more uniform and smooth.

(a) Topview. A photo of a developed SU-8 photoresist (b) Topview. Close up on figure 3.17(b). which defines the microfluidic channel with integrated waveguides. The narrowed vertical trench is the laser cavity. The width of the laser cavity is 100 µm. The horizontal trenches perpendicular to the narrowed microfluidic channel defines a SU-8 waveguide on both sides of the microfluidic channel.

(c) Topview. A photo of a bonded and sealed microﬂuidic (d) Topview. Close up on ﬁgure 3.17(d). channel with integrated waveguides fabricated with SU-8 photoresist. The width of the laser cavity is 100 µm.

Figure 3.17: Comparison of the SU-8 structure before and after bonding for a 100 µm wide channel.

A drawback of this baking step was that the small 100 µm wide channels were filled up with some of the bonding layer of SU-8. This can be seen from the four photos in figure 3.17. Figure 3.17(a) and figure 3.17(b) shows the developed 100 µm wide cavity SU-8 structure before bonding. There are no metallic mirrors present on these photos. Figure 3.17(c) and figure 3.17(d) show a bonded 100 µm wide SU-8 structure. The channel is closed due to the bonding. It is believed that the SU-8 bonding layer gets too soft during the bonding and will be able to float due to capillary forces. 3.9Chapter Summary 63

The wider channels of 500 µm and 1000 µm were nearly all of them open for flow. The manually localized pressure on the bonding structure could also be an important factor in the filling of the small channel. It can also be seen from the photos that the integrated SU-8 waveguides will not be very efficient do to filling of the trenches which defines the waveguides. Improvements for the fabrication of the device will obviously be to prevent the SU-8 from blocking smaller channels or cavities. The next design could contain small holes in crosslinked SU-8 which could suck some of the soft SU-8 during the bonding, which might prevent the small channel from being blocked. Another interesting detail could be to use dielectric mirrors as substrates in order to increase the optical feedback and decrease the absorption of both pump light and laser emission light. This would most likely give better possibilities for characterization of the laser dye.

3.9Chapter Summary

The chapter starts with considerations about the device requirements. The most important requirement was to have enough round trip gain in the micron sized laser cavity. A calculation of the critical inversion surface revealed that lasing should be possible in 10 µm long cavity. Different mirror types for the micro laser was discussed and the choice of using metallic gold/chromuium mirrors in favour of dielectric mirrors was a combination of time and fabrication options. Dielectric materials can not really be deposited in MIC’s cleanroom with precise control of the layer thickness. It was also reasoned that the laser should be a vertical emitting device simply due to the easy fabrication, which horizonal emitting devices would not provide. The requirement of a 10 µm long cavity puts a limit on how fast the dye solution can flow though the cavity. The limit will be determined by the fluidic connections. The maximum flow speed has to be tested experimentally. Different fabrication schemes were considered and the KOH etch of silicon based was fabricated initially. It turned out that the KOH etch of silicon was not optical smooth, which was important since the bottom of the channel should be the substrate for the bottom mirror. The problem of a rough etch is believed to be a micro-masking effect from the etch released hydrogen, which forms bubbles on the silicon surface. Another and more severe problem became clear after the initial anodic bonding experiments. The high temperature needed for making the ions mobile in the glass wafer resulted in diffusion of the gold/chromium mirror layer into the silicon substrate. The result was a very rough and dull surface, no good for use as an optical mirror. Instead a SU-8 photoresist device was fabricated with success. The devices was based on SU-8 structured channels on glass substrates. The microfluidic channels was sealed by means of a bonded glass lid with a thin layer of SU-8 as the bonding glue. The bottom and top mirrors were fabricated by e-beam evaporation of gold and chromium on the glass substrates. A minor problem with the bonding method applied was filling of the smallest channels with the SU-8 bonding glue. The larger channels were not blocked. Chapter 4 Optical Characterization 64

Chapter 4

Optical Characterization

4.1 Chapter Outline

• Section 4.2 presents the optical measurement setups used for the optical character- izations. An argon ion laser is used for pumping the KOH etch based device and a pulsed Nd:YAG laser is used for pumping of the SU-8 photoresist device.

• Section 4.3 discusses the important parameters, which can be adjusted in order to compare the device performance under diﬀerent conditions. Facts about the performed measurement series are presented.

• Section 4.4 presents some of the optical spectra measured from the micro-cavity ﬂu- idic dye lasers, and illustrates how the important measurements data are extracted from the measured spectra.

• Section 4.5 discusses the results obtained by examining the important optical parameters from the measured spectra. The results are compared with theory.

• Section 4.6 discusses the near future perspectives of micro-cavity ﬂuidic dye lasers and takes a near future outlook into the upcoming experiments.

• Section 4.7 summarizes the chapter. 4.2 Experimental Measurement Setups 65

4.2 Experimental Measurement Setups

Chapter 3 explained that initially a KOH etched micro-cavity fluidic dye laser device was designed and fabricated. At that time there was only a cw argon ion laser at 488 nm available for laser excitation of the fluorescent dyes. The maximum output of the laser is 130 mW. Later on a new micro-cavity fluidic dye laser device was fabricated based on SU-8 photoresist for fabrication of the microfluidic channel and also for bonding. At that time a new more powerful laser for excitation had arrived from the supplier. The laser is a pulsed frequency doubled Nd:YAG (neodymium doped yttrium aluminum garnet) laser with a maximum pulse energy of 200 mJ. The pulse length is 5 ns with a repetition rate of 10 Hz. The wavelengths of the Nd:YAG laser are 1064 nm and 532 nm. Only the secondary harmonic wave (532 nm) has been used in this project.

4.2.1 Argon Ion Laser Setup

Sideview

Mirror Mirror

Power Meter

Lens Optical Fiber to Spectrometer

Argon Ion Laser λ = 488 nm

Dye Inlet Dye Outlet

Mirror Mount + Laser Chip

Figure 4.1: The sketch is showing a sideview of the experimental setup. The cw argon ion laser with a wavelength of 488 nm is used as the optical pump source for the dye Rhodamine 110 dissolved in ethanol. The dye solution is pumped through the KOH etched micro-cavity fluidic dye laser by means of a syringe pump. The light is guided with mirrors so that the pump laser light can enter the microfluidic channel from above through the chip glass lid. The excitation power is measured with a power meter before the light beam is focused. The laser light is focused by a lens to increase the excitation intensity. The emitted light from the micro-cavity laser device is transmitted through the chip glass lid and collected by an optical fiber connected to a spectrometer. The spectrometer is photomultiplier based and has therefore a high sensitivity. The spectrometer can only be used in cwmode.

The initial optical experiments were performed with the KOH etched microfluidic channel laser device with an argon ion laser used for excitation. The dye pumped through the micro-cavity was Rhodamine 110, which has an absorption maximum at 510 nm as shown Chapter 4 Optical Characterization 66 in figure 2.12(b). The choice of Rhodamine 110 was simple due the fact that the only laser available for excitation was an argon ion laser. The fluorescent maximum of Rhodamine 110 is around 530 nm. Ethanol was chosen as solvent since it has a fairly low viscosity which makes pumping of the dye solution through the microfluidic channel easier than for instance with ethylene glycol often used in jet dye lasers. Ethanol compared to water allows for higher dye concentrations due to lower dielectrical constant which prevents aggregation of the dye molecules. However, since the argon ion laser is a continuous wave laser, the greater heat capacity of water is an advantage. Figure 4.1 shows a sketch of the experimental optical setup used for measurements. The light from the argon ion laser is guided by optical mirrors down onto the dye laser chip. Before the beam enters the dye laser, the laser beam is focused by means of a lens in order to increase the excitation intensity. The excitation power is measured before the beam is focused in order not to harm the detector. The laser beam is transmitted through the glass lid of device and into the micro-cavity containing the dye solution. The laser device used for measurements did not have a metallic top mirror deposit on the downwards side of the lid. The reason was that this would decrease the useful amount of excitation power too much. The bottom of the channel was deposited with a gold/chromium layer to increase the cavity reflectivity. The dye solution was contained in a syringe and pumped through thedevicebemeansofasyringepump.Theflowratewas10µL/min. The emitted light from the device was collected with an optical fiber placed above the glass lid of the device. The collected light was analyzed with a spectrometer based on photomultipliers for high sensitivity. 4.2 Experimental Measurement Setups 67

4.2.2 Nd:YAG Laser Setup

Topview λ λ Frequency Splitter Attenuator Nd:YAG Laser 1= 1064 nm, 2 = 532 nm λ 2 = 532 nm Mirror

Sideview Mirror

Optical Fiber Mirror to Spectrometer

Slit 1.5 mm Edge Filter

Dye Inlet Dye Outlet Mirror Mirror Power Meter Mount + Laser Chip

Figure 4.2: The sketch is showing a topview of the experimental setup. The dashed box indicates that the view is shifted to a sideview. The pulsed frequency doubled Nd:YAG laser with a wavelength of 532 nm, a pulse length of 5 ns, and a repetition rate of 10 Hz is used as the optical pump source for the dye Rhodamine 6G dissolved in ethanol or water. The dye solution is pumped through the SU-8 photoresist based micro-cavity fluidic dye laser by means of a syringe pump. The emitted laser light from the Nd:YAG laser crystal consists of the fundamental infra-red wave with a wavelength of 1064 nm. A frequency doubler inside the laser generates a secondary harmonic wave with a wavelength of 532 nm. A frequency splitter in front of the Nd:YAG laser splits up the two wavelengths and only permits the secondary harmonic wave (532 nm) to pass. The high power 532 nm beam is then reduced significantly by an attenuator. The light is then guided by mirrors through a slit in order to reduce the beam size from 6 mm to 1.5 mm. The excitation power is measured with a power meter before the light beam is reduced with an optical slit. After the slit the light is guided into the microfluidic channel from above through the chip glass lid by means of mirrors. The emitted light from the micro-cavity laser device is transmitted through the chip glass lid and further transmitted through an edge filter, which reduces the power of scattered pump light. The emitted light is then collected by an optical fiber connected to a spectrometer. The spectrometer is a CCD array type. The spectrometer can be used in both cwand pulsed mode.

The SU-8 photoresist device was fabricated during the time the Nd:YAG laser from Continuum (Surelite I-10) arrived to MIC. The MEMS group at MIC got, shortly after, the possibility of using a demo model of a spectrometer. This spectrometer is a charge couple device (CCD) array type from Avantes (AVS-USB2000), which does not have the high sensitivity as a photomultiplier based spectrometer but can provide realtime measurement meaning that all wavelengths are measured instantly. This allows for measuring in a pulsed mode. The photomultiplier spectrometer could not measure the short nanosecond pulses. The Nd:YAG laser with the secondary harmonic wave with a wavelength of 532 nm Chapter 4 Optical Characterization 68 is ideal for pumping the dye Rhodamine 6G since the absorption maximum is around 530 nm as shown in figure 2.12(a). The fluorescent maximum of Rhodamine 6G dissolved in ethanol is around 560 nm as shown in figure 2.13. A sketch of the experimental measurement setup used for the SU-8 photoresist based device is shown in figure 4.2. The fundamental harmonic wave with a wavelength of 1064 nm was removed by a frequency splitter and was not used for excitation. The second harmonic wave with a wavelength of 532 nm was significantly damped by an attenuator. The damped beam is guided by mirrors through a slit, which reduced the the beam diameter from 6 mm to 1.5 mm corresponding to a transmittance of 7 %. The excitation power was measured before the slit since the detector can not measure below an average power of 1-2 mW, which is need. The detector is a low power thermal detector from Coherent. The power-meter is also from Coherent (FieldMaster-GS). After the the beam has passed the slit, it was guided by mirrors down onto the micro-cavity fluidic dye laser. The beam was not focussed due to the high intensity provided by the Nd:YAG laser even though the laser power was heavily reduced by the attenuator. The emitted light from the micro-cavity dye laser and the scattered pumping laser light was filtered with an edge filter in order to damp the scattered pump laser light. Above the edge filter an optical fiber collected the emitted light, which then was analyzed by the spectrometer. The dye solution was contained in a syringe and pumped through the SU-8 laser device at a flowrate of 10 µL/min by means of a syringe pump. Figure 4.3 shows a sketch of the SU-8 laser device under optical pumping. The optical pump laser beam is transmitted through the chip glass lid and through the top cavity metallic mirror. The emitted laser light from the micro dye laser is transmitted through the top mirror.

Pump Light Dye Laser Light

Dye Inlet Top Mirror Dye Outlet

Glass Z SU-8 SU-8 Y Glass

Bottom Mirror

Figure 4.3: Sideviewsketch of the SU-8 micro-cavity ﬂuidic dye laser under optical pumping. The pump light is transmitted through the glass lid and the top mirror. The emitted laser light is transmitted through the top mirror. The dye solution is pumped through the microﬂuidic channel by means of a syringe pump. 4.3 Important Device Parameters 69

4.3 Important Device Parameters

The important parameters, which can be adjusted for different experimental measurement series are the optical pump excitation power, the dye concentration, the mirror reflectance, and the cavity length or dye cell length. The dye flowrate can also be important but only in cw measurements. The dye concentration, the dye cell length, and the cavity mirror reflectance are important for the round-trip gain. Increasing the concentration compared to a macro sized dye laser will be necessary due to the fact that the cavity length is very short in a micron sized device, which results in a low round-trip gain. Varying these three parameters for different measurement series will give information about the usable range of the parameters, and how the device is operating under different conditions. The flowrate will also be fixed to point of safe operation meaning that there should not be any leakage in the system. It turned out that the flowrate of 10 µL/min could fulfill this requirement of safe operation. Increase the flowrate too much above 10 µL/min would in most cases lead to a leakage in the system.

4.3.1 Facts About the Optical Measurements The measurements were performed with a steady flow of dye through the micro-cavity with a flowrate of 10 µL/min. The width of the cavity for all devices used in the measurements was 1 mm. The concentration of the dye solution, the cavity length (not the dye layer thickness), and the reflectance of the cavity mirrors were varied for the different measurement series. For each measurement series an optical spectrum is recorded for every excitation power used. A measurement series usually contain 15-16 recorded spectra for the SU-8 based laser device. For the KOH etch based device the number is 13. A complete list of the measurement series is shown in appendix K. Chapter 4 Optical Characterization 70

4.4 Measurement Results

4.4.1 KOH Etched Chip with Argon Ion Laser Pumping

Optical Spectrum Output Power vs Pump Power 0.14 0.14 10 mW 0.001 mol/L 40 mW 0.005 mol/L 0.12 70 mW 0.12 0.01 mol/L 100 mW 0.02 mol/L 130 mW 0.1 0.1

0.08 0.08

0.06 0.06

0.04 0.04 Normalized Output Power [A.U.] Normalized Output Power [A.U.] 0.02 0.02

0 0 480 490 500 510 520 530 540 550 560 570 580 590 600 610 620 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Wavelength [nm] Optical Pump Power [mW]

(a) Measured optical spectra from Rhodamine 110 dis- (b) The peak output versus optical pump power for solved in ethanol with a concentration of 2·10−2 mol/L. different concentrations of Rhodamine 110 dissolved in The flowrate is 10 µL/min. The first peak is scattered ethanol. The measured points can be fitted almost lin- light from the argon ion laser at 488 nm. The second early indicating that the measured spectra only represent peak is the fluorescent output from the micro-cavity flu- fluorescence. A knee in the input/output curve would in- idic dye laser with a peak wavelength of around 533 nm. dicate the onset of lasing, which is not seen in any of the curves.

Figure 4.4: Measured optical spectra from a KOH etched micro-cavity fluidic dye laser device with Rho- damine 110 dissolved in ethanol flowing with a flowrate of 10 µL/min. The optical pump source is a continuous wave argon ion laser with a wavelength of 488 nm.

