Optical Sources

Wei-Chih Wang Department of Power Mechanical Engineering National Tsinghua University

w.wang 1 Week 9 • Course Website: http://courses.washington.edu/me557/sensors • Reading Materials: - Week 9 reading materials are from: http://courses.washington.edu/me557/reading/ • HW #3 is assigned and due Week 13 • HW #2 due next Monday • Final Project proposal: Due Monday Week 13 • Set up a time to meet for final project Week 12 • Oral Presentation on 12/23 in class w.wang 2 Light source

- Broadband light source - Light emitting diode - Laser

w.wang 3 What is Light?

All the fifty years of conscious brooding have brought me no closer to the answer to the question, “what are light quanta?” Of course today every rascal thinks he knows the answer, but he is deluding (fooling) himself. - Albert Einstein, 1951

Early days, a light beam was thought to consist of particles. Later, the phenomena of interference and diffraction were demonstrated which could be explained only by assuming a wave model of light. Much later, it was shown that phenomena such as photoelectric effect and Compton effect could be explained on if we assume a particle model of light.

* Ajoy Ghatak, Optics, Macgraw Hill, 2010

* The photoelectric effect is the observation that many metals emit electrons when light shines upon them. Electrons emitted in this manner can be called photoelectrons. The phenomenon is commonly studied in electronic physics, as well as in fields of chemistry, such as quantum chemistry or electrochemistry (Wikipedia). * *Compton scattering is the inelastic scattering of a photon by a charged particle, usually an electron. It results in a decrease in energy (increase in wavelength) of the photon (which may be an X ray or gamma ray photon), called the Compton effect. (Wikipedia) 4 W. Wang Summarize What is Light

- Behave like mechanical particle (reflection, refraction) - Behave like wave (interfere, diffract, partly electric, partly magnetic) - Energy transfer between light and matter (behave like particle with wavelike behavior) Light as particle • A photon is like a particle, but it has very small mass • Think of a photon as a grain of sand. • We see so many photons at the same time it’s like seeing all the sand on a beach; we don’t notice the single grains • When it hit a reflective surface, it reflects like particle and when it travel through different optical medium, it refract to different angle. W. Wang 6 Light as a wave

• But sometimes light acts like a wave • A wave has a wavelength, a speed and a frequency. • We’ll learn more about wave behavior when we talk about polarization, interference and diffraction. • All light travels same speed (in vacuum, index independent of wavelength) • Wavelength gets shorter as frequency goes up (C=f)

W. Wang 7 Light as a photon

• Photon as particle with zero mass • Particles which have wavelike properties • Consisting of a quantum of electromagnetic radiation where energy is concentrated • The energy goes up as frequency goes up (E = h x f

= h x Co/  • Color depends on frequency in non vacuum

medium (E = h x f = h x Co/ 

W. Wang 8 Wave and Quantum

• When you think of light as a wave of E and B fields, I advice you to not think of them as photons or particles. But just visualize circular waves in water. This is similar except for the fact that it is in three dimensions and that the waves are not through any medium but these 'fields'. So all you need is to disturb a field, and you’ll always get a light wave just like when u disturb water.

2 asin • It’s interesting that field lines are just a mathematical sin ( ) I  I   o asin convenience or abstract model invented by Faraday, and (  )2 are no more real in the physical sense than isobars on weather  maps or contour lines on maps

• On the other hand. When you think of light as a photon, you go quantum. Here everything goes crazy. You no longer use fields. But purely use concept of energy, and discrete orbits (Bohrs model) When electron falls from one discrete orbit to another (and this is not a classical fall, fall, but a quantum fall, where it disappears from one orbit and appears in another). And this leads to generation of photons, or light! w.wang 9 The Bohr Theory of Atomic Structure – Quantum Mechanics

1. The energy of electrons in atoms is QUANTISED,i.e the electron energy cannot vary continuously but is confined to definite fixed energy levels. 2. The electron can only change in energy by moving from one energy level to another. 3. If an electron moves from a lower to a higher level the atom absorbs energy corresponding to the difference in energy between the two levels. 4. If an electron moves from a higher to a lower level the atom emits energy, again corresponding to the difference in energy between the two levels concerned. In the latter Quantum Mechanics has the electron case the energy is emitted as electromagnetic radiation of in a bound state as a probability density function so there is no 'circular frequency given by the equation E = h. orbit'. In these orbits electron do not emit energy in the form of EM radiation w.wang 10 Quantum Theory of Light

