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 dif- ferent 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 pho- toresist 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. Resum´e 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........................