The measurements performed on the KOH etch based device did not show any signs of lasing. The measured spectra for the highest concentration used 2·10−2 mol/L are shown in figure 4.4(a). The measured spectra pose no sharp peaks and it can be concluded that only fluorescence is present. This can be confirmed with the graph shown in figure 4.4(b). Here are shown the input/output power curves extracted for the measurement series with different concentrations. All four curves represents almost linear curves, which indicates that there is no gain present system and thereby no lasing. There can be several reasons why lasing was not achieved. First and most obviously is that the excitation power was too low. Besides the low pump power, the quality of the bottom mirror was very pour as shown in figure 3.14. The dye concentration could also be too low and thereby the round- trip gain would be too low to compensate the cavity loss. The last reason could be that the triplet absorption is too high due to a low flowrate. The triplet absorption is known to be a major obstacle for obtaining lasing in a dye laser operated in a continuous wave mode. The reason or reasons, which were preventing lasing could not be determined. One could chose to optimize the fabrication process or develop a new type of device. The last option was chosen and the project continued with the development of the SU-8 photoresist based micro-cavity fluidic dye laser. 4.4 Measurement Results 71

4.4.2 SU-8 Chip with Nd:YAG Laser Pumping

500 µm Pyrex glass wafer n=1.474 5nmCr

Top mirror Z 10 µm dye solution n=1.36 40 nm Au Cavity Length 5µmSU-8n=1.59 150 nm Au X Bottom mirror

500 µm Pyrex glass wafer n=1.474 10 nm Cr

Figure 4.5: Sketch of the micro-cavity in the SU-8 chip with the expected thicknesses for the diﬀerent layers. The expected refractive indices are also shown. The channel width for all devices used for measurements is 1 mm.

The SU-8 photoresist micro-cavity chip containing Rhodamine 6G dissolved in ethanol is sketched in ﬁgure 4.5. The refractive indices and the diﬀerent layer thicknesses shown are the expected values. The refractive index of the Rhodamine 6G is supplied by Bar- roso et al. [4]. The value of n =1.36 for the dye solution reassembles the value for pure ethanol. The refractive index of the dye solution will probably not stay the same value for high concentrations. The refractive index of SU-8 has been measured here at MIC (K. B. Mogensen and J. El-Ali) at the helium-neon laser wavelength of 632.8 nm and was found to be 1.59. The weakness of the cavity structure is that the SU-8 thicknesses of the bonding layer and the channel layer is not known very precisely. The problem gets a bit complicated when the optical path length consist of two layers where the thicknesses and the refractive indices are not known precisely. If one measures a series of Fabry-Perot cavity modes only the total optical path length can be estimated. The optical path length for a two layer structure for orthogonal incidence of the light is given by:

Lop = n1L1 + n2L2 (4.1) The Fabry-Perot condition from equation 2.38 can now be rewritten to: 2 λ = L q =0, 1, 2, 3... (4.2) q op (q +1)

Here q is the mode number and λq is the vacuum wavelength for the cavity mode q. The optical path length Lop can be determined from the cavity mode peaks by linearly fitting the λq versus 2/(q +1). The q should be adjusted so that the error of fit is minimized. The slope of the fitted curve will then be the estimated optical path length Lop. Having considered this, one should be able to see an increase in the optical path if the concentration is increased. Figure 4.6 shows four subfigures where subfigure 4.6(a) shows the measured spectrum from a SU-8 laser containing a dye solution with a concentration of 10−2 mol/L. The device is expected to have a dye layer of 10 µm and a SU-8 bonding layer of 5 µm. The reflectance of the bottom mirror is expected to be 83 % and the top mirror is expected to have a reflectance of 72 %. Chapter 4 Optical Characterization 72

Optical Spectrum 4000 Output Power and FWHM vs Optical Pump Power 0.14 mW 50 5000 0.84 mW FWHM 3500 2.66 mW 45 Power 4500 4.76 mW Fitted Curves 3000 6.51 mW 40 4000 35 3500 2500 30 3000 2000 25 2500

1500 FWHM [nm] 20 2000 Output Power [counts]

15 1500 Output Power [counts] 1000 10 1000 500 5 500

0 0 0 520 530 540 550 560 570 580 590 600 610 620 630 640 0 1 2 3 4 5 6 7 Wavelength [nm] Optical Pump Power [mW]

(a) Measured optical spectra for different excitation pow- (b) The full width at half maximum linewidth together ers emitted from the micro-cavity fluidic dye laser. The with output peak power plotted against the optical pump first peak is scattered light form the Nd:YAG laser at power for the 570 nm peak shown in figure 4.6(a). In the 532 nm. The second peak is the laser output from the input/output curve there is a knee indicating a threshold micro-cavity fluidic dye laser at a wavelength of 570 nm. for lasing at an excitation power of 0.6 mW. Around the threshold point the FWHM curve has the largest negative slope indicating the threshold for lasing. The linewidth for the laser output is 5.7 nm for an excitation power of 6.5 mW.

Optical Spectrum with Cavity Modes Linear Fit of the Cavity Modes 4000 650 Lasing Mode 640 Cavity Modes 3500 Linear Fit x14 630 3000 620 2500 610 2000 600 1500 Wavelength [nm] 590 Output Power [counts] 1000 580

500 570

0 560 540 550 560 570 580 590 600 610 620 630 640 0.028 0.029 0.03 0.031 0.032 Wavelength [nm] 2/(q+1)

(c) A zoom in on the measured spectrum from fig- (d) A linear fit of cavity mode wavelengths against 2/(q+ ure 4.6(a) where the cavity modes are weakly visible. 1) where q is the cavity mode number. The fitted linear The vertical dashed lines are the estimated cavity modes curves passes through (0,0). The slope will then represent based on the fitted optical path length from figure 4.6(d). the best estimate of the optical path length for the micro- cavity fluidic dye laser. The triangle on the plot is the lasing mode which falls out of the cavity mode series, which is an indication of frequency pulling of the lasing mode. The estimated optical path length is 20.11 µm.

Figure 4.6: Measured optical spectra from a SU-8 photoresist based micro-cavity fluidic dye laser device with Rhodamine 6G dissolved in ethanol with a concentration of 10−2 mol/L. The dye flowrate is 10 µL/min. The optical pump source is a pulsed Nd:YAG laser with a wavelength of 532 nm. The expected thicknesses of the cavity layers are a 10 µm layer of dye solution and a 5 µm layer of SU-8 as shown on figure 4.5. The bottom mirror reflectance is expected to be 83 % and for the top mirror it is 72 %. 4.4 Measurement Results 73

Two peaks are visible in the spectra shown in figure 4.6(a). The first peak is the the scattered pump laser light with a wavelength of 532 nm. The second peak at 570 nm is the emitted light from the dye. As seen in figure 4.6(b) the FWHM linewidth at the highest excitation power of 6.5 mW is 5.7 nm. The excitation power could not be increased further due to saturation of the spectrometer. Increasing the excitation to 9-10 mW will start burn off the top mirror. The excitation diameter is 1.5 mm. It should be noted that the shown excitation powers are the corrected ones for the slit. It should also be noted that the measured power is an average of the pulse power. A simple calculation tells us that the peak power is 20·106 higher since the pulse length for the Nd:YAG laser is 5 ns 1 · 6 and the repetition rate is 10 Hz giving the factor: 5ns· 10 s−1 =20 10 . Figure 4.6(b) also shows the input/output power curve for the dye laser. Two linear fits of the input/out curve revealed that a bend or a knee is present. The bend is indicating the threshold for lasing and is estimated to be 0.6 mW. In figure 4.6(c) is shown a zoom in on the spectrum from figure 4.6(a). The zoom reveals that there are some weak cavity modes present. For reference a spectrum with the right scale is shown. The vertical dashed lines mark the estimated cavity modes calculated from the estimated optical path length Lop. Figure 4.6(d) shows the measured peak wavelength of the cavity modes fitted with a strait line as discussed earlier. The fit reveals that the lasing mode indicated with a triangle is not part of the cavity mode series. This indicates that the lasing mode has been pulled away from the cavity mode also known as frequency pulling. The frequency pulling, the bend in the input/output curve, and the linewidth narrowing are all three signs of lasing. Figure 4.7 shows the optical spectra obtained from the same device as the one rep- resented in figure 4.6 but with a dye concentration of 10−3 mol/L which is ten times lower than before. The spectra in figure 4.7(a) show the clear existence of multiply cavity modes. The cavity modes measured have been extracted and the optical path length have been estimated as shown in figure 4.7(b). The vertical dashed lines in figure 4.7(a) indicates the cavity modes calculated on the basis of the estimated optical path length. The estimated cavity modes reassembles the cavity modes measured at lower excitation powers. At the highest excitation power the dominant peak starts to deviate from the cavity mode. This could be an indication of frequency pulling and the onset of laser oscillation. In figure 4.7(c) the input/output power curve is shown, which reveals some sort of saturation effect since the slope decreases at higher excitation powers. This could indicate that the spectra shown are near threshold but can not operate above or much above the threshold. The reason being a too low dye concentration. The device used for the measurements discussed so far has been tested with other concentrations as well. Figure 4.8 shows three input/output power curves for three different dye concentrations. The 10−2 mol/L has the lowest the threshold power of 0.6 mW as expected. The 2·10−2 mol/L seemed to have the highest gain due to the steepest slope above threshold. The concentration of 5·10−2 mol/L has the largest threshold value of 3.4 mW, which is significantly higher than the others. This might indicate a starting process of dimerization in the dye due to the high concentration. Having considered a device with mirrors with as high reflectance as possible by means of gold mirror let us consider the opposite case. A device without metallic mirror will only for the top mirror have the dye/glass interface and the SU-8/glass interface for the bottom mirror. Chapter 4 Optical Characterization 74

Optical Spectrum with Cavity Modes Linear Fit of the Cavity Modes 650 670 0.35 mW 600 0.63 mW 660 550 1.68 mW 650 3.50 mW 500 7.28 mW 640 450 630 400 620 350 610 300 600 250 590 Wavelength [nm] 200 580 Output Power [counts] 150 570 100 560 50 550 0 540 540 550 560 570 580 590 600 610 620 630 640 650 660 670 0.026 0.027 0.028 0.029 0.03 0.031 0.032 0.033 Wavelength [nm] 2/(q+1)

(a) The measured output spectra for different excitation (b) A linear fit of cavity mode wavelengths against powers. The vertical dashed lines are the estimated cav- 2/(q + 1) where q is the cavity mode number. The fitted ity modes. It can be seen for higher excitation powers linear curves passes through (0,0). The slope will then that the peak is starting to deviate from the cavity mode. represent the best estimate of the optical path length for This is an indication of the onset of lasing. the micro-cavity fluidic dye laser. The estimated optical path length is 20.26 µm.

Output Power at 563 nm vs Pump Power 650 600 550 500 450 400 350 300 250 200 Output Power [counts] 150 100 50 0 0 1 2 3 4 5 6 7 8 Optical Pump Power [mW]

(c) The plotted input/output power curve reveals some sort of saturation (decreasing curve slope) beyond 1 mW of excitation power. It is therefore questionable wether the dye has actually reached threshold and is lasing.

Figure 4.7: Measured optical spectra from a SU-8 photoresist based micro-cavity fluidic dye laser device with Rhodamine 6G dissolved in ethanol with a concentration of 10−3 mol/L. The dye flowrate is 10 µL/min. The optical pump source is a pulsed Nd:YAG laser with a wavelength of 532 nm. The device used for measurements is the same one as presented in figure 4.6. 4.4 Measurement Results 75

Output Power vs Pump Power 4000

3500

3000

2500

2000

1500 Output Power [counts] 1000

0.01 mol/L 500 0.02 mol/L 0.05 mol/L 0 0 1 2 3 4 5 6 7 8 9 Optical Pump Power [mW]

Figure 4.8: The three input/output power measurements are fitted linearly in two parts. The three curves represents measurements with different concentrations of Rhodamine 6G dissolved in ethanol. The concentrations are 10−2 mol/L, 2·10−2 mol/L, and 5·10−2 mol/L. The measurements are performed on a SU-8 photoresist based micro-cavity fluidic dye laser device. The dye flowspeed is 10 µL/min. The optical pump source is a pulsed Nd:YAG laser with a wavelength of 532 nm. The device used for measurements is the same one as presented in figure 4.6 and figure 4.7.

These interfaces results in a reflectance of 0.16 % for the top mirror and 0.14 % for the bottom mirror. Another option is that the cavity mirrors will be the glass/air interface resulting in reflectances of 3.67 % for top and bottom interface. Figure 4.9(a) shows a measured spectrum for this low reflectance device containing a dye solution with a concentration of 10−2. As seen the low mirror reflectance was no obstacle for obtaining lasing. There were no cavity modes present so the optical path length could not be determined. That observation could have revealed which interfaces that provided the optical feedback. The input/output power curve in figure 4.9(b) does not reveal any value of the threshold. The linewidth narrowing is also shown in figure 4.9(b) and reveals a linewidth below 5 nm. Measurements with water as solvent were also conducted but they did not show any signs of lasing. The peak output was seen at a wavelength of around 580 nm for a concentration of 10−2. This could indicate that dimerization is playing a role. For a teen times lower concentration lasing was not observed either. The peak was then seen at around 577 nm. In both cases the peak wavelength was significantly longer than for the same concentrations in ethanol. Chapter 4 Optical Characterization 76

Optical Spectrum Output Power and FWHM vs Optical Pump Power 4000 0.14 mW 40 4000 0.49 mW FWHM Power 3500 0.91 mW 35 3500 3.71 mW 7.28 mW 3000 30 3000

2500 25 2500

2000 20 2000

1500 FWHM [nm] 15 1500 Output Power [counts] Output Power [counts] 1000 10 1000

500 5 500

0 0 0 520 530 540 550 560 570 580 590 600 610 620 630 640 0 1 2 3 4 5 6 7 8 Wavelength [nm] Optical Pump Power [mW]

(a) Measured optical spectra with different excitation (b) The input/output curve together with the full powers. The first peak is scattered light form the width at half maximum linewidth are plotted. The in- Nd:YAG laser. The second peak is the dye laser output put/output power curve is very linear and a clear thres- with a wavelength of 570 nm. At low excitation powers hold knee can not be identified. Only the FWHM curve the fluorescent spectrum is clearly visible. can reveal that the linewidth is narrowed.

Figure 4.9: Measured optical spectra with different excitation powers from a SU-8 photoresist based micro-cavity fluidic dye laser device with Rhodamine 6G dissolved in ethanol with a concentration of 10−2 mol/L. The dye flowspeed is 10 µL/min. The optical pump source is a pulsed Nd:YAG laser with a wavelength of 532 nm. The expected cavity lenght dimensions of the micro-cavity are shown on figure 4.5 however this device has no metallic mirrors. 4.5 Discussion of Results Obtained from the SU-8 Dye Laser 77

4.5 Discussion of Results Obtained from the SU-8 Dye Laser

In the previous section some of the optical spectra measured were shown to get a picture of how the spectra have been analyzed and to see the quality of the dye laser output. However in order to see some generel tendencies in the spectra an other way of presenting the results is necessary. I have chosen to put the important parameters into tables so that diﬀerent measurement series can be compared and a conclusion can be made.

4.5.1 Optical Path Length The optical path length can be determined from the cavity modes. Unfortunately the fabricated device have two layers with different refractive indices. The geometrical value of the thicknesses is not determined to a very precise degree since spin coating of SU-8 will give variations across a whole wafer. The refractive index of SU-8 has been measured at a wavelength of 632.8 nm. The refractive index of the dye solution is in literature given as the value of ethanol’s refractive index. Since we can not determine the exact values for the different layers we can only see changes, if any, in the refractive index for different concentrations. Also a large increase in the cavity size should be seen in the spectra. Table 4.1 shows the estimated optical path length for two devices containing different concentrations of Rhodamine 6G in ethanol.

Conc. [mol/L] 10−4 10−3 10−2 5·10−1 10−1 10−2 Dye layer [µm] 10 10 10 10 10 10 SU-8 bonding layer [µm] 5 5 5 5 5 10 Estimated optical path [µm] 20.31 20.26 20.11 21.90 22.37 30.33 Standard deviation [nm] ±4.6 ±4.4 ±4.3 ±2.9 ±8.5 ±3.8

Table 4.1: The estimated optical path lengths determined from the cavity peak wavelengths extracted from the measured spectra. The first five optical paths are estimated for the same device operated with different dye concentrations. The last value for the optical path length is estimated for a device with a 10 µm thick SU-8 bonding layer. Both devices used for this table had the reflectance of R1 =83%and R2 = 72 %. For devices with lower cavity mirror reflectance, the cavity modes were not visible. The expected optical path length for the device with a 5 µm SU-8 bonding layer is 21.55 µm. For the device with a 10 µm SU-8 bonding layer the expected optical path length is 29.50 µm.