Wave-Particle Duality of Light

Quantum theory tells us that both light and matter consists of tiny particles which have wavelike properties associated with them. Light is composed of particles called photons, and matter is composed of particles called electrons, protons, neutrons. It's only when the mass of a particle gets small enough that its wavelike properties show up.

w.wang 11 Particle with Wave-Like Behavior

e - e Photoelectric Effect - At this point you may think that it's pretty obvious that light behaves like a wave. An important feature of this experiment So, why how do we know is that the electron is emitted from the that light is really composed metal with a specific kinetic energy (i.e. a specific speed). (if velocity doesn't of particles called photons? change, kinetic energy doesn’t change) Support for this idea comes from an experiment that is When light intensity increased (brighter light), the kinetic energy of the emitted called the photoelectric electron did not change! The numbers effect. of electrons instead increased! So energy conserved!

w.wang 12 There is a critical frequency for each metal, fo , below which no electrons are emitted. This tells us that the kinetic energy is equal to the frequency of the light times a constant (i.e., the slope of the line). That constant is called Planck's Linear Constant and is given the symbol h= 6.626 x 10-34 J s. equation . 𝑊=hfo

From the experiment, the kinetic energy of emitted electron shows that it varied with the frequency of the light (f )and this changed the kinetic energy of the

emitted electron (K.Emax)

w.wang 13 EM Theory

The EM radiation emitted by the black body is discrete packets of energy known as quanta..

w.wang 14 w.wang 15 w.wang 16 i

i i∝𝐼 𝐾. 𝐸𝑚𝑎𝑥 ℎ𝑓𝑊𝑜

is fixed so more e- more intensity eV Reaching isatuaration

An important feature of this experiment is that the electron is emitted from the metal with a specific kinetic energy (i.e. a specific speed). (if velocity doesn't change, kinetic energy doesn’t change)

When light intensity increased (brighter light), the kinetic energy of the emitted electron did not change! The numbers of electrons instead increased! So energy conserved!

w.wang 17 Atoms and Electrons

• All matter is made up of elements. They are all made of one type of atom, • The three main particles making up an atom are the proton, the neutron and the electron. • Electrons spin around the center, or nucleus, of atoms. The nucleus is made up of neutrons and protons. • Electrons contain a negative charge, nucleus protons a positive charge. Neutrons are neutral HELIUM ATOM Elements are completely pure.

• All Matter is made up of Elements • They are only made of one type of ATOM

w.wang HELIUM ATOM 19 There are many different kinds of atoms, one for each type of element, There are 118 different known elements that make up every thing! w.wang 20 Elements make up all living things. O: Oxygen C: Carbon H: Hydrogen N: Nitrogen

w.wang 21 Atom Smallest particle of an element that still has the characteristics of that element

Planetary Model of Sodium Atom w.wang 22 Actual photograph of atoms of germanium in an ink-blot. w.wang 23 Structure of an atom

Electron cloud or Shell or Level Electron (e-) Surrounded by a cloud of electrons reside at different shell or energy level

• Proton (p+) • Neutron (n)

w.wang 24 Electron energy levels •Are regions where electrons travel around the nucleus . • Each energy level can only hold a certain number of electrons • 1st Shell = 2 electrons • 2nd Shell = 8 electrons • 3rd Shell = 8 electrons

w.wang 25 3 Li

w.wang 26 Groups/Families • The elements in a column are called a group. • Groups are also known as families. • Elements in a group have similar characteristics. • The OUTER SHELL = VALENCE SHELL

w.wang 27 Periods • Each horizontal row is called a period. • A period contains a series of different type of elements from different family. • Row # represent the # of SHELLS (shell #).