The first five measurements are performed on the same device containing different concentrations. The estimated optical path length for the three lowest concentrations only deviate with 200 nm, which is below half a wavelength. The standard deviation shows that the estimations are very precise. It can then be concluded that the refractive index of the dye solution does not change very much at low concentrations. The high concentrations of 5·10−2 mol/L and 10−1 mol/L however shows a significantly increase. Assume that the refractive index of the dye solution at low concentration reassembles the value of pure ethanol of 1.36 and assume that the refractive index of SU-8 is 1.59. Chapter 4 Optical Characterization 78

The expected optical path length is 21.55 µm but the average of the three with the lowest concentrations is 20.22 µm. Taking the ratio between these two values we can estimate the thickness of the two layers. The dye solution layer will then be 9.38 µmthickand the SU-8 bonding layer will then be 4.69 µm. Assuming that we now know the thickness of both layers and the refractive index of SU-8, we can know predict the value of the refractive index of the dye solution at higher concentrations. The refractive index of the dye solution at a concentration of 5·10−2 mol/L will then be 1.54 and for the concentration of 10−1 mol/L the value of the refractive index of the dye solution will then be 1.59. The last device with an expected bonding layer thickness of 10 µm shows an increase in the optical path length as expected when increasing the SU-8 bonding layer thickness. The estimated optical path length is in fact a little above the expected only illustrating that the SU-8 thickness can vary from one fabrication run to another.

4.5.2 Peak Wavelength and Linewidth The lasing wavelength depends on the concentration according to [13, 44, 48]. If that is also the case for a micro-cavity ﬂuidic dye laser, it could prove to be an alternative way of tuning the lasing wavelength. Table 4.2 shows the peak wavelength for devices operating under lasing and also for those only emitting ﬂuorescent light.

Conc. [mol/L] 10−4 10−3 10−2 2·10−2 5·10−2 10−1 R: 0.14 % + 0.16 % 570 nm 575 nm R: 83 % + 0.16 % 561 nm* 571 nm R: 83 % + 72 % 560 nm* 562 nm* 570 nm 572 nm 578 nm 581 nm R: 83 % + 72 %** 559 nm* 569 nm 573 nm

Table 4.2: Peak wavelengths emitted from a micro-cavity fluidic dye laser for different concentrations. Data in the same roware extracted from measurements performed on the same device. The wavelengths marked with * indicates that under these conditions the devices did not show clear or any signs of lasing and the output is expected to be fluorescent or only operating around threshold. ** indicates that the SU-8 bonding layer is 10 µm thick for the particular device in the last row. The first column shows the expected reflectance of the cavity mirrors. The first rowshowsthe concentration of the dye in ethanol used in the particulary measurement.

It can be seen as a general tendency without exception that the lasing wavelength is increasing with concentration which is in agreement with literature as mentioned before. This could turn out to be an useful property for creating a tunable micro dye laser since movable mirrors especially vertical mirrors for tuning will hard be to fabricate. The linewidth of the laser output is also an important parameter, of cause depending on the application. It can not be expected that the linewidth of the lasing peaks emitted from the fabricated devices will be extremely narrow due to the low reflectance of the metallic mirrors compared to commercial available dielectric mirrors. The short cavity length will also decrease the Q-factor of the device and thereby increase the linewidth of the laser output. The fluorescent band of a dye is very wide compared to for instance a gas laser. This means that the wide band of allowed optical transitions in the dye will 4.5 Discussion of Results Obtained from the SU-8 Dye Laser 79 not help reducing the linewidth. Table 4.3 shows the measured linewidths obtained from different devices and dye concentrations.

Conc. [mol/L] 10−4 10−3 10−2 2·10−2 5·10−2 10−1 R: 0.14 % + 0.16 % 4.4 nm 5.8 nm R: 83 % + 0.16 % 13.8 nm* 5.1 nm R: 83 % + 72 % ? 14.8 nm* 5.7 nm 6.4 nm 7.8 nm 14.8 nm R: 83 % + 72 %** 16.0 nm* 15.8 nm 4.8 nm

Table 4.3: Extracted linewidths for micro-cavity fluidic dye lasers for different concentrations under maximum possible excitation powers. Data in the same row are extracted from measurements performed on the same device. The output spectra with linewidths marked with * did not show a clear dominant peak. ** indicates that the SU-8 bonding layer is 10 µm thick for the particular device in the last row. The first column shows the expected reflectance of the cavity mirrors. The first row shows the concentration of dye in ethanol used in the particulary measurement. ? indicates that the linewidth could not be determined.

Surprisingly the smallest linewidth measured was emitted from a device with no metallic mirrors. There does not seem to be any consensus in the linewidths obtained. The linewidths measured might be too large because of the spectrometer. The resolution of the spectrometer should be around 0.38 nm that is at least the separation between two data points in the measured spectra. However the linewidth of the Nd:YAG laser should be 1 cm−1 given by the manufacturer, which corresponds to 4.5 · 10−12 nm. The measured spectra suggests a linewidth for the Nd:YAG laser to be around 2 nm. These numbers suggest that the measured linewidths of the dye laser spectra should be smaller.

4.5.3 Threshold Power and Critical Inversion Concentration

Conc. [mol/L] 10−4 10−3 10−2 2·10−2 5·10−2 10−1 R: 0.14 % + 0.16 % 0.9 mW* 0.8 mW R: 83 % + 0.16 % ? 0.8 mW* R: 83 % + 72 % ? ? 0.6 mW 0.8 mW 3.4 mW 8.6 mW R: 83 % + 72 %** ? 8.5 mW 0.7 mW

Table 4.4: Extracted optical threshold pump powers for micro-cavity fluidic dye lasers for different concentrations. Data in the same roware extracted from measurements performed on the same device. The threshold powers marked with * did not show a clear knee in the input/output curve but the spectra revealed a clear narrowing of the output linewidths and the threshold values were taken there. ** indicates that the SU-8 bonding layer is 10 µm thick for the particular device in the last row. The first column shows the expected reflectance of the cavity mirrors. The first row shows the concentration of dye in ethanol used in the particulary measurement. ? indicates that the particulary measured spectrum is a fluorescent output. It should be noted that the power values is the average measured power and not the peak power.

The threshold powers for lasing have in most cases been estimated by means of two linear ﬁts. In other cases where the input/output curve did not have any visible bend, the threshold was determined from the linewidth narrowing. Table 4.4 shows the estimated Chapter 4 Optical Characterization 80 threshold powers for diﬀerent devices and concentrations. The high value of 8.5 mW in the last row falls out of the general tendency. At lower concentrations the threshold value does not seem to change very much. At high concentrations the value is increased quite a lot and this might be an indication of dimerization. In order to check the measured results a stationary solution of the critical inversion ratio might come in handy. The stationary solution of the critical inversion ratio was derived in section 2.5.2. For recapitulation the result is given here.

N σ − 1 ln(R R ) 1 = 0l 2LN 1 2 (4.3) N σ0l + σe + kST τT (σ0l − σTl) Two reasons exist in this case for applying a stationary solution to a time depended problem. The stationary solution can be expressed analytically, which gives a quick and easy understanding of the problem. The second reason is that the lifetime of the photons in the cavity is much smaller than the pulse length of the pump laser. The lifetime in the cavity can be calculated by this expression: n 2L t = = − op =0.26 ps (4.4) c − 1 2L ln(R1R2)c ln(R1R2)c It seems that the photon lifetime in the cavity will be much shorter than the duration of the pump pulse. This means that there will properly not be a very large peak build up of photons in the cavity during the pump pulse which is 5 ns long. On this basis it is assumed that the photon concentration and thereby the molecule concentration in the diﬀerent states will be fairly constant during a pump pulse. A stationary solution can therefore be applied although it can only deliver an estimation on the critical inversion ratio. Having justiﬁed the use of a stationary solution a table with the dye constants for use in the stationary model is given in table 4.5.

σe σ0l σTl τ0 τT kST 2.1·10−16 cm2 1.0·10−19 cm2 5.9·10−17 cm2 6.0 ns 50 ns 1.3·107 s−1

Table 4.5: Important constants for the Rhodamine 6G dye dissolved in ethanol at a lasing wavelength of 580 nm [4, 48].

Using the constants given in table 4.5 yet another approximation has to be made. The constant shown only applies for the wavelength of 580 nm. In order to use these constants we have to assume that they are constant throughout the optical spectrum. It is well known that it is not the case as seen at ﬁgure 2.13. Figure 4.10 shows some calculated critical inversion ratios by use of equation 4.3 based on the numbers in table 4.5. The curves are truncated when the critical inversion ratio N1/N equals one. It is not physical possible to excite more dye molecules than the number of dye molecules present in the dye solution. The curves tells us that it should not be possible to obtain lasing with a dye concentration of 10−3 mol/L since the reﬂectance squared value for the gold/chromium mirrors is around 0.6. This is in agreement with the measurements performed. The 10−2 mol/L curve reveals that it is not possible to 4.5 Discussion of Results Obtained from the SU-8 Dye Laser 81

Critical Inversion Ratio Curves 1 10

0 10

−1 10

−2 10

Critical Inversion Ratio N1/N −3 10 0.0001 mol/L 0.001 mol/L 0.01 mol/L

−4 0.1 mol/L 10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Mirror Reflectance Squared R²

Figure 4.10: Calculated critical inversion ratios for the stationary approximation. The dye constants used are shown in table 4.5. The dye cell length for these calculations is 10 µm long. The triplet state has been included in the calculation giving a higher estimate on the critical inversion ratio. achieve lasing from the devices without metallic mirrors at that concentration. This is in disagreement with measurements. If one neglects the triplet state (which should be legal with short pulse pumping) it will improve the situation a bit but not nearly enough to account for the observed lasing at a concentration of 10−2 mol/L with a very low reﬂectance. The measured threshold pump powers can be used to calculate backwards from the threshold pump power to the concentration of excited molecules. Equation 4.5 [48] from section 2.5.2 can be used for calculating the threshold pump power needed to obtain lasing if one knows the critical inversion concentration of excited molecules.

N1hcLA Pinv = (4.5) λpτ0 Using the measured threshold powers in table 4.4 and transform them into peak pump powers instead of average powers one can calculate the concentration of excited molecules. Equation 4.5 can be rewritten so that the average power can be used. 20 · 106P Tλ τ N = inv p 0 (4.6) 1 hcLA 6 The factor 20 · 10 converts the average pump power into a peak pump power. Pinv is the measured average threshold pump power. T is the transmittance of the pump power light through the top lid, which in this case is 7 % for the metallic mirrors. For top mirror without metallic layers T is one. λp is the wavelength of the pump light. τ0 is ﬂuorescent lifetime. h is Planck’s constant and c is the speed of light in vacuum. L is the dye cell or cavity length and A is the area of excitation. The excitation diameter in our case is 1.5 mm. Table 4.6 shows the calculated critical inversion concentrations N1∗∗ and the indirectly measured N1∗ by means of the measured threshold pump power. Chapter 4 Optical Characterization 82

Dye Conc. [mol/L] 10−2 2·10−2 5·10−2 10−1 N1 [m mol/L] N1* N1** N1* N1** N1* N1** N1* N1** R: 0.14 % + 0.16 % (27.2) (62.7) (24.2) (62.7) R: 83 % + 0.16 % (24.2) (31.0) R: 83 % + 72 % 1.27 2.49 1.69 2.50 7.19 2.52 18.2 2.55 R: 83 % + 72 %*** (18.0) 2.49 1.48 2.50

Table 4.6: N1** is the calculated critical inversion concentration based on the stationary model. N1* is the indirectly measured critical inversion concentration obtained from backwards calculation from the excitation power. Data in the same row are calculations and measurements performed on the same device. *** indicates that the SU-8 bonding layer is 10 µm thick for the particular device in the last row. The first column shows the expected reflectance of the cavity mirrors. The first row shows the concentration of dye in ethanol used in the particular measurement. The numbers in () are not physical possible since the concentration of excited molecules is larger than the concentration of molecules present in the dye solution.

The values in parentheses indicates that the concentration of excited molecules exceeds the total dye concentration. The lasing obtained by devices with low mirror reflectance devices can not be explained by neither the stationary model or the threshold pump power. Both numbers indicate that it should be impossible. However the model does not take the fluorescent contribution to the lasing mode into account. This contribution might be high in a micro-cavity. The devices with a high reflectance shows a good agreement between the model and the threshold power except for high concentrations. The reason probably being the effects of dimerization, which increases with dye concentration. This effect is not accounted for in the stationary model. Even though the threshold pump power for high concentrations leads to a much higher result of the critical concentration, it is still physical possible since the critical inversion ratio is still well below one.

Dye Conc. [mol/L] 10−2 2·10−2 5·10−2 10−1 N1 [m mol/L] N1* N1** N1* N1** N1* N1** N1* N1** R: 0.14 % + 0.16 % (27.2) (51.3) (24.2) (51.3) R: 83 % + 0.16 % (24.2) (26.1) R: 83 % + 72 % 1.27 2.04 1.69 2.04 7.19 2.05 18.2 2.08 R: 83 % + 72 %*** (18.0) 2.04 1.48 2.04

Table 4.7: The same as in table 4.6 except that for the calculated critical concentrations N1** the triplet inﬂuence has been ignored resulting in a lower estimate of the critical inversion concentration.

Table 4.7 shows the calculated critical inversion concentration when the triplet state is ignored. This gives better agreement between the value obtained from the threshold pump power and the calculated from the stationary model at least for low dye concentrations. All together it seems reasonable to use a stationary solution to predict how large a concentration that should be used in order to overcome the cavity losses and reach threshold for lasing. 4.6 Perspectives and Outlook 83

4.6 Perspectives and Outlook

On the basis of the results presented in this chapter, it can initially be concluded that lasing in micron sized fluidic cavities is possible. Lasing is even possible in structures where the cavity mirror reflectance is small, provided that the dye concentration is large. This is in agreement with the results presented by Klebniczki et al. [23]. Lasing in continuous wave mode has not yet been verified and needs to be further investigated. CW operation will be useful for absorption measurements and other bio/chemical measurements. The measurements showed that the lasing wavelength depends on the concentration meaning that the maximum gain for the system is moved towards longer wavelengths as verified by [13, 44, 48]. The same applies for the dye cell/cavity length however that has not been verified experimental in this thesis. This means that the lasing wavelength goes towards longer wavelengths when the round-trip gain is increased. The feature of wavelength dependency on the dye concentration could prove to a useful feature and making tunability possible in simple micron sized cavities. One could imaging a device with two inlets and one outlet. In one of the inlets a concentrated dye solution is pumped through. In the other inlet a pure solvent is pumped through. The two channels will merge and be transported to a passive mixer before transported to the laser cavity. The pumping pressure difference will determine the ratio between the dye solution and the solvent and thereby the final dye concentration. The lasing wavelength can then be controlled by the pump pressures. Such a device is currently being designed by masters student B. B. Olsen. The fact that lasing can be obtained in cavities with a low reflectance might be useful for horizontal emitting devices. This will only require smooth sidewalls. Or one could imagine a device which makes use of fluidic interfaces by means of pumping a high index liquid through two inputs, which will surround and laminate the dye solution, which is flowing between the two high index liquid flow streams. This laminating effect can be realized due to the fact that the Reynolds’ number is very low in a microfluidic channel and the flow will be laminar. Klebniczki et al. [23] showed that the interface between a thin dye solution layer and the air was sufficient to produce lasing in pulsed mode. The situation might be different in continuous wave operation where the triplet absorption should be much more dominant and thereby reduce the round-trip gain. The next step in the project (beyond the scope of this thesis) is testing of the possibilities for wavelength tuning by means of varying the concentration. After having tested the tunability, one can begin to consider and test if cw operation is possible. Fabrication and test of horizontal emitting devices should be considered later on since this probably requires a better process control for obtaining smooth sidewalls together with integration of waveguides. The question is then if SU-8 micro-fabrication techniques can fulfill the demands or another fabrication technique should be applied? For now the SU-8 photoresist provides easy and fast fabrication, which enables faster obtainment of experimental results. Chapter 4 Optical Characterization 84

4.7 Chapter Summary

The chapter starts with a description of the two measurement setups used for the two types of fabricated devices. The KOH etch based device was optically pumped by an continuous wave argon ion laser with a wavelength of 488 nm. The dye used was Rhodamine 110 dissolved in ethanol. The dye has an absorption maximum at a wavelength of 510 nm. The fluorescent maximum of the dye is located at a wavelength of around 530 nm. The device was tested with four different dye concentrations but only fluorescent spectra were measured. The SU-8 photoresist based devices were optically pumped with a pulsed frequency doubled Nd:YAG laser with a wavelength of 532 nm. The micro-cavity contained Rho- damine 6G dissolved in ethanol as active laser medium. Rhodamine 6G has an absorption maximum around 530 nm and a fluorescent maximum at a wavelength of around 560 nm. Lasing was obtained from several devices containing different concentrations of dye solution. Lasing was not obtained for concentrations below 10−2 mol/L. Lasing was observed for concentrations from 10−2 mol/L to 10−1 mol/L. The increase of the dye concentration resulted in an increase in the lasing wavelength. The cavity mirror reflectance did seem to have a great influence wether lasing was obtained or not at least for those concentrations tested. The mirror reflectance had a great influence on the cavity modes. The cavity modes were only visible in the spectrum for the device having the largest cavity reflectance. From the cavity modes the optical path length could be determined. There were some minor deviations from the expected to measured. This could be explained by fabrication variations, which could differ from device to device. The optical path length was increased at higher concentrations indicating that the refractive index of the dye solution was increased. The threshold pump power was used to calculate backwards in order to find the concentration of excited molecules. These numbers was compared to those obtained with a stationary model for the threshold concentration derived earlier from time-depended rate-equations. The agreement was large for devices with a high reflectance and at low dye concentrations. At high concentrations the measured threshold pump power was increased significantly, which resulted in a high estimation of excited molecules compared to the stationary model. The possible explanation was that dimerization at high concentration had increased the cavity losses and a larger threshold pump power was needed. The stationary model could not explain the lasing obtained in the device with low cavity mirror reflectance, but the contribution of fluorescence into the cavity mode might explain the phenomenon. The chapter was ended with a short discussion about future work in this field of micro-cavity dye lasers. For the moment a design is under development, which allows for concentration changes, which can be used for tuning of the lasing wavelength. Chapter 5 Conclusion 85

Chapter 5

Conclusion

Micro-cavity fluidic dye lasers fabricated with standard microfabrication techniques have been investigated and described in this thesis. The main goal for this thesis is to demon- strate that lasing is possible in a micron sized laser cavity with a continuous flow of organic laser dye through the cavity. The minor goals are to get an overview of the theory that is important for dye lasers and establish a theoretical and experimental basis for further work in the area of micron sized dye lasers. All the goals are basically reached. Organic dye molecules contain chains of conjugated double bonds, which enables the bonding π electrons to be delocalized and be free to move in the chain. The effective length of the conjugated chain of double bonds will to a first approximation determine the absorption and fluorescent wavelengths. Detailed quantum mechanical treatments given in literature reveal that three energy states are important for the optical and electronic transitions in a dye molecule. The singlet ground state S0, the first excited singlet state S1 and the first excited triplet state T1. Most of the absorption and fluorescence transitions take place between the single states. However the excited molecules can by an intercrossing transition decay to the triplet state. The triplet state can absorb both excitation light and emitted dye laser light and thereby introduce an extra loss in the laser system. The loss is only important in the continuous wave optical pump mode since the intercrossing decay time is fairly long. In pulsed optical pump mode the triplet absorption of laser light is prevented due to the short optical pump pulses. The optical and electronic transitions can be modelled by time-depended coupled rate equations. Those can not be solved analytically. A stationary model was derived and compared with existing theory. The stationary model can predict the critical inversion ratio which is the ratio between the concentration of excited molecules and the total concentration of dye molecules present in the solvent for instance ethanol. This ratio can be used to predict the optical pump power needed to reach the threshold for lasing. Other subjects were presented like modelling of mirrors for calculation of the reflectance of laser cavity mirrors both metallic and dielectric. Simple hydrodynamic theory was also presented for calculating the counter pressure developing in microchannels when a liquid is flowing through a microchannel with a constant velocity. It was reasoned that with a vertical emitting device, it would be a lot easier to fabricate the cavity mirrors. It was decided to use e-beam evaporation of gold/chromium on horizontal surfaces for mirror fabrication. Different fabrication schemes were consid- Chapter 5 Conclusion 86 ered and a fabrication process involving KOH etching of a microchannel in silicon was realized. The idea was to use the bottom (100) plane of the KOH etched channel as deposition surface for a gold/chromium mirror. It turned out that the etched surface became rough. Hydrogen bubbles formed during the etch were sticking to the surfaces. The hydrogen bubbles prevents KOH from attacking silicon uniformly. The device was finished with an anodic bonding of a glass wafer on top of the microfluidic channel. The thermal heating during the bonding process made the gold/chromium layer diffuse into the silicon substrate and leaving the surface very rough. This might be circumvented by decreasing the temperature of the anodic bonding, but was not tested. The reason being that the roughness from the KOH etch also should be optimized, which would be too time-consuming. The device was optical characterized by means of optically pumping of Rhodamine 110 with a cw argon ion laser (488 nm). The dye Rhodamine 110 dissolved in ethanol was pumped through the micro-cavity. The measured optical spectra revealed that only fluorescence was present, probably due to triplet absorption and the rough cavity mirror. The second micro-cavity laser device was based on a SU-8 photoresist with integrated gold/chromium mirrors. The microfluidic channel was defined by an UV-lithographic step, baking, and development of a 10 µm thick SU-8 photoresist, which had been spin-coated on the top glass wafer. On the top glass wafer a semi-transparent gold/chromium mirror was defined by e-beam evaporation before the spin-coating of the SU-8. The bottom glass wafer with a non-transparent gold/chromium mirror was spin-coated with a layer of SU-8 photoresist acting as a bonding glue. The wafers were bonding together with manually applied pressure at a temperature of 75 ◦C as recommended by Jackman et al. [21]. An additional baking step at 90 ◦C for an hour was introduced to increase the bonding strength. The SU-8 bonding layer was crosslinked by UV-exposure and a post exposure bake. Two problems exist for this device. The spin-coating will not be uniform over whole wafer leading thickness variations, which will effect the output spectrum. The variants in the SU-8 layer will be something one has live with. The other problem was that the narrow channels were blocked with the bonding SU-8. This might be circumvented by making small holes in the cross-linked SU-8 layer, which could suck some of the soft SU-8 during the bonding instead of the soft SU-8 fills up the microfluidic channels. The SU-8 photoresist based micro-cavity device was successfully characterized by means of Rhodamine 6G dissolved in ethanol pumped through the device and used as the active laser medium. For optical pumping a pulsed frequency doubled Nd:YAG laser (532 nm) was used. Lasing was obtained with dye concentrations ranging from 10−2 to 10−1 mol/L. The lasing wavelength spanned from from 570 nm to 581 nm for the just mentioned concentration range. It was seen that the lasing wavelength was increased with increasing concentration. This is in agreement with literature [13, 44, 48]. Lasing was achieved from devices with different mirrors where the reflectance spanned from 83 % to below 1 %. Common for all devices were that clear signatures for lasing were not seen below a concentration of 10−2 mol/L. In some spectra a frequency pulling effect of the lasing mode was seen, which is a sign for lasing. The optical path length was extracted from the cavity modes visible in some of the measured optical spectra. The optical path length revealed an increase for high concentrations indicating that the refractive index of the dye solution was increased. Chapter 5 Conclusion 87

The threshold pump powers were estimated from bends in the input/output power curves. These threshold powers were converted by calculation into the threshold concentrations of excited molecules. These numbers were compared to the calculated concentrations from the stationary model. There was a large agreement for devices with high cavity mirror reflectance and with low dye concentrations. For larger concentrations the model predicted too low concentrations compared to the measured. This can be explained by dimerization of dye molecules at higher concentrations resulting in larger cavity losses, which increases the threshold pump power. The model could not explain the lasing obtained for devices with low cavity mirror reflectance. According to both the measured pump power and the stationary model, lasing for these devices would require a larger concentration of excited molecules than dye molecules present in the solution. The only explanation could be that the fluorescent contribution to the lasing mode was very large. The fact that the SU-8 fabricated devices were successfully optical characterized, fulfills the main goal of creating a lasing device with a fixed wavelength. The thesis gives a basic overview of the theory behind dye lasers and with the knowledge about the characterized devices one gets quickly a feeling for the device parameters, which can be adjusted for future devices. The SU-8 photoresist has many interesting properties. One of them is the high refractive index, which allows fabrication of waveguides. SU-8 waveguides can be fabricated in the same layer of SU-8 as the microfluidic channel layer. SU-8 allows for fast, easy, and fairly cheap fabrication and is therefore ideal for prototyping. The fabrication and experimental results of this thesis are already being transferred to a new device. The device will contain a passive mixing unit, which can mix and dilute a concentrated dye solution with a pure solvent. The idea is to make use of self-tuning by the dye itself, which depends on the concentration. The device will then offer the possibility of easy wavelength tuning. With this device being in the design process, the work in this thesis is continued in a natural way. Bibliography 88

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List of Figures

1.1 Spectra for different laser dyes. Supplied by Exciton Inc...... 2 1.2 Sketch of a microfluidic dye laser with horizontal emission through a waveguide with external optical pumping through the glass lid which seals the microfluidicchannel...... 3 1.3 Sketch of a microfluidic dye laser with vertical emission through the glass lid with external optical pumping through the glass lid which seals the microfluidicchannel...... 4

2.1 Schematics of the light interaction with matter and the appropriate Ein- stein coefficients for light absorption, spontaneous, and stimulated emission. 8 2.2 Ideal laser cavity with concave mirrors separated with a distance L and with a mirror reflectance R1 = 100 % and for R2 slightly below a 100 % ideally. The active medium is pumped with an external energy source for instance light. A strong laser field is build up in the cavity due to mirror feedback. A small fraction of the laser light is emitted through the R2 mirrorasusefullightoutput...... 10 2.3 To the left a sketch of electron transitions and photon emission in a four level laser. To the right a sketch of the gain curve of the active medium together with the light output of the laser in the three characteristic situations...... 11 2.4 Sketch of a round-trip for laser light in a laser cavity...... 12 2.5 Drawings of electron probability functions for two different σ bonds namely ssσ and ppσ and a drawing of a electron probability function for a π bond namely the ppπ. The signs shown in electron clouds indicate the sign of theelectronwavefunction[5]...... 13 2.6 Drawings of some bonds in organic molecules...... 14 2.7 The picture shows the chemical structure of two common dyes. (a) 3,3´ diethyl thiatricarbocyanine iodide. (b) Rhodamine 6G. The heavy lines indicate the chromophoric regions of the dyes [53]...... 15 2.8 Chemical structure of phenolphthalein. The arrows indicate that parts of themoleculecanrotate[48]...... 15 2.9 The π electron cloud is shown in a) and b) for a simple cyanine dye. In c) the potential energy of the dye is shown as a function of the dye length. Beneath in d) an approximation of the potential is shown as an infinite potentialquantumwell[48]...... 16 List of Figures 94

2.10 Energy levels for a typical organic dye [53]...... 17 2.11 Chemical structure of three Xanthene dyes [48]...... 18 2.12 Absorbance spectra for the dyes Rhodamine 6G and Rhodamine 110. . . . 19 2.13 Absorption, emission and triplet spectra defined as capture cross sections for Rhodamine 6G dissolved in ethanol. [53]...... 19 2.14 Model for the energy levels for a typical dye molecule [4]...... 22 2.15 Calculated surface for the critical inversion concentration. The minimum on the surface represents the ratio between the critical inversion population and the dye concentration for a given cavity loss and wavelength. − 1 The parameter r is the cavity loss 2L ln(R1R2). This graph is calculated for Rhodamine 6G in water plus Ammonyx LO and is copied from ”Dye Lasers”[48]...... 26 2.16 Calculation of the critical inversion surface depending on the product of reflectance of the cavity mirrors and the dye concentration. The ratio between the critical concentration of excited molecules N1 and the total concentration N of dye molecules is plotted. The dye simulated is Rhodamine 6G dissolved in ethanol and the cavity length is 10 µm. The data for the calculation of rate equation 2.23 are from Barroso et al. [4]. The values used correspond to a wavelength of λ = 580 nm for lasing and thereby ignoring self-tuning of the dye. Every value above 1 for the ratio N1/N is ignored and assigned the value of 1. Critical inversion can in theory of cause only happen when the ratio is below 1. Dimerization at higher concentrations areignoredinthiscalculation...... 27 2.17 Sketch for optical transmission and reflection for multilayer films both dielectric and metallic films...... 29 2.18 Dielectric stack mirror with calculated reflectance spectra...... 31 2.19 Plot of the complex refractive indices in the range from 200 nm to 1550 nm of gold, aluminum, and chromium. The values for the refractive indices for the various metals are listed in the appendices F,G, and H [36]...... 32 2.20 Reflectance dependencies of the wavelength for tree different metallic materials and also reflectance and transmittance dependencies of the layer thicknessforthemetals...... 32 2.21 A Fabry-Perot cavity with some of the allowed cavity modes...... 34 2.22 Plot of the transmittance through a Fabry-Perot with the reflectance R1 = 0.8andR2 =0.8. The transmittance curves are called Airy functions. The medium between the mirror is transparent with a refractive index of n =1.36. 35 2.23 Calculated cavity modes for a cavity containing Rhodamine 6G...... 36 2.24 Sketch of frequency pulling of the lasing mode away from the cavity mode towardsthegainmaximum[53]...... 37 2.25 Pressure drops across 1 mm long micro-channels with ethanol flowing at a velocity of 10 m/s. Solid curves represent channels with rectangular shape while the dashed curve represents a circular shape...... 39 2.26 Reynolds’ number for 1 mm long micro-channels with ethanol flowing at a velocity of 10 m/s. Solid curves represent channels with rectangular shape while the dashed curve represents a circular shape...... 40 List of Figures 95

2.27 Entrance length for developing laminar flow in 1 mm long micro-channels with ethanol flowing at a flow velocity of 10 m/s. Solid curves represent channels with rectangular shape while the dashed curve represents a circu- larshape...... 41

3.1 Calculated critical inversion surface with a maximum threshold ratio value of 1/10. See figure 2.16 for further information...... 45 3.2 Counter pressure in a channel calculated for different water flow speeds or flowrates...... 47 3.3 Sketch of the fabricated flexible chip mount system, which allows for fast change of the chip. O-rings have been applied for ensuring a tight interface betweenthechipandthechipmountsystem...... 48 3.4 Sideview of a laser cavity. Sketch of a KOH etched micro-cavity with gold/chromium mirrors as top and bottom. The Pyrex glass lid is anodic bonded onto the silicon wafer. The dye flow is in the y-direction...... 50 3.5 Sideview of a laser cavity. Sketch of a micro-cavity defined in a SOI wafer. The micro-cavity is defined by a deep dry etch of the top silicon layer and then a wet oxide etch. The mirrors are e-beam deposit gold/chromium as the top and bottom. The Pyrex glass lid is anodic bonded onto the silicon wafer.Thedyeflowisinthey-direction...... 51 3.6 Sideview of a laser cacity. Sketch of a micro-cavity defined with SU-8 photoresist. The mirrors are e-beam deposit gold/chromium as the top and bottom. The substrate wafer is bonded onto the SU-8 defined micro- cavity by means of a SU-8 bonding layer. The dye flow is in the y-direction. 51 3.7 Layouts for the KOH etched and SU-8 based microfluidic channels. . . . . 52 3.8 The process of anodic bonding. The anode is placed on the silicon substrate and the cathode is placed on the sodium doped glass. Thermal heating is applied in the temperature range 200 ◦Cto450◦C. The applied field can be ranging from 100 V to 1000 V depending on the wafer or film thicknesses. The illustrated principal is proposed by Berthold et al. [6]...... 53 3.9 Sideview sketch of two SU-8 bonded structures bonded with different procedures proposed by Jackman et al. [21]...... 54 3.10 Fabrication process steps. 500 µm thick Pyrex glass wafers are used as top and bottom substrates. 1-4: Metallic mirrors are deposited on the top- and bottom wafers by standard UV-lithography, electron-beam evaporation and lift-off. 5-6: The micro-flow channels are defined in SU-8 photoresist on the top wafer by spin-coating, soft-baking baking, UV-exposure, post exposure baking and development. A 5 µm thick SU-8 bonding layer is deposited on the bottom wafer by spin-coating and soft-baking. 7: The wafers are bonded by a manually applied pressure at 75 ◦C. The wafers are further sealed by a bake at 90 ◦C for an hour and then cooled down before cross-linking of the SU-8 by an UV flood-exposure, followed by a post exposure bake. 8: The chips are diced and inlet/outlet holes are drilled. 56 3.11 Comparison of the optical properties at wavelength of 570 nm (lasing) and a of wavelength 532 nm (pumping) of the top mirror...... 57 List of Figures 96

3.12 Photos of fabricated KOH-etched based devices. See the layout sketch in ﬁgure3.7(a)forcomparison...... 58

3.13 The measured peak to peak roughness on the (100) plane in a KOH etched channelfordiﬀerentetchdepth...... 59

3.14 Photo of a the bottom mirror surface in the cavity after the anodic bonding. The wide black vertical line is the oblique (111) plane leading down to the deep transport channels. The visual appearance, seen with the naked eye, of the mirror surface seemed dull and the yellow appearance of gold is gone. 59