w.wang 28 Chemistry 101

When sub-atomic particles were first discovered in 1897, they were thought to orbit the nucleus like planets orbit the sun. Unfortunately, this is the atomic model still taught in many secondary schools. In the mid-20th. century, it was discovered that the structure of the electronic shells was somewhat more complex consisting of: - up to at least seven capital-lettered shells (1=K, 2=L, 3=M, 4=N, 5=O, 6=P, 7=Q) - up to at least 4 small-lettered orbitals (s, p, d, f) - each orbital is further divided into sub-orbitals: s has 1, p has 3, d has 5, f has 7 - each of these sub-orbitals can accommodate two electrons. 29 w.wang w.wang 30 Helium

Helium (He) with its two electrons, has filled its s-orbital and its K-shell. Because its outer shell is filled, Helium (He) does not have any valance electrons. As a result, it tends to be nearly perfectly inert or non-reactive

w.wang 31 What is Light? Matter is composed of atoms, ions or molecules and it is light’s interaction with matter which gives rise to the various phenomena which can help us understand the nature of matter. The atoms, ions or molecules have defined energy levels usually associated with energy levels that electrons in the matter can hold. Light can be generated by the Examples of Light absorption of a matter or a photon of light can photon to raise atom or molecule to interact with the energy levels in higher energy levels and emitting a photon and instead relaxes to the a number of ways. lower energy state w.wang 32 Light Source Electromagnetic waves travel through space at a single constant speed, c=3.00x108 msec-1 sometimes referred to as the 'speed of light' although it is, in fact, the speed of all electromagnetic radiation. The wavelength and frequency of electromagnetic radiation are related to the velocity by the equation: c=

Electromagnetic waves are a form of energy and the energy varies with wavelength and frequency according to the relationship:

E = h = hc /  (h = Planck's constant = 6.63 x 10-34 Js) i.e. energy is directly proportional to frequency (high frequency = high energy and vice-versa) but inversely proportional to wavelength (long wavelength = low energy and vice-versa).

The hydrogen emission spectrum is a line spectrum, i.e. only particular frequencies (wavelengths, energies) are observed.

w.wang 37 The Hydrogen Emission Spectrum

The lines in the hydrogen emission spectrum are found to be grouped into line series: Lyman (Ultra Violet), Balmer (Visible), Brackett, Paschen and Pfund (all in the Infra Red region). w.wang 38 WHAT IS GOING ON HERE?

H2  2 H (energy) Molecule Atoms

H  H* Atom in 'excited state', i.e. with increased Energy (Hydrogen can be excited by heat or electromagnetic energy)

w.wang 39 H*  H + h

The excited atom returns to the 'normal' state (ground state)by releasing the excess energy as electromagnetic radiation of energy (E= h) corresponding to the energy difference between the ground state and the excited state. The fact that the hydrogen emission spectrum is a line spectrum – only radiation of very specific frequencies being emitted – means that only excited states of very specific energies are being formed.

w.wang 40 By inspection an empirical mathematical formula – the Rydberg Equation - was found which predicted the position of all the lines in the spectrum:

When n1 =1andn2 =2,3,4……∞ the positions of the lines in the Lyman (UV) series is predicted. The Balmer (visible) series is predicted by n1 =2,n2 = 3, 4, 5 etc. The Brackett, Paschen and Pfund series (IR) are predicted by n1 =3,n1 =4andn1 = 5 respectively.

w.wang 41 Current • Electrons can be made to move from one atom to another. When those electrons move between the atoms, a current of electricity is created. The electrons move from one atom to another in a "flow." One electron is attached and another electron is lost. It is a situation that's very similar to electricity passing along a wire and a circuit. The charge is passed from atom to atom when electricity is "passed." When electrons move among the atoms of matter, a current of electricity is created. This is what happens in a piece of wire. The electrons are passed from atom to atom, creating an electrical current from one end W. Wang 42 to other, just like in the picture. z x B =  H =  (1+) H = o(M+H) y

(current density, amp/m2)