3.15PhotosofafabricatedSU-8bondeddevice...... 61

3.16 Comparison of two drilled holes indicating the strength of two diﬀerent polymerbondingmethods...... 61

3.17 Comparison of the SU-8 structure before and after bonding for a 100 µm widechannel...... 62

4.1 The sketch is showing a sideview of the experimental setup. The cw argon ion laser with a wavelength of 488 nm is used as the optical pump source for the dye Rhodamine 110 dissolved in ethanol. The dye solution is pumped through the KOH etched micro-cavity fluidic dye laser by means of a syringe pump. The light is guided with mirrors so that the pump laser light can enter the microfluidic channel from above through the chip glass lid. The excitation power is measured with a power meter before the light beam is focused. The laser light is focused by a lens to increase the excitation intensity. The emitted light from the micro-cavity laser device is transmitted through the chip glass lid and collected by an optical fiber connected to a spectrometer. The spectrometer is photomultiplier based and has therefore a high sensitivity. The spectrometer can only be used in cwmode...... 65 List of Figures 97

4.2 The sketch is showing a topview of the experimental setup. The dashed box indicates that the view is shifted to a sideview. The pulsed frequency doubled Nd:YAG laser with a wavelength of 532 nm, a pulse length of 5 ns, and a repetition rate of 10 Hz is used as the optical pump source for the dye Rhodamine 6G dissolved in ethanol or water. The dye solution is pumped through the SU-8 photoresist based micro-cavity fluidic dye laser by means of a syringe pump. The emitted laser light from the Nd:YAG laser crystal consists of the fundamental infra-red wave with a wavelength of 1064 nm. A frequency doubler inside the laser generates a secondary harmonic wave with a wavelength of 532 nm. A frequency splitter in front of the Nd:YAG laser splits up the two wavelengths and only permits the secondary harmonic wave (532 nm) to pass. The high power 532 nm beam is then reduced significantly by an attenuator. The light is then guided by mirrors through a slit in order to reduce the beam size from 6 mm to 1.5 mm. The excitation power is measured with a power meter before the light beam is reduced with an optical slit. After the slit the light is guided into the microfluidic channel from above through the chip glass lid by means of mirrors. The emitted light from the micro-cavity laser device is transmitted through the chip glass lid and further transmitted through an edge filter, which reduces the power of scattered pump light. The emitted light is then collected by an optical fiber connected to a spectrometer. The spectrometer is a CCD array type. The spectrometer can be used in both cwandpulsedmode...... 67

4.3 Sideview sketch of the SU-8 micro-cavity ﬂuidic dye laser under optical pumping. The pump light is transmitted through the glass lid and the top mirror. The emitted laser light is transmitted through the top mirror. The dye solution is pumped through the microﬂuidic channel by means of a syringepump...... 68

4.4 Measured optical spectra from a KOH etched micro-cavity fluidic dye laser device with Rhodamine 110 dissolved in ethanol flowing with a flowrate of 10 µL/min. The optical pump source is a continuous wave argon ion laser withawavelengthof488nm...... 70

4.5 Sketch of the micro-cavity in the SU-8 chip with the expected thicknesses for the diﬀerent layers. The expected refractive indices are also shown. The channel width for all devices used for measurements is 1 mm...... 71

4.6 Measured optical spectra from a SU-8 photoresist based micro-cavity fluidic dye laser device with Rhodamine 6G dissolved in ethanol with a concentration of 10−2 mol/L. The dye flowrate is 10 µL/min. The optical pump source is a pulsed Nd:YAG laser with a wavelength of 532 nm. The expected thicknesses of the cavity layers are a 10 µm layer of dye solution and a5µm layer of SU-8 as shown on figure 4.5. The bottom mirror reflectance is expected to be 83 % and for the top mirror it is 72 %...... 72 List of Figures 98

4.7 Measured optical spectra from a SU-8 photoresist based micro-cavity fluidic dye laser device with Rhodamine 6G dissolved in ethanol with a concentration of 10−3 mol/L. The dye flowrate is 10 µL/min. The optical pump source is a pulsed Nd:YAG laser with a wavelength of 532 nm. The device used for measurements is the same one as presented in figure 4.6...... 74 4.8 The three input/output power measurements are fitted linearly in two parts. The three curves represents measurements with different concentrations of Rhodamine 6G dissolved in ethanol. The concentrations are 10−2 mol/L, 2·10−2 mol/L, and 5·10−2 mol/L. The measurements are performed on a SU-8 photoresist based micro-cavity fluidic dye laser device. The dye flow speed is 10 µL/min. The optical pump source is a pulsed Nd:YAG laser with a wavelength of 532 nm. The device used for measurements is the same one as presented in figure 4.6 and figure 4.7...... 75 4.9 Measured optical spectra with different excitation powers from a SU-8 photoresist based micro-cavity fluidic dye laser device with Rhodamine 6G dissolved in ethanol with a concentration of 10−2 mol/L. The dye flow speed is 10 µL/min. The optical pump source is a pulsed Nd:YAG laser with a wavelength of 532 nm. The expected cavity lenght dimensions of the micro-cavity are shown on figure 4.5 however this device has no metallic mirrors...... 76 4.10 Calculated critical inversion ratios for the stationary approximation. The dye constants used are shown in table 4.5. The dye cell length for these calculations is 10 µm long. The triplet state has been included in the calculation giving a higher estimate on the critical inversion ratio...... 81

A.1 Fabrication process sequence for SU-8 photoresist based device...... 102

B.1 Fabrication process sequence for KOH etch based device...... 107 B.2 Fabrication process sequence for KOH etch based device...... 108 B.3 Fabrication process sequence for KOH etch based device...... 109 B.4 Fabrication process sequence for KOH etch based device...... 110

D.1 Sketch for optical transmission and reflection for multilayer films both dielectric and metallic films...... 114

E.1Electronicstatesfordyemolecules...... 117

I.1 Lithographic masks for the laser fabricated with SU-8...... 122 I.2 ChiplayoutforthelaserfabricatedwithSU-8...... 122

J.1 Lithographic masks for the laser fabricated in KOH etched silicon and anodicbonded...... 123 J.2 Chip layout for the laser fabricated in KOH etched silicon and anodic bonded.123 List of Tables 99

List of Tables

4.1 The estimated optical path lengths determined from the cavity peak wavelengths extracted from the measured spectra. The first five optical paths are estimated for the same device operated with different dye concentrations. The last value for the optical path length is estimated for a device with a 10 µm thick SU-8 bonding layer. Both devices used for this table had the reflectance of R1 =83%andR2 = 72 %. For devices with lower cavity mirror reflectance, the cavity modes were not visible. The expected optical path length for the device with a 5 µm SU-8 bonding layer is 21.55 µm. For the device with a 10 µm SU-8 bonding layer the expected optical path length is 29.50 µm...... 77

4.2 Peak wavelengths emitted from a micro-cavity fluidic dye laser for different concentrations. Data in the same row are extracted from measurements performed on the same device. The wavelengths marked with * indicates that under these conditions the devices did not show clear or any signs of lasing and the output is expected to be fluorescent or only operating around threshold. ** indicates that the SU-8 bonding layer is 10 µm thick for the particular device in the last row. The first column shows the expected reflectance of the cavity mirrors. The first row shows the concentration of the dye in ethanol used in the particulary measurement...... 78

4.3 Extracted linewidths for micro-cavity fluidic dye lasers for different concentrations under maximum possible excitation powers. Data in the same row are extracted from measurements performed on the same device. The output spectra with linewidths marked with * did not show a clear dominant peak. ** indicates that the SU-8 bonding layer is 10 µm thick for the particular device in the last row. The first column shows the expected reflectance of the cavity mirrors. The first row shows the concentration of dye in ethanol used in the particulary measurement. ? indicates that the linewidthcouldnotbedetermined...... 79 List of Tables 100

4.4 Extracted optical threshold pump powers for micro-cavity fluidic dye lasers for different concentrations. Data in the same row are extracted from measurements performed on the same device. The threshold powers marked with * did not show a clear knee in the input/output curve but the spectra revealed a clear narrowing of the output linewidths and the threshold values were taken there. ** indicates that the SU-8 bonding layer is 10 µm thick for the particular device in the last row. The first column shows the expected reflectance of the cavity mirrors. The first row shows the concentration of dye in ethanol used in the particulary measurement. ? indicates that the particulary measured spectrum is a fluorescent output. It should be noted that the power values is the average measured power and not the peakpower...... 79 4.5 Important constants for the Rhodamine 6G dye dissolved in ethanol at a lasingwavelengthof580nm[4,48]...... 80 4.6 N1** is the calculated critical inversion concentration based on the stationary model. N1* is the indirectly measured critical inversion concentration obtained from backwards calculation from the excitation power. Data in the same row are calculations and measurements performed on the same device. *** indicates that the SU-8 bonding layer is 10 µm thick for the particular device in the last row. The first column shows the expected reflectance of the cavity mirrors. The first row shows the concentration of dye in ethanol used in the particular measurement. The numbers in () are not physical possible since the concentration of excited molecules is larger than the concentration of molecules present in the dye solution...... 82 4.7 The same as in table 4.6 except that for the calculated critical concentrations N1** the triplet influence has been ignored resulting in a lower estimate of the critical inversion concentration...... 82

F.1Thecomplexrefractiveindexofgold[35,36]...... 119

G.1Thecomplexrefractiveindexofaluminum.[35,36]...... 120

H.1 The complex refractive index of chromium [35, 36]...... 121

K.1 The complete sheet of performed optical measurements during the thesis. The last column indicates the number of optical spectra measured with different excitation power in the particulary measurement series. The dye flow channels were in all cases 10µm high and 1 mm wide. The dye flowrates were in all cases 10 µL/min...... 124 Appendix A Fabrication Process Sequence for SU-8 Resist Based Chip 101

Appendix A

Fabrication Process Sequence for SU-8 Resist Based Chip

A.1 Process Sequence

1. Cleaning of a 500 µm thick Pyrex glass wafer as top wafer.

2. HMDS backing and spinning on 1.5 µm thick resist plus pre exposure bake.

3. UV exposure though top mirror mask. Post exposure bake. UV ﬂoat exposure. Development.

4. 5/40 nm Cr/Au e-beam evaporation for top mirror.

5. Lift oﬀ the metal in acetone plus ultra sonic.

6. 12-18 of hours dehydrating bake at 120 ◦C. Spinning on 10 µm thick SU-8 resist and soft baking.

7. UV-exposure through microﬂuidic channel mask, and post exposure baking for crosslinking of the SU-8. Development of the SU-8 resist for microﬂuidic channels in PGMEA and rinse in IPA.

8. Cleaning of a 500 µm thick Pyrex glass wafer as bottom wafer.

9. 10/150 nm Cr/Au e-beam deposition for bottom mirror.

10. 12-18 hours of dehydrating bake. Spinning on 5 µm thick SU-8 resist and a soft baking.

11. Heating both wafers to 75 ◦C. Manually pressing the two glass wafers together. 90 ◦C bake for an hour. UV-ﬂoat-exposure and post exposure bake for crosslinking of the SU-8 bonding layer.

12. Dicing out to chips. Drilling inlet and outlet holes. Appendix A Fabrication Process Sequence for SU-8 Resist Based Chip 102

1) Pyrex Glass wafer

2) 1.5 µm photoresist

3) UV-exposure and development

4) E-beam evaporated 5/40 nm Cr/Au

5) Lift off metal

6) 10 µm SU-8 photoresist and softbake

7) UV-exposure, post exposure bake, and development

8) Pyrex Glass wafer

9) E-beam evaporated 10/150 nm Cr/Au

10) 5 µm SU-8 photoresist and softbake

11) Bonding, UV-exposure, and post exposure bake

12) Dicing and drilling of inlet and outlet

Figure A.1: Fabrication process sequence for SU-8 photoresist based device. Appendix A Fabrication Process Sequence for SU-8 Resist Based Chip 103

A.2 Process Recipes

Pyrex glass wafer cleaning Scrubbing the glass wafer on a rotating plate with DI-water and Triton-X100 soap followed by a standard 7-Up etch.

7-Up etch (hot sulphuric acid + ammoniumperoxodisulphate) for cleaning Temperature: 80 ◦C. Time: 10 min. DI-water rinse: 5 min.

Standard photolithography negative process HMDS baking: 35 min. SSI spinner resist recipe: PR1-5. Thickness: 1,5 µm. Exposure time 1: 6.5 sec. (Karl Suss Alinger). Reversal bake: 120 ◦C for 2 min. Exposure time 2: 35 sec. (Karl Suss Alinger). Development: 60 sec in NaOH. DI-water rinse: 5 min.

Leybold e-beam evaporation of Cr/Au Combination for top mirror: 5/40 nm Cr/Au. Combination for bottom mirror: 10/150 nm Cr/Au.

Lift-oﬀ metal with hot acetone and ultra sonic Temperature: 50 ◦C. Time 1: 20 min. DI-water rinse. Emptying, cleaning and reﬁling of bath. Time 2: 10 min. DI-water rince: 10 min.

SU-8 photoresist spin-coating 12-18 hours dehydrating bake of the wafer at 120 ◦C. Spin-coating of SU-8 with manually dispensing. Rotational speed: 3000 RPM. Time: 30 sec. Softbake on KS-spinner hotplate with elevator ramping. Temperature: 90 ◦C. Appendix A Fabrication Process Sequence for SU-8 Resist Based Chip 104

SU-8 photoresist exposure and post exposure bake UV-lithography: Exposure time: 60 sec. (Karl Suss aligner). Post exposure bake: Bake time: 65 ◦C for 2 min + 90 ◦C for 15 min. Slow cooling.

SU-8 photoresist development in propylene glycol monomethyl ether acetate (PGMEA) Rough development time: 1 min. Fine development time: 2 min. Rinse time in isopropyl alcohol (IPA): 5 min.

SU-8 photoresist bonding procedure 1. SU-8 photoresist spin-coating procedure for SU-8 bonding layer on bottom wafer. 2. Heating of both wafers on a hotplate at 75 ◦C. 3. Manually pressing the two wafers together so most of the Newton rings disappear. 4. 90 ◦C bake for an hour for increasing the strength and hermetic quality of the bonding. 5. UV-exposure for 1 min for crosslinking. (Karl Suss Aligner). 6. Post exposure bake: 65 ◦C for 2 min + 90 ◦C for 15 min. 7. Slow cooling. Appendix B Fabrication Process Sequence for KOH Etch Based Chip 105

Appendix B

Fabrication Process Sequence for KOH Etch Based Chip

B.1 Process Sequence

1. (100) silicon wafer.

2. 500 nm thick wet thermal oxidation.

3. Deposit of 50 nm thick LPCVD silicon rich nitride.

4. HMDS backing and spinning on 1.5 µm thick resist and a softbake.

5. UV exposure though 1. mask. Post exposure bake. UV ﬂoat exposure. Develop- ment.

6. RIE etch of the nitride for deﬁning the total device area.

7. Removal of resist with acetone.

8. HMDS backing and spinning on 1.5 µm thick resist and a softbake.

9. UV exposure though 2. mask. Post exposure bake. UV ﬂoat exposure. Develop- ment.

10. BHF etch of oxide for deﬁning deep transport channel areas.

11. Removal of resist with acetone. Cleaning of the wafers in 7-Up. BHF etch of native oxide.

12. Deep KOH etch of deep transport channels. Cleaning of the wafer in 7-Up.

13. BHF etch of oxide for deﬁning the laser cavity areas. Cleaning of the wafers in 7-Up. Afterwards BHF of native oxide.

14. KOH etch of laser cavity.

15. Cleaning of the wafers in 7-Up. Removal of nitride in hot H3PO4. Appendix B Fabrication Process Sequence for KOH Etch Based Chip 106

16. Oxide etch with BHF during RCA cleaning.

17. HMDS backing and spinning on 1.5 µm thick resist and a softbake.

18. UV exposure though 3. mask. Post exposure bake. UV ﬂoat exposure. Develop- ment.

19. E-beam evaporation of 10/150 nm chromium/gold for bottom cavity mirror.

20. Lift oﬀ with acetone plus ultra sonic for deﬁning bottom cavity mirror.

21. Cleaning of Pyrex glass wafer.

22. Covering the Pyrex glass wafer with blue plastic ﬁlm.

23. CO2 laser pattering of inlets and outlets in the blue plastic ﬁlm covering the glass wafer.

24. HF etch of inlets and outlets in the glass wafer. Removal of the blue plastic ﬁlm.

25. Anodic bonding of the silicon and the glass wafer at 350 ◦C plus 1000 V.

26. Dicing of the bonded wafer into chips. Appendix B Fabrication Process Sequence for KOH Etch Based Chip 107

1) (100) Silicon wafer

2) 500 nm wet thermal oxide

3) 50 nm LPCVD Si-rich nitride

4) 1.5 µm photoresist

5) UV-exposure and development

6) RIE etch of nitride

7) Removal of resist

Figure B.1: Fabrication process sequence for KOH etch based device. Appendix B Fabrication Process Sequence for KOH Etch Based Chip 108

8) 1.5 µm photoresist

9) UV-exposure and development

10) BHF etch of oxide

11) Removal of resist

12) KOH etch of transport channels

13) BHF etch of oxide

14) KOH etch of laser cavity

Figure B.2: Fabrication process sequence for KOH etch based device. Appendix B Fabrication Process Sequence for KOH Etch Based Chip 109

15) H3PO4 etch of nitride

16) Etch of oxide during RCA cleaning

17) 1.5 µm photoresist

18) UV-exposure and development

19) E-beam evaporated 10/150 nm Cr/Au

20) Lift off metal

21) Pyrex glass wafer

22) Blue plastic film

23) CO2 laser writing in plastic film

Figure B.3: Fabrication process sequence for KOH etch based device. Appendix B Fabrication Process Sequence for KOH Etch Based Chip 110

24) HF etch of inlet and outlet

25) Anodic bonding

Figure B.4: Fabrication process sequence for KOH etch based device. Appendix B Fabrication Process Sequence for KOH Etch Based Chip 111

B.2 Process Recipes

Thermal wet oxidation Temperature: 1000 ◦C. Thickness: 500 nm.

Low pressure chemical vapour deposition (LPCVD) of Si-rich nitride (Si3N4) Gases: 93 sccm DCS and 13 sccm NH3. Pressure: 112 mTorr. Temperature 835 ◦C. Time: 12 min. Thickness: 50 nm.

Reactive ion etch (RIE) of nitride Recipe: OH-POLYA. Gases: 32 sccm SF6 and 8 sccm O2. Time: 1 min and 30 sec.

Photoresist strip in acetone Rough strip: 30 sec. Fine strip: 5 min with ultra sonic. DI-water rinse: 10 min.

Buﬀered hydroﬂuoric acid etch (BHF) of oxide Time: 15 min. DI-water rinse: 5 min.

7-Up etch (hot sulphuric acid + ammoniumperoxodisulphate) for cleaning Temperature: 80 ◦C. Time: 10 min. DI-water rinse: 5 min. Appendix B Fabrication Process Sequence for KOH Etch Based Chip 112

Potassium hydroxide (KOH) anisotropic silicon etch KOH weight-percent: 28 %. Temperature: 60 ◦C. Initial BHF etch of native oxide in 45 sec. DI-water rinse: 2 min. KOH etch: Variable time. DI-water rinse: 5 min.

Nitride etch with hot phosphoric acid (H3PO4) Temperature: 180 ◦C. Time: 30 min. DI-water rinse: 5 min.

RCA cleaning ◦ RCA1: NH4OH+H2O2+DI-water for 10 min at 70 C. DI-water rinse: 2 min. HF:30sec. ◦ RCA2: HCL+H2O2+DI-water for 10 min at 70 C. DI-water rinse: 2 min. HF:30sec. DI-water rinse: 5 min.

Leybold e-beam evaporation of Cr/Au Combination for bottom mirror: 10/150 nm Cr/Au.

Lift-oﬀ metal with hot acetone and ultra sonic Temperature: 50 ◦C. Time 1: 20 min. DI-water rinse. Emptying, cleaning and reﬁling of bath. Time 2: 10 min. DI-water rince: 10 min.

Pyrex glass wafer cleaning Scrubbing the glass wafer on a rotating plate with DI-water and Triton-X100 soap followed by a standard 7-Up etch.

Anodic bonding Pressure: 1 atm. Temperature: 350 ◦C. Voltage: Up to around 1000 V. Appendix C Fresnel and Power Coeﬃcients 113

Appendix C

Fresnel and Power Coeﬃcients

In this appendix the Fresnel coefficients are presented. These coefficients are useful when calculating reflection and transmission of different materials. The Fresnel coefficients tell you how the E-field of the light is changed in magnitude and phase across a boundary. Also the power reflection and transmission are stated here in this appendix. These formulas together with appendix D are used to calculate the reflection and transmission of the mirrors used in the fabricated devices.

Fresnel reflection coefficient for TE polarization: E n cos(θ ) − n cos(θ ) TE: r = r = 1 1 2 2 (C.1) E n1 cos(θ1)+n2 cos(θ2) Fresnel reflection coefficient for TM polarization: E n cos(θ ) − n cos(θ ) TM: r = r = 2 1 1 2 (C.2) E n2 cos(θ1)+n1 cos(θ2) Fresnel transmission coefficient for TE polarization: E 2n cos(θ ) TE: t = t = 1 1 (C.3) E n1 cos(θ1)+n2 cos(θ2) Fresnel transmission coefficient for TM polarization: E 2n cos(θ ) TM: t = t = 1 1 (C.4) E n2 cos(θ1)+n1 cos(θ2) Power reflection: P E 2 R = r = r = r2 (C.5) P E Power transmission: P n cos(θ ) E 2 n cos(θ ) T = t = 2 2 t = 2 2 t2 (C.6) P n1 cos(θ1) E n1 cos(θ1) Appendix D Modelling of Optical Reflection and Transmission 114

Appendix D

Modelling of Optical Reﬂection and Transmission

δ δ −i 2 −i3 2 2 2 t32t21e t32r21r23t21e θ 1 T 1 2 1 r21 ,t21 θ 2 2 d2 3 r32 ,t32 3 θ θ 3 3 = E 1 R2

−iδ E = 1 r32 2 t32r21t23e (a) (b)

Figure D.1: Sketch for optical transmission and reflection for multilayer films both dielectric and metallic films.

In order to calculate the Fresnel reﬂection and transmission of both dielectric and metallic materials one have to introduce a complex refractive index. The complex refractive index is given by:

n = nRe − ikIm (D.1) Considering multiply layers of different materials one need a systematically procedure to do the calculation. An iterative method has been developed by H. Cory et al. [10]. Figure D.1 shows a light beam with a E-field magnitude of E travelling through medium 3. The beam be partly reflected back and partly transmitted through to medium 2. The light that is transmitted through medium 2 will experience infinite number of reflections and transmissions with medium 1 and 3. One should take care because it is the E-field magnitude that is shown in the figure, not the power magnitude. Appendix D Modelling of Optical Reflection and Transmission 115

The job is then to calculate the total reﬂection and transmission as shown in the right side of the ﬁgure. At all the interfaces Snell’s law shall be obeyed:

n1 sin(θ1)=n2 sin(θ2)=... = nN sin(θN ) (D.2)

In fact one needs to calculate all the angles, which the light is refracted with before any calculation of reflection and transmission. This can of cause be avoided, if the light is striking orthogonal to the interface. Then you do not need to care about the polarization of the light, which can be seen from the Fresnel coefficients given in appendix C. The Fresnel coefficients for orthogonal incidence for the interface between medium 3 and 2 is given here in equations D.3 and D.4.

n3 − n2 r32 = (D.3) n3 + n2

2n3 t32 = (D.4) n3 + n2 Because of the orthogonal incidence the Fresnel coefficients are reduced from four to two. Now one can sum op all the reflections and transmission, which will contribute to the total reflection R2 for the whole structure.

−iδ2 −i2δ2 R2 = r32 + t32r21t23e + t32r21r23r21t23e + ... r + r e−iδ2 = 32 21 (D.5) −iδ2 1+r32r21e

As expected this sum is converging to a finite expression. The capital R2 should not be confused with the power reflection. It is only determining the magnitude of the reflected E- field to the incoming E-field. The same summing can be done with the total transmission T2.

−iδ2/2 −i3δ2/2 −i5δ2/2 T2 = t32t21e + t32r21r23t21e + t32r21r23r21r23t21e + ... t t e−iδ2/2 = 32 t21 (D.6) −iδ2 1+r32r21e The argument in the complex exponential function determines the phase, which the light will strike the interface with and the damping of the ﬁeld through the medium:

4πn2d2 cos(θ2) δ2 = (D.7) λ0

Here λ0 is the wavelength of the light in free space. The structure can be expanded with a fourth layer called medium 4 which means the light will enter medium 4 first then medium 3 and so on. The same formulas can be used for the reflection and transmission. However now the results of the former calculation R2 or T2 have to be inserted into formula instead of the Fresnel coefficient r21 or t21. The total reflection for the four layer structure will then be: r + R e−iδ3 R = 43 2 (D.8) 3 −iδ3 1+r43R2e Appendix D Modelling of Optical Reflection and Transmission 116

The same can be done for the transmission: t T e−iδ3/2 T = 43 2 (D.9) 3 −iδ3 1+r43R2e The argument will of cause contain the relevant data for layer three.

4πn3d3 cos(θ3) δ3 = (D.10) λ0 A general calculation formula for each layer can now be written in systematical way for both the reﬂection, the transmission, and the argument:

−iδj r + R − e R = (j+1,j) (j 1) (D.11) j −iδj 1+r(j+1,j)R(j−1)e

−iδj /2 t T − e T = (j+1,j) (j 1) (D.12) j −iδj 1+r(j+1,j)R(j−1)e

4πnjdj cos(θj) δj = (D.13) λ0 The calculated E-field coefficients for reflection and transmission are complex numbers. When for instance all N layers have been calculated, one can use the equations C.5 and C.6 to find the power reflection and the power transmission of the whole structure: P E 2 R = r = r = R2 (D.14) P E N 2 Pt n2 cos(θ2) Et n2 cos(θ2) 2 T = = = TN (D.15) P n1 cos(θ1) E n1 cos(θ1) With the power coefficients the power absorption can be found:

A =1− R − T (D.16)

This equation can be used as an control if one is modelling layers with only the real part of the refractive index. Then this equation should be fulﬁlled R + T =1. Appendix E Gain and Laser Threshold for cw Lasers 117

Appendix E

Gain and Laser Threshold for cw Lasers

Figure E.1: Electronic states for dye molecules.

To predict the gain and thereby the laser threshold one needs to know the concentration of molecules in the different states. The three most important states are the ground singlet state S0, the first excited singlet state S1 and the first exited triplet state T1.The molecules in the T1 can absorb some of the laser light. The lifetime for the decay from S1 to S0 is τ, which is called the spontaneous emission life time. Figure E.1 shows the intercrossing rate kST from state S1 to state T1. Also the rate from state T1 to state S0 denoted 1 is shown. τ is the lifetime of the triplet state T . The intercrossing rate is τT T 1 defined as this [48]: 1 − φ k = (E.1) ST τ φ is the fluorescence quantum efficiency. The population of T1 can now be calculated by this rate equation [48]:

dN N T1 − T1 = NS1 kST (E.2) dt τT Appendix E Gain and Laser Threshold for cw Lasers 118

Above the threshold the concentration of molecules, which are excited into the state S1 will not increase much with more pumping power. τT is considered to be fairly large. Then the number of molecules in the triplet state at inversion will be in steady state [48]: ≈ NT1 NS1 kST τT = NinvkST τT (E.3) This expression is also called the equilibrium triplet approximation. It is also necessary to know the capture cross sections for the different states. The molecular absorption cross section σ(λ) is defined by this intensity relation according to [48]: I(λ, x)=I(λ, 0)e−Nσ(λ)x (E.4) This equation can be extended to describe a round trip in the cavity with a length of L, mirror reflections R1 and R2, and the different molecular absorption and emission mechanisms. When skipping the wavelength dependency one can write:

(g−ε)x I(x)=R1R2I0e (E.5)

Here g = NS1 σem is the gain in dye and ε = NS0 σS + NT1 σT is the loss in the dye. For a round trip in the cavity one will get this expression for the light intensity:

(g−ε)2L I (g−ε)2L I = R1R2I0e ⇒ G = = R1R2e (E.6) I0 In order to reach inversion population the overall gain should be one or higher. With a overall gain of one G = 1, the threshold condition will be: 1 g = ε − ln(R R )(E.7) 2L 1 2 Putting in the diﬀerent molecular concentrations and molecular cross sections and equat- ing to zero one gets the expression for the threshold condition: 1 N σ − N σ − N σ + ln(R R )=0 (E.8) S1 em S0 S T1 T 2L 1 2

Using equation E.3 in the threshold condition and the the condition N = NS0 +NS1 +NT1 one gets an equation: 1 [σ + σ + k τ (σ − σ )]N − σ N + ln(R R )=0 (E.9) em S ST T S T inv S 2L 1 2

The concentration of needed molecules excited to the S1 state can be written as: − 1 σSN 2L ln(R1R2) Ninv = (E.10) σem + σS + kST τT (σS − σT ) The needed pumping power can now approximately be written as [48]:

∼ NinvhcLA Pp,inv = (E.11) λpτ Appendix F Refractive Index of Gold (Au) 119

Appendix F

Refractive Index of Gold (Au)

λ0 [nm] n k λ0 [nm] n k λ0 [nm] n k 200.0 1.427 1.215 310.0 1.830 1.916 619.9 0.194 3.050 206.6 1.422 1.306 317.9 1.840 1.904 652.6 0.166 3.150 210.1 1.430 1.334 326.3 1.824 1.878 688.8 0.160 3.800 213.8 1.320 1.364 335.1 1.798 1.860 729.3 0.164 4.350 217.5 1.438 1.388 344.4 1.766 1.846 774.9 0.174 4.860 221.4 1.442 1.418 354.2 1.740 1.848 826.6 0.188 5.390 225.4 1.452 1.442 364.7 1.716 1.862 885.6 0.210 5.880 229.6 1.454 1.478 375.7 1.696 1.906 953.7 0.236 6.470 233.9 1.462 1.510 387.5 1.674 1.936 1033.0 0.272 7.070 238.4 1.470 1.550 400.0 1.658 1.956 1127.0 0.312 7.930 243.1 1.478 1.590 413.3 1.636 1.958 1240.0 0.372 8.770 248.0 1.484 1.636 427.5 1.616 1.940 1265.0 0.389 8.090 253.0 1.490 1.698 442.8 1.562 1.904 1291.0 0.403 8.250 258.3 1.504 1.748 459.2 1.426 1.846 1319.0 0.419 8.420 263.8 1.546 1.784 476.9 1.242 1.796 1384.0 0.436 8.590 269.5 1.598 1.822 495.9 0.916 1.840 1409.0 0.473 8.960 275.5 1.648 1.852 516.6 0.608 2.120 1442.0 0.493 9.150 281.8 1.690 1.882 539.1 0.402 2.540 1476.0 0.515 9.360 288.3 1.742 1.900 563.9 0.306 2.880 1512.0 0.537 9.580 295.2 1.776 1.918 590.4 0.236 2.950 1550.0 0.559 9.810 302.4 1.812 1.920

Table F.1: The complex refractive index of gold [35, 36]. Appendix G Refractive Index of Aluminum (Al) 120

Appendix G

Refractive Index of Aluminum (Al)

λ0 [nm] n k λ0 [nm] n k λ0 [nm] n k 206.6 0.130 2.39 495.9 0.755 6.03 799.9 2.800 8.45 215.6 0.141 2.51 500.0 0.769 6.08 825.0 2.750 8.31 225.4 0.155 2.64 506.1 0.789 6.15 826.6 2.740 8.31 236.2 0.172 2.79 516.6 0.826 6.28 850.0 2.610 8.22 248.0 0.190 2.94 527.6 0.867 6.42 855.1 2.580 8.21 261.0 0.209 3.11 539.1 0.912 6.55 875.0 2.380 8.18 275.5 0.233 3.30 550.0 0.958 6.69 885.6 2.240 8.21 291.7 0.261 3.51 551.0 0.963 6.70 900.0 2.060 8.30 300.0 0.276 3.61 563.6 1.020 6.85 918.4 1.860 8.44 310.0 0.294 3.74 576.7 1.080 7.00 925.0 1.770 8.49 317.9 0.310 3.84 590.4 1.150 7.15 950.0 1.490 8.88 326.3 0.326 3.95 600.0 1.200 7.26 953.7 1.470 8.95 335.1 0.344 4.06 604.8 1.220 7.31 991.9 1.370 9.49 344.4 0.364 4.17 619.9 1.300 7.48 1000.0 1.350 9.58 350.0 0.375 4.24 635.8 1.390 7.65 1033.0 1.260 10.00 354.2 0.385 4.30 650.0 1.470 7.79 1078.0 1.210 10.60 364.7 0.407 4.43 652.6 1.490 7.82 1127.0 1.200 11.20 375.7 0.432 4.56 670.2 1.600 8.01 1181.0 1.210 11.80 387.5 0.460 4.71 688.8 1.740 8.21 1200.0 1.210 12.00 400.0 0.490 4.86 700.0 1.830 8.31 1240.0 1.210 12.50 413.3 0.523 5.02 708.5 1.910 8.39 1305.0 1.230 13.20 427.5 0.558 5.20 729.3 2.140 8.57 1378.0 1.260 14.00 442.8 0.598 5.38 750.0 2.400 8.62 1459.0 1.330 14.90 450.0 0.618 5.47 751.4 2.410 8.62 1500.0 1.380 15.40 459.2 0.644 5.58 774.9 2.630 8.60 1550.0 1.440 16.00 476.9 0.695 5.80