W. Wang 43 Displacement current is electric field generated by B field that was radiating in air

Induced current is from dispersive medium like dielectric material

D  H  J  t

use 1 c     r o ro D=  E W. Wang 44 Electromagnetic Wave

W. Wang 45 Light Sources

Broadband light sources: incoherent, intensity distribution no uniform across all spectrum (white light source) Hologen:250-1100nm Krpton:350-1700nm Zenon (Xenon) :180- 2200nm Deuterium: 190-500nm Mercury+Argon: 253.65 –1013.98nm Deuterium + halogen:190-900nm LED: range from 400 to 1800nm Tungsten+ Hologen: 350-2000nm

Narrow band light source: Laser- 200 to 1800nm w.wang 46 Noble Gas

Noble Gases • Non-reactive • Full outer shell

w.wang 47 Broadband light source

w.wang 48 Broadband light source

• Incandescent filament Lamps • Gas Discharge light • LED

w.wang 49 Incandescent Filament Lamps In conventional filament lamps or light bulbs, electric current is passed through a coiled tungsten filament, contained in a glass envelope that is filled with an inert gas. When heated by an electrical current, the filament emits electromagnetic radiation. At lower temperatures, radiation is mainly emitted in the infrared part of the spectrum as heat. At Tungsten higher temperatures, the proportion of radiation filament lamp at wavelengths ranging from 380 to 780 nm increases and visible light is produced. In a conventional lamp, the filament temperatures are limited to about 2700 Kelvin. The tungsten filament starts to evaporate and as a result it leads to the blackening of the inside of the lamp envelope. The Great Lightbulb Conspiracy 50 https://spectrum.ieee.org/tech-history/dawn-of-electronics/the-great- w.wang lightbulb-conspiracy Light Bulb Conspiracy (planned Obsolescence) • Intentionally design to last 1000 hours (1924, disbanded 1939) by an organization which name was Phoebus Cartel – memebrships such as Osram, Philips, and General Electric. operated under the premise that their only wish was to standardize the whole light bulb industry • One of the oldest light bulbs that is still in use today is called the “Centennial Light,” and it has been in use for around 108 years. • German Watchmaker Dieter Binninger made light filament light bulb to last 150,000hours (17 years) • Bernard London, who famously proposed ending the Great Depression by mandating planned obsolescence, and Brook Stevens, whose post-war ideas became the gospel of the 1950’s and helped shape the throwaway The Centennial Light, consumer society of today. burning since 1901 • LED bulb designed to last 25 years. Halogen

Halogen lamps are also filament lamps. However, halogen is added to the fill gas to prevent evaporated tungsten from condensing on the inside of the lamp envelope. This feature is used to exploit higher filament temperatures of 3000K and beyond and allows the size of tungsten halogen, quartz- the lamp envelope to be halogen or quartz iodine significantly reduced lamp

w.wang 52 Halogen group

Halogens • Highly Reactive with alkali metals •Missing 1 electron in outer shell

w.wang 53 Halogen Spectrum

W. Wang w.wang 54 Tungsten Halogen Lamp

Fiber light or spectrometer light w.wang 55 Gas discharged lamp

In a gas discharge lamp, once a sufficient voltage is applied, electrons are emitted from a heated electrode, creating a plasma or a gas capable of conducting electricity. In the plasma mobile electrons collide with atoms (predominantly mercury), transfer energy to the atoms and elevate them to an excited state. When these atoms fall back to their original status they emit photons (packages of energy). In many low pressure gas discharge lamps the wavelength of the emitted photon is not in the range of visible light. Mercury, for example, has its major emission in the ultraviolet at 254 nm. w.wang 56 Noble Gas

Noble Gases • Non-reactive • Full outer shell

w.wang 57 high intensity discharge lamp (e.g. metal halide lamp) 6,000 to 15,000 hours High pressure gas discharge lamps emit radiation directly as visible light. In this type of lamp the combination of different element atoms in the hot gas plasma, each emitting at specific wavelengths, determines the color characteristics of the lamp as a whole, as well as the quality of color rendition properties. A metal-halide lamp is an electrical lamp that produces light by an electric arc through a gaseous mixture of vaporized mercury Most gas discharge lamps need at least one free due to excited electrons (mixed with argon electron combined with a high pulse to start the or xenon gas) and metal halides (e.g. lamp operation and to produce light. Usually minute sodium iodide and scandium iodide- compounds of metals with bromine or quantities of materials like tritium or krypton-85 iodine). It is a type of high-intensity are applied either in the lamps themselves or in discharge (HID) gas discharge lamp. starter devices as a source for electrons. High luminous white light -sport stadium, retail sign, head light (xenon) w.wang 58 Sodium Lamp