Table G.1: The complex refractive index of aluminum. [35, 36]. Appendix H Refractive Index of Chromium (Cr) 121

Appendix H

Refractive Index of Chromium (Cr)

λ0 [nm] n k λ0 [nm] n k λ0 [nm] n k 200.0 0.89 1.69 307.7 1.02 2.76 815.7 4.23 4.34 202.9 0.92 1.72 316.3 1.06 2.85 826.6 4.27 4.33 207.0 0.94 1.73 323.7 1.12 2.95 849.2 4.31 4.32 210.1 0.96 1.73 333.3 1.18 3.04 861.0 4.33 4.32 214.1 0.97 1.74 341.6 1.26 3.12 885.6 4.38 4.31 217.1 0.97 1.75 351.2 1.33 3.18 911.0 4.42 4.30 221.0 0.95 1.74 362.5 1.39 3.24 939.3 4.47 4.29 225.0 0.94 1.76 372.3 1.43 3.31 968.6 4.49 4.28 230.0 0.93 1.80 385.0 1.44 3.40 999.9 4.50 4.28 233.9 0.90 1.83 396.1 1.48 3.54 1033.0 4.52 4.29 238.0 0.87 1.87 409.2 1.54 3.71 1069.0 4.53 4.30 243.1 0.86 1.94 424.6 1.65 3.89 1107.0 4.53 4.31 248.0 0.85 2.01 438.1 1.80 4.06 1148.0 4.53 4.34 253.0 0.86 2.07 455.8 1.99 4.22 1192.0 4.51 4.36 258.3 0.86 2.13 471.4 2.22 4.36 1240.0 4.47 4.43 263.8 0.86 2.21 490.1 2.49 4.44 1292.0 4.47 4.50 269.5 0.88 2.28 512.3 2.75 4.46 1348.0 4.45 4.56 275.5 0.89 2.35 532.1 2.98 4.45 1378.0 4.43 4.60 281.8 0.90 2.42 558.5 3.18 4.41 1442.0 4.35 4.66 287.0 0.91 2.49 582.1 3.34 4.38 1512.0 4.24 4.81 293.8 0.94 2.58 610.8 3.48 4.36 1590.0 4.13 5.03 300.2 0.98 2.67 700.5 3.84 4.37

Table H.1: The complex refractive index of chromium [35, 36]. Appendix I Lithographic Masks for Chips Fabricated with SU-8 122

Appendix I

Lithographic Masks for Chips Fabricated with SU-8

Figure I.1: Lithographic masks for the laser fabricated with SU-8.

Figure I.2: Chip layout for the laser fabricated with SU-8. Appendix J Lithographic Masks for Chips Fabricated in KOH Etched Si 123

Appendix J

Lithographic Masks for Chips Fabricated in KOH Etched Si

Figure J.1: Lithographic masks for the laser fabricated in KOH etched silicon and anodic bonded.

Figure J.2: Chip layout for the laser fabricated in KOH etched silicon and anodic bonded. Appendix K Measurement Series 124

Appendix K

Measurement Series

Name SU-8 Mirrors Dye/Solvent Conc. Num. Spectra-6-28 5 µm 83 % + 72 % Rho-6G/Ethanol 10−2 mol/L 6 Spectra-7-4 5 µm 83 % + 72 % Rho-6G/Ethanol 10−2 mol/L 16 Spectra-7-8 5 µm 0.14 % + 0.16 % Rho-6G/Ethanol 10−2 mol/L 16 Spectra-7-29 5 µm 83 % + 72 % Rho-6G/Ethanol 10−3 mol/L 16 Spectra-8-2 5 µm 83 % + 72 % Rho-6G/Ethanol 10−4 mol/L 16 Spectra-8-5-1 5 µm 83 % + 72 % Rho-6G/Ethanol 10−1 mol/L 16 Spectra-8-5-2 10 µm 83 % + 72 % Rho-6G/Ethanol 10−2 mol/L 15 Spectra-8-7 10 µm 83 % + 72 % Rho-6G/Ethanol 10−3 mol/L 15 Spectra-8-10-1 5 µm 83 % + 72 % Rho-6G/Ethanol 2·10−2 mol/L 15 Spectra-8-10-2 5 µm 83 % + 72 % Rho-6G/Ethanol 5·10−2 mol/L 16 Spectra-9-10-1 10 µm 83 % + 72 % Rho-6G/Ethanol 2·10−2 mol/L 15 Spectra-9-10-2 5 µm 0.14 % + 0.16 % Rho-6G/Ethanol 2·10−2 mol/L 16 Spectra-9-11-1 5 µm 83 % + 0.16 % Rho-6G/Ethanol 10−2 mol/L 15 Spectra-9-11-2 5 µm 83 % + 0.16 % Rho-6G/Ethanol 10−3 mol/L 16 Spectra-9-23-1 5 µm 83 % + 72 % Rho-6G/Water 10−2 mol/L 16 Spectra-9-23-1 5 µm 83 % + 72 % Rho-6G/Water 10−3 mol/L 16 KOH-1 0 ?%+0.16 % Rho-110/Ethanol 10−3 mol/L 13 KOH-2 0 ?%+0.16 % Rho-110/Ethanol 5·10−3 mol/L 13 KOH-3 0 ?%+0.16 % Rho-110/Ethanol 10−2 mol/L 13 KOH-4 0 ?%+0.16 % Rho-110/Ethanol 2·10−2 mol/L 13

Table K.1: The complete sheet of performed optical measurements during the thesis. The last column indicates the number of optical spectra measured with different excitation power in the particulary measurement series. The dye flowchannels werein all cases 10 µm high and 1 mm wide. The dye flowrates were in all cases 10 µL/min. Appendix L Publications 125

Appendix L

Publications

The following papers have been submitted. The papers are based on parts of the theoretical and experimental work described in this masters thesis. The papers are shown on the following pages.

1. Abstract accepted for the international conference IEEE MEMS 2003 held in Kyoto, Japan. January 19-23. Bjarne Helbo, Anders Kristensen, and Aric Menon. Micro-Cavity Fluidic Dye Laser.

2. Paper manuscript submitted to Journal of Micromechanics and Microengineering in the beginning of September for revision. B Helbo, A Kristensen and A Menon. A Micro-Cavity Fluidic Dye Laser. Micro-Cavity Fluidic Dye Laser

Category: 7. Micro-Optical Devices/Systems, Poster Bjarne Helbo, Anders Kristensen, and Aric Menon Mikroelektronik Centret (MIC), Technical University of Denmark (DTU)

We have successfully fabricated and characterized a micro-cavity fluidic dye laser with metallic mirrors, which can be integrated with other µ-TAS systems without further processing steps. In our design, the laser dye is continuously pumped through the laser cavity. This allows for cw laser operation and a long lifetime for the system. Other groups have previously demonstrated micro-cavity dye lasers [1, 4], but to our knowledge, our design is the first one using standard micro-fabrication techniques for fluidic dye lasers.

In our design, the laser dye solution is pumped through a microfluidic channel, formed in SU-8 photoresist. The SU-8 channel structure is sandwiched between Pyrex glass wafers, and the structure is bonded together using SU-8. The laser cavity is formed inside the microfluidic channel, by means of mirrors deposited on the Pyrex glass top and bottom surfaces, as outlined in figure 1 and 2. The micro-cavity fluidic dye laser is optically pumped by an external laser, and the output is emitted through the semi-transparent top-mirror.

The process sequence is shown in figure 3. Pyrex glass wafers are used as top and bottom wafers, and in order to simplify the process we used Cr/Au films for the mirrors. For the Cr/Au films we have calculated the reflectance R1=83% (bottom), R2=72% (top) and the transmittance T1=0%, T2=6%. Our process also allows for dielectric mirrors to be used. The semitransparent top mirror is defined in a lift off process. The microfluidic channel is defined in a 10 µm thick SU-8 photoresist. The non transparent bottom mirror is deposited on the bottom wafer and a 5 µm thick layer of SU-8 is spun on as a bonding glue. Our lowtemperature SU-8 bonding technique, which was partly adapted from Jackman et al. [2] and Shen et al. [3] requires no expensive bonding equipment and yet has a good hermetic quality. Both wafers in process step 6 are heated to 75o and manually pressed together. The bonded wafers are baked at 90o for an hour. This step improved the hermetic quality of the bonding. The wafers are again exposed to UV-light in order to cross-link the SU-8 on the bottom wafer and further cross-linked by a post exposure bake. The bonded wafers are diced before drilling inlet and outlet holes for the dye solution. A photo of a fabricated device is shown in figure 4. The channel at the center of the chip is 1 mm wide and 10 µm high.

The laser was characterized using Rhodamine 6G dissolved in ethanol as active medium, and optically pumped by a pulsed, frequency doubled Nd:YAG laser (wavelength 532 nm). The spot diameter of the pumping laser beam on the micro-cavity fluidic dye laser cavity is approximately 1.5 mm. The emitted light is collected with an optical fiber coupled to a spectrometer. We have measured on six different dye concentrations ranging from 10−4 to 10−1 mol/l. The lowest concentration for which we were able to observe lasing was 10−2 mol/l. Figure 5 shows a measured spectrum for the dye concentration 10−2 mol/l. The first peak, at 532 nm is due to scattered light from the pumping laser and the dye laser peak is seen at a wavelength of 570 nm with a Full Width Half Maximum (FWHM) of 5.7 nm. From figure 6 we estimated that the lasing threshold power was 0.6 mW. The insert in figure 6 shows the threshold optical pump power and lasing wavelength as a function of the dye concentration. Both wavelength and threshold power increases with dye concentration and an optimized minimum for the threshold power was found around 10−2 mol/l.

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References

[1] F. De Martini, G. Innocenti, G.R. Jacobovitz, and P. Mataloni. Physical Review Letters, 59(26):2955–2958, 1987.

[2] R.J. Jackman, T.M. Floyd, R. Ghodssi, M.A. Schmidt, and K.F. Jensen. Journal of Micromechanics and Microengineering, 11:263–269, 2001.

[3] S.C. Shen, C.T. Pan, and H.P. Chou. Proceedings of SPIE, 4407:185–192, 2001.

[4] H. Yokoyama, M. Suzuki, and Y. Nambu. Applied Physics Letters, 58(23):2598–2600, 1991. Corresponding author: Bjarne Helbo, Mikroelektronik Centret (MIC), Technical University of Denmark, Ørsteds Plads, Bldg. 345east, DK-2800 Kgs Lyngby, Denmark. Tel: +45 4525 5777, Fax: +45 4588 7762, E-mail: [email protected]

Pump Light Dye Laser Light

Dye Inlet Top Mirror Dye Outlet 500 µm Pyrex glass wafer n=1.474 5nmCr

Top mirror Glass

10 µm dye solution n=1.36 40 nm Au Cavity Length SU-8 150 nm Au 5µmSU-8n=1.59 SU-8 Bottom mirror Glass 10 nm Cr 500 µm Pyrex glass wafer n=1.474 Bottom Mirror Figure 1: Layer structure of the microfluidic dye laser. Figure 2: Cross-sectional drawing of the microfluidic laser The laser cavity is formed between the gold mirrors. showing the dye flow through the laser cavity during optical pumping.

1: Top: 1.5 µm photoresist 5: Top: 10 µm SU-8

2: Top: Expose and develop 6: Top: Expose, bake, develop

Inlet Top Mirror Outlet

6: Bottom: 5µmSU-8 3: Top: Cr/Au5/40nm 7: Bonding, expose, bake Fluidic Channel 5mm

Figure 4: Photo of fabricated glass/SU-8 chip seen from 4: Top: Lift off metal above. 8: Dicing. Drill inlet and outlet

4: Bottom: Cr/Au 10/150 nm

Figure 3: Fabrication process sequence.

Optical Spectrum 4000 Output Power and FWHM vs Optical Pump Power 0.1 mW 50 Threshold Power and Wavelength vs Dye Conc. 5000 580 10 0.8 mW Threshold Power FWHM 3500 Lasing Wavelength Power 2.7 mW 45 578 8 4500 4.8 mW Fitted 1 3000 6.5 mW 40 576 6 Fitted 2 4000 35 574 4 3500

2500 Wavelength [nm] 572 2 30 Threshold Power [mW] 3000 570 0 2000 0.01 0.02 0.05 0.1 25 Dye Conc. [mol/l] 2500

1500 FWHM [nm] 20 2000 Output Power [counts]

15 1500 Output Power [counts] 1000 10 1000 500 5 500

0 0 0 520 530 540 550 560 570 580 590 600 610 620 0 1 2 3 4 5 6 7 Wavelength [nm] Optical Pump Power [mW]

Figure 5: Measured spectrum emitted from the laser Figure 6: Output power and FWHM vs excitation power dye pumped through the device with a concentration of for a laser dye concentration of 10−2 mol/l. The insert 10−2 mol/l. shows the threshold power and the lasing wavelength dependencies on the dye concentration. A Micro-Cavity Fluidic Dye Laser

B Helbo, A Kristensen§ and A Menon Mikroelektronik Centret, Technical University of Denmark, Bldg. 345east, Ørsteds Plads, DK-2800 Kgs. Lyngby

Abstract. We have successfully fabricated and characterized a micro-cavity fluidic dye laser with metallic mirrors, which can be integrated with other microfluidic systems without adding further process steps. A laser dye solution is pumped through a microfluidic channel containing the laser-cavity. The microfluidic channel structure, which is formed in SU-8 photoresist, is sandwiched between Pyrex glass wafers, bonded together at lowtemperature by means of SU-8. The laser wascharacterized using Rhodamine 6G laser dye dissolved in ethanol as active medium, and optically pumped by a frequency doubled Nd:YAG laser. The dye solution was optimized, and lasing was observed at a wavelength of 570 nm with a full width half maximum linewidth of 5.7 nm and the optical pump power density threshold for lasing was 34 mW/cm2.

Submitted to: J. Micromech. Microeng.

PACS numbers: 42.55.Mv, 42.60.By

§ E-mail: [email protected] A Micro-Cavity Fluidic Dye Laser 2

1. Introduction

The functionality of ”lab-on-a-chip” systems and other types of microsystems can be enhanced significantly by integration of lasers and other active optical components. The fabrication of lab-on-a-chip systems is in most cases based on silicon- and polymer micro- fabrication. Currently, the most commonly used miniaturized laser sources in the visible to infra-red wavelength range are made from III-V semiconductor crystals. These types of devices cannot be easily integrated with the silicon or polymer-based microsystems. Miniaturized, optically pumped dye lasers may solve this problem of integration. Conjugated organic molecules, so-called dye molecules, are used as active medium, dissolved in a liquid solution or cast into a solid matrix [1]. A variety of laser dyes with optical gain covering the wavelength range from below 300 nm to above 900 nm are commercially available. Solid state dye lasers can be fabricated by doping transparent, solid polymers with dye molecules. Polymethyl methacrylate (PMMA) is often used as host matrix [2]. This approach is very appealing for making integrable micro-lasers, but bleaching of the dye molecules due to large intensities of the optical pump excitation can limit the life-time of the laser. In liquid dye lasers, the problems with bleaching can be avoided by a regenerating continuous flow of dye through the laser cavity. Micro-cavity liquid dye lasers, with dimensions of the laser cavity comparable to the wavelength for spontaneous emission [3, 4, 5], have been fabricated and used to investigate zero-threshold laser action. Klebniczki et al. [3] created a thin sheet of dye solution in the air, and used the dye solution/air interfaces as cavity mirrors. De Martini et al. [4] assembled a microscopic Fabry-Perot cavity, which could be tuned by means of a piezo-electric transducer, and laser dye was pumped through the laser cavity. None of these approaches are compatible with integration on micro-chips. Yokoyama et al. [5] assembled a Fabry-Perot micro-cavity of two glass wafers with dielectric multi-layer mirrors. The cavity length was defined by Ti spacers deposited on one of the glass wafers. A continuous flow of dye was not possible in this design, where the capillary forces were used to pull the laser dye into the micro-cavity. In this paper we present a micro-cavity fluidic dye laser, fabricated by standard micro-fabrication techniques. The design combines the possibility of easy integration with other microsystems and a long life-time due to a liquid dye flow. The liquid laser dye solution is pumped through a microfluidic channel, which contains a laser resonator as shown in figure 1. The microfluidic channel structure is defined in SU-8 photoresist, sandwiched between Pyrex glass wafers and the laser is optically pumped by an external laser. Output from the dye laser is emitted through the semi-transparent top mirror. Lasing at a wavelength of 570 nm is demonstrated, using the laser dye Rhodamine 6G dissolved in ethanol as active medium, optically pumped by a pulsed, frequency doubled Nd:YAG laser. A Micro-Cavity Fluidic Dye Laser 3

Pump Light Dye Laser Light

Dye Inlet Top Mirror Dye Outlet

Glass Z SU-8 SU-8 Y Glass

Bottom Mirror

Figure 1. Cross-sectional outline of the micro-cavity fluidic dye laser. The laser dye is pumped through a microfluidic channel formed in SU-8 photoresist, sandwiched between Pyrex glass wafers. The laser cavity is formed inside the microfluidic channel by metal mirrors, deposited on the top and bottom glass wafers. The micro-cavity fluidic dye laser is pumped optically by an external laser, and output is emitted through the semi-transparent top mirror.

2. Fabrication

The micro-cavity fluidic dye laser structure is formed in SU-8 photoresist [6], sandwiched between Pyrex glass wafers. The laser cavity is formed inside the microfluidic channel by metal mirrors (Cr/Au), deposited on the top and bottom wafers. The fabrication process is shown in figure 2. In order to obtain good quality of the wafer bonding, the Pyrex glass wafers are cleaned with Triton X100 soap and then in a 7Up etch (sulphuric acid and ammoniumperoxodisulphate at 80 ◦C) prior to the fabrication. First the Cr/Au mirrors are deposited on the top and bottom wafers by UV- lithography, electron-beam evaporation and lift-off. A thin layer of Cr is used to improve the adhesion of the Au layer. The top mirror is designed to be semi-transparent, with a transmittance of app. 5 % in the yellow part of the visible spectrum, while the bottom mirror should have maximal reflectance. The thickness of the metallic layers was selected by calculating the reflectance and transmittance from the refractive indices of the metals [7] and the dielectric materials: n = 1.474 at a wavelength of 588 nm for the Pyrex wafers and n = 1.59 at a wavelength of 632.8 nm for the SU-8 photoresist. In the design we neglected the wavelength dependence of the refractive indices for the dielectric materials. Following the procedure of Cory et al. [8] we found a reflectance for the bottom mirror R1 = 83 % and a transmittance T1 = 0 % for a layer structure of 10 nm Cr and 150 nm. For the top mirror we got R2 =72%andT2 = 6 % for a layer structure of 5 nm Cr and 40 nm Au. The microfluidic channel structure is then fabricated on the top wafer. In order to achieve good adhesion of the SU-8 photoresist, the wafers are dehydrated by baking in an oven at 120 ◦C for 12 hours. A 10 µm thick layer of SU-8 photoresist is spun on and pre-exposure baked on a hotplate (65 ◦C for 2 min plus 90 ◦C for 2 min). The channel structure is defined by UV exposure (dose: 200 mJ/cm2), cross-linking by post-exposure baking (65 ◦C for 2 min plus 90 ◦C for 15 min) and development in propylene glycol A Micro-Cavity Fluidic Dye Laser 4

1: Top: 1.5 µm photoresist 5: Top: 10 µm SU-8

2: Top: Expose and develop 6: Top: Expose, bake, develop

6: Bottom: 5µmSU-8 3: Top: Cr/Au5/40nm 7: Bonding, expose, bake

4: Top: Lift off metal 8: Dicing. Drill inlet and outlet

4: Bottom: Cr/Au 10/150 nm

Figure 2. Fabrication process steps. 500 µm thick Pyrex glass wafers are used as top and bottom substrates. 1-4: Metallic mirrors are deposited on the top- and bottom wafers by standard UV-lithography, electron-beam evaporation and lift-off. 5-6: The micro-flowchannels are defined in SU-8 photoresist on the top waferby spin-coating, pre-exposure baking, UV-exposure, post exposure baking and development. A 5 µm thick SU-8 bonding layer is deposited on the bottom wafer by spin-coating and pre- exposure baking. 7: The wafers are bonded by a manually applied pressure at 75 ◦C. The wafers are further sealed by a bake at 90 ◦C for an hour and then cooled down before cross-linking of the SU-8 by an UV flood-exposure, followed by a post exposure bake. 8: The chips are diced and inlet/outlet holes are drilled. monomethyl ether acetate (PGMEA) [6] for 3 min. The wafers are bonded together using SU-8 photoresist as bonding glue. Our bonding method is partly adopted from Jackman et al. [9] and Shen et al. [10]. The bottom wafer is dehydrated, and spin-coated with a 5 µm thick layer of SU-8 photoresist, which is used as bonding glue. The SU-8 is pre-baked on a hotplate (65 ◦C for 2 min plus 90 ◦C for 2 min) in order to remove the solvents. The two wafers are now heated to 75 ◦C and manually pressed together until all Newton rings have disappeared. The bonded wafers are then baked on a hotplate for an hour at 90 ◦C. This step improves the hermetic quality and strength of the wafer bonding at the price of a larger minimal achievable channel width of around 200 µm. The bonding is followed by an UV flood- exposure (dose: 200 mJ/cm2) and a post exposure bake on a hotplate (65 ◦C for 2 min plus 90 ◦C for 15 min) in order to cross-link the SU-8 bonding layer. Finally the bonded chips are diced, and inlet/outlet holes for the laser dye are drilled. In figure 3 we show a photo of a finished device. The microfluidic channel with the laser cavity underneath the top-mirror is 10 µm high and 1 mm wide. A Micro-Cavity Fluidic Dye Laser 5

Inlet Top Mirror Outlet Y

X Fluidic Channel 5mm

Figure 3. Photo of a glass/SU-8 chip with a micro-cavity ﬂuidic dye laser, seen from above.

3. Optical characterization

For the initial characterization of the device, we used the laser dye Rhodamine 6G dissolved in ethanol. This laser dye has an absorption maximum at a wavelength around 530 nm and a fluorescent maximum around 560 nm. For optical pumping we used a frequency doubled Nd:YAG laser, emitting at a wavelength of 532 nm. The laser was pulsed with a pulse length of 5 ns and a repetition rate of 10 Hz. The spot diameter of the pumping laser beam on the micro-cavity fluidic dye laser was approximately 1.5 mm. The dye laser output was collected by an optical fiber and feeded into an optical spectrum analyzer (Avantes AVS-USB2000). An optical edge filter was used to attenuate the scattered light from the pumping laser. An emission spectrum from the micro-cavity fluidic dye laser is shown in figure 4. A dye solution of 10−2 mol/L Rhodamine 6G in ethanol was pumped through the 10 µm deep and 1 mm wide microfluidic channel at a flow rate of 10 µL/min, by means of a syringe pump. The power density of the Nd:YAG pumping laser beam was 368 mW/cm2.

Optical Spectrum with Cavity Modes

3500 650 Lasing Mode 640 Cavity Modes 630 Linear Fit [nm]

3000 N 620 λ 610 x14 600 2500 590 580 Wavelength 570 560 2000 0.028 0.029 0.03 0.031 0.032 2/(N+1)

1500

Output Power [counts] 1000

500

0 540 550 560 570 580 590 600 610 620 630 640 Wavelength [nm]

Figure 4. Emission spectrum from the micro-cavity fluidic dye laser. The Rhodamine 6G laser dye fluoresces in the wavelength interval from 540 nm to 650 nm. The peaks at 582.8, 591.3, 600.5, 610.1, 619.1, 628.5 and 638 nm are identified as longitudinal cavity modes of the Fabry-Perot laser-resonator (see inset), while the peak at 570 nm is a laser mode. Inset: Wavelength of emission peaks λN versus 2/(N + 1), where N =62, 63, .., 68 is the longitudinal mode number. See text. A Micro-Cavity Fluidic Dye Laser 6

A strong peak is observed at a wavelength of 570 nm, and a series of seven smaller peaks are observed on the blown up emission spectrum at wavelengths of 582.8, 591.3, 600.5, 610.1, 619.1, 628.5, and 638 nm. The plot in the inset is used to identify the seven weaker peaks at 582.8 to 638 nm as longitudinal modes of the Fabry-Perot laser- resonator, with mode numbers N =68, 67, .., 62. The peak wavelengths of all eight peaks, λN are plotted versus 2/(N + 1). A straight line, corresponding to the Fabry-

Perot condition, λN =2Lopt/(N + 1), can be ﬁtted to the 7 data points at wavelength 582.8 to 638 nm (circles). The last data-point (triangle), from the strong peak at 570 nm, falls outside this series. From the linear ﬁt, we extract an optical path length,

Lopt =(20.1±0.2) µm, of the laser-resonator. We calculated the expected peak positions from the linear ﬁt, and they are plotted as vertical dashed lines on ﬁgure 4.

500 µm Pyrex glass wafer n=1.474 5nmCr

Top mirror Z 10 µm dye solution n=1.36 40 nm Au Cavity Length 5µmSU-8n=1.59 150 nm Au X Bottom mirror

500 µm Pyrex glass wafer n=1.474 10 nm Cr

Figure 5. Layer structure of the micro-cavity ﬂuidic dye laser.

A cross-sectional outline of the laser-resonator with thicknesses and refractive index, n for the laser dye [1] and the different layers is shown in figure 5. From these numbers, we calculate an optical path-length of 21.6 µm. The discrepancy between this value and the value extracted from the linear fit in figure 4 can be accounted for by an uncertainty of 1 µm in the thickness of the bonding SU-8 layer. Figure 6 shows a series of five emission spectra from the same run of the micro-cavity fluidic dye laser. The power density of the Nd:YAG pumping laser beam is increased from 8 mW/cm2 to 368 mW/cm2. The peak at 532 nm is due to scattered light from the Nd:YAG pumping laser, and the strong peak at 570 nm emerges from the Rhodamine fluorescence as the optical pumping power density is raised. In figure 7 we have plotted the peak value (output power) and full width at half maximum (FWHM) of this peak as a function of optical pumping power density. The FWHM drops by nearly an order of magnitude as the pumping power density is raised from 8 mW/cm2 to 60 mW/cm2. This is accompanied by a knee on the output power density trace around 30 mW/cm2. These two observations, a shift in the output power to pumping power density ratio and a shrinking linewidth (linewidth narrowing), indicate the onset of laser oscillations. The observation from figure 4, that the wavelength, 570 nm, is shifted from a cavity mode is another indication of lasing (frequency pulling). In order to extract the optical pumping power density threshold for lasing, we made linear fits to the two parts of the output power to pumping power density trace above and below the knee, as shown in figure 7. The threshold could then be determined from A Micro-Cavity Fluidic Dye Laser 7

Optical Spectrum 4000 Pump Power Density 8 mW/cm² 3500 48 mW/cm² 151 mW/cm² 3000 269 mW/cm² 368 mW/cm² 2500

2000

1500 Output Power [counts] 1000

500

0 520 530 540 550 560 570 580 590 600 610 620 630 640 Wavelength [nm]

Figure 6. Measured spectra emitted from the micro-cavity fluidic dye laser, using a dye solution of 10−2 mol/L Rhodamine 6G in ethanol and a flowrate of 10 µL/min. The peak at 532 nm is due to scattered light from the frequency doubled Nd:YAG pumping laser. The laser peak emerges at 570 nm as the pumping power density is raised. With the largest pumping power density shown here, 368 mW/cm2 ,thepeak has a FWHM of 5.7nm. the intersection of the two fitted lines. In this way we found a threshold pumping power density of 34 mW/cm2 for the data in figure 6 and 7. Data sets were taken with different concentrations of Rhodamine 6G in ethanol, varying from 10−4 to 10−1 mol/L. We were not able to observe clear signatures of lasing for concentrations below 10−2 mol/L. The inset of figure 7 shows the measured threshold pumping power density versus Rhodamine 6G concentration in ethanol. The device was operated for several hours without showing any signs of degradation of the output signal. The pumping laser pulse length of 5 ns is short enough to avoid problems with triplet excitation of the dye molecules [11], and a flow rate of 10 µL/min through the 10 µm deep and 1 mm wide micro-flow channel provides a sufficient replacement of dye molecules in the laser cavity to avoid problems with heating or bleaching of the dye.

4. Conclusion

A micro-cavity fluidic dye laser has been designed and fabricated using standard micro- fabrication techniques. In our design, a microfluidic channel structure is formed in SU-8 photoresist, and sandwiched between two Pyrex glass wafers. A low temperature SU-8 bonding technique was employed to bond the Pyrex wafers together and seal the microfluidic channels. Lasing from the device was demonstrated using Rhodamine 6G dissolved in ethanol as active medium, and a pulsed, frequency doubled Nd:YAG laser for optical pumping. Laser emission was observed at a wavelength of 570 nm with a full width half maximum A Micro-Cavity Fluidic Dye Laser 8

Output Power and FWHM vs Optical Pump Power Density 50 5000 Threshold Power Density vs Dye Conc. 500 FWHM 45 Power 4500 ] ² 400 Fitted 1

40 300 Fitted 2 4000

35 200 3500

30 Threshold [mW/cm 100 3000

0 0.01 0.02 0.05 0.1 25 Dye Conc. [mol/L] 2500

FWHM [nm] 20 2000

15 1500 Output Power [counts]

10 1000

5 500

0 0 0 50 100 150 200 250 300 350 400 Optical Pump Power Density [mW/cm²]

Figure 7. Output power and FWHM for the emission peak at 570 nm versus optical pumping power density obtained with 10−2 mol/L of Rhodamine 6G in ethanol and a dye flowrate of 10 µL/min. The spot size of the Nd:YAG pumping laser beam on the micro-cavity fluidic dye laser cavity is app. 1.5 mm. The threshold optical pumping power density for lasing is estimated to 34 mW/cm2 by linear fits to the high- and low power parts of the output power data. The measured dependency on dye concentration of the threshold pumping power density for lasing is plotted in the inset. linewidth of 5.7 nm for a dye concentration of 10−2 mol/L. The optical pump power density threshold for lasing was estimated to 34 mW/cm2. Micro-cavity dye lasers have been demonstrated before [4, 5], but to our knowledge, our design is the first one using standard micro-fabrication techniques for fluidic dye lasers. Furthermore, the device can readily be integrated with other microfluidic systems, since the process flow allows for fabrication of microfluidic channels and optical wave-guides in the same process steps, which are used for the micro-cavity fluidic dye laser.

Acknowledgments

This work was supported by the Danish Technical Research Council, STVF (grant number 26-02-0064) and by the H.C. Ørsteds Foundation.

References

[1] Barroso J, Costela A, Garc´ia-Moreno I and Sastre R 1998 Chemical Physics 238 257-272 [2] Li Y, Sasaki M and Hane K 2001 J. Micromech. Microeng. 11 234-238 [3] Klebniczki J, Hebling J, Hopp B, Hajs G and Bor Z 1994 Meas. Sci. Technol. 5 601-603 [4] De Martini F, Innocenti G, Jacobovitz G R and Mataloni P 1987 Phys. Rev. Lett. 59 2955-2958 [5] Yokoyama H, Suzuki M and Nambu Y 1991 Appl. Phys. Lett. 58 2598-2600 [6] XP SU-8 photoresist and PGMEA developer from MicroChem Corp. [7] Palik E D 1998 Handbook of Optical Constants of Solids (NewYork: Academic Press) [8] Cory H, Shiran S and Heilper M 1993 IEEE Transactions on Electromagnetic Compatibility 35 451-456 A Micro-Cavity Fluidic Dye Laser 9

[9] Jackman R J, Floyd T M, Ghodssi R, Schmidt M A and Jensen K F 2001 J. Micromech. Microeng. 11 263-269 [10] Shen S C, Pan C T and Chou H P 2001 Proceedings of SPIE 4407 185-192 [11] Sch¨afer F P 1990 Dye Lasers (Berlin: Springer-Verlag)