An amalgam is an alloy of mercury with another metal, which may be a liquid, a soft paste or a solid, depending upon the proportion of mercury

The lamp is operating with liquid amalgam in the tube. Standard: North American / European • Street light Lamp watts: 70-1000w Base type: E27, 40/MOG Bulb shape: TD/ED/TD/E • horticulture LUM. output: 6500-130000lm Color temp.: 2000k (yellow) Rated life: 18000-24000hr w.wang 59 Mercury Lamp

• Gas charging lamp • 10 and 100 times brighter than incandescent lamps (such as the tungsten-halogen) and can provide intense illumination over selected wavelength bands throughout the visible spectral region when combined with the appropriate filters (e.g.. Photolithography)

w.wang 60 low-pressure mercury gas discharge fluorescent lamp

Phosphor coating Ultraviolet photons have the capability to excite fluorescent powders, which are coated on the inside of the tube, with a high degree of efficiency. As a result, these powders emit visible radiation in a range of colors. Lamps based on these principles and operating at low internal gas pressure are called “fluorescent lamps”.

w.wang 61 •Regular •Halogen Incandescent •Nernst •Parabolic aluminized reflector (PAR)

•Fluorescent • Fluorescent lamp (compact) • Fluorescent induction •Photoluminescent • laser lamp Luminescent •Solid-state • LED bulb •Cathodoluminescent • Electron-stimulated •Electroluminescent • field-induced polymer

•Acetylene/Carbide Methods of generation •Argand •Candle •Diya •Flare •Gas •Kerosene Combustion •Lantern •Limelight •Oil •Rushlight •Safety •Tilley •Torch

•Carbon arc Electric arc •Klieg light •Yablochkov candle

•Deuterium arc •Neon •Plasma Gas discharge •Sulfur •Xenon arc •Xenon flash

•Hydrargyrum medium-arc iodide (HMI) •Hydrargyrum quartz iodide (HQI) High-intensity •Mercury-vapor discharge (HID) •Metal-halide • ceramic w.wang •Sodium vapor wikipedia 62 Application

•Floodlight •Footlight •Gobo •Theatrical •Scoop •Cinematic •Spotlight •ellipsoidal reflector •Stage lighting instrument

•Aircraft warning •Balanced-arm lamp •Chandelier •Emergency light •Gas lighting •Gooseneck lamp •Intelligent street lighting •Light tube Stationary •Nightlight •Neon lighting •Pendant light •Recessed light •Sconce •Street light •Torchère •Track lighting •Troffer

•Flashlight •tactical •Glow stick •Headlamp (outdoor) Mobile •Lantern •Laser pointer •Navigation light •Searchlight •Solar lamp

•Germicidal •Grow light •Industrial •Infrared lamp •Scientific •Stroboscope •Tanning

•Aroma lamp •Black light •Bubble light •Christmas lights •Crackle tube •Display •DJ lighting •Decorative •Electroluminescent wire •Lava lamp •Marquee •Plasma globe •Strobe light

•Bioluminescence •Chemiluminescence •Electroluminescence Related topics •Laser w.wang •Photoluminescence 63 •Radioluminescence wikipedia Ideas for final projects- using different Types of lights for unseen application

w.wang 64 Light Emitting Diode

w.wang 65 Light Emitting Diode (LED) The electroluminescent process of LED is to covert input electrical energy into output optical radiation in the visible or infrared (heat) portion of the spectrum, depending on the semiconductor material.

LEDs and laser diodes are very similar devices. In fact, when operating below their threshold current, all laser diodes act as LEDs.

w.wang 66 Light Emitting Diodes

– The Light-Emitting Diode (LED) is a semiconductor pn junction diode that emits visible light or near- infrared radiation when forward biased. – Visible LEDs emit relatively narrow bands of green, yellow, orange, or + - red light (tens of nm). Infrared LEDs emit in one of several bands just beyond red light. w.wang 67 Light Source

Electromagnetic waves travel through space at a single constant speed, c = 3.00 x 108 msec-1 sometimes referred to as the 'speed of light' although it is, in fact, the speed of all electromagnetic radiation. The wavelength and frequency of electromagnetic radiation are related to the velocity by the equation:

c = 

Electromagnetic waves are a form of energy and the energy varies with wavelength and frequency according to the relationship:

The entire range of electromagnetic radiation from the lowest energy (low frequency, long wavelength) to the highest energy (high frequency, short wavelength) is called the electromagneticw.wang spectrum. 68 The energy conversion takes place in two stages: first, the energy of carriers in the semiconductor is raised above their equilibrium value by electrical input energy, and second, most of these carriers, after having lived a mean lifetime in the higher energy state, give up their energy as spontaneous emission of photons with energy nearly equal to the bandgap Eg of the semiconductor:

Ego = h hc where h is plank constant and is frequency of emitting light.

The choice of LED materials requires the wavelength light emission to be within visible light or infrared light region. This means that the bandgap of the semiconductor has to be roughly around 2 eV. The most frequency used binary compounds for LED applications are III-V compounds such as GaAs and GaP. In your case, the red diode could be made of a homojunction GaAsP (650nm) diode and the yellow diode made of a homojunction GaAsP:N (585nm) diode, where N representing doping level.

w.wang 69 The typical spectral output of a LED might looks like:

Spectra of different color LEDs

w.wang 70 Why Semiconducting Materials? Rectification and control electrical current with charges and also longer life time • On/off switching (e.g. passing current more easily in one direction than the other) (diode) • Current or voltage driven amplification (diode) • Energy converter (solar cell, photo detector etc.) • Nonlinear IV characteristic (diode) • Easily doped or change layer structure to change electromagnetic properties (mu, epsilon etc.) to create variable resistance, capacitance and its sensitivity to light or heat (e.g. different wavelengths)

w.wang 71 Linear and Nonlinear electronics

current voltage voltage voltage Vaccum tube Thermistor Diode (i.e. type 2A3) (i.e. PN diode, LED, (large negative laser diode, phtodiode) temperature v  D  nV T coefficient of i D  I S  e  1  resistivity)  

current Normal resistor Ohm’s Law: i=v/R w.wang voltage 72 Introduction to Diodes D1 + ANODE CATHODE - DIODE

• A diode can be considered to be an electrical one-way valve. • They are made from a large variety of materials including silicon, germanium, gallium arsenide, silicon carbide …

W. Wang 73 Introduction to Diodes

• In effect, diodes act like a flapper valve – Note: this is the simplest possible model of a diode

W. Wang 74 Introduction to Diodes

•Fortheflapper valve,asmall positive pressure is required to open. • Likewise, for a diode, a small positive voltage is required to turn it on. This voltage is like the voltage required to power some electrical device. It is used up turning the device on so the voltages at the two ends of the diode will differ. – The voltage required to turn on a diode is typically around 0.6 - 0.8 volt for a standard silicon diode and a few volts for a light emitting diode (LED)

W. Wang 75 Introduction to Diodes

10V

-5V

-10V 0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms V(D1:1) • 10 volt sinusoidal voltage Time source

D1N4002 VAMPL = 10V V1 R1 FREQ = 1k 1k

0 • Connect to a resistive load through a diode

W. Wang 76 Introduction to Diodes D1

V D1N4002 V VAMPL = 10V V1 Only positive R1 FREQ = 1k 1k current flows

0 10V

-5V

-10V 0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms V(D1:1) V(D1:2) Time w.wang 77 What is Semiconductor material?

• The ability to permit the flow of electrons through a substance is its conductivity. The conductivity of conductors is highest, while it is lowest for an insulator, as the electrons flowing through it are negligible. However, as the name suggests, the conductivity of a semiconductor is moderate. • Another interesting thing about a semiconductor is that the current is carried not only by electrons, but by the vacancies they leave behind, which are known as holes. The holes left behind in the valence band can be occupied by electrons from lower states and contribute to the flow of current, thereby leaving a hole in these deeper states as well, which will be occupied by electrons beneath, and so on. • In a typical conductor, like copper, electrons carry the current and act as the charge carrier. In semiconductors, both electrons and holes — the absence of an electron — act as charge carriers. By controlling the doping of the semiconductor, the conductivity and the charge carrier tailored to be either electron or hole based.

w.wang 78 Semiconductor

Variable conductivity A pure semiconductor is a poor electrical conductor as a consequence of having just the right number of electrons to completely fill its valence bonds. Through various techniques (e.g., doping or gating), the semiconductor can be modified to have excess of electrons (becoming an n-type semiconductor) or a deficiency of electrons (becoming a p- type semiconductor). In both cases, the semiconductor becomes much more conductive (the conductivity can be increased by a factor of one million, or even more). Semiconductor devices exploit this effect to shape electrical current. Junctions When doped semiconductors are joined to metals, to different semiconductors, and to the same semiconductor with different doping, the resulting junction often strips the electron excess or deficiency out from the semiconductor near the junction. This depletion region is rectifying (only allowing current to flow in one direction), and used to further shape electrical currents in semiconductor devices. Energetic electrons travel far Electrons can be excited across the energy band gap of a semiconductor by various means. These electrons can carry their excess energy over distance scales of microns before dissipating their energy into heat, significantly longer than is possible in metals. This effect is essential to the operation of bipolar junction transistors. Light energy conversion Electrons in a semiconductor can absorb light, and subsequently retain the energy from the light for a long enough time to be useful for producing electrical work instead of heat. This principle is used in the photovoltaic cell (e.g. solar cell). Conversely, in certain semiconductors, electrically excited electrons can relax by emitting light instead of producing heat. This is used in the light emitting diode. Thermal energy conversion Semiconductors are good materials for thermoelectric coolers and thermoelectric generators, which convert temperature differences into electrical power and vice versa. Peltier coolers use semiconductors for this reason.

w.wang 79 Semiconductor

w.wang 80 Doping and the control of current through a device

To contemplate its utility, one must understand that the current flowing through a semiconductor, unlike a conductor, is not an uncontrollable surge of electrons, but rather a delicate combination of charges and their steady flow. Innovative engineering put forward the idea of contaminating a silicon or germanium atom on purpose to induce new energy levels. The amount of contamination or doping provides the means of controlling current

w.wang 81 Doping

Silicon atoms are arranged in a definite symmetrical pattern making them a crystalline solid structure. A crystal of pure silica (silicon dioxide or glass) is generally said to be an intrinsic crystal (it has no impurities) and therefore has no free electrons. But simply connecting a silicon crystal to a battery supply is not enough to extract an electric current from it. To do that we need to create a “positive” and a “negative” pole within the silicon allowing electrons and therefore electric current to flow out of the silicon. These poles are created by doping the silicon with certain impurities. w.wang 82 P type and N type Materials The semiconductor can be modified to have excess of electrons (becoming an n-type semiconductor) negatively charge ion or a deficiency of electrons (becoming a p-type semiconductor) positively charged ion.

In both cases, the semiconductor becomes much more conductive (the conductivity can be increased by a factor of one million, or even more). Semiconductor devices exploit this effect to shape electrical current. w.wang 83 Donor ( n type semiconductors)

In semiconductor physics, a donor is a dopant atom that, when added to a semiconductor, can form a n-type region. For example, when silicon (Si), having four valence electrons, Extra e- needs to be doped as an n-type semiconductor, elements from group V like phosphorus (P) or arsenic (As) can be used because they have five valence electrons. A dopant with five valence electrons is also called a pentavalent impurity. Other Group pentavalent dopants are antimony (Sb) and bismuth (Bi). V

When substituting a Si atom in the crystal lattice, four of the valence electrons of phosphorus form covalent bonds with the neighbouring Si atoms but the fifth one remains weakly bonded. At room temperature, all the fifth electrons are liberated, can move around the Si crystal and can carry a current and thus act as charge carriers. The initially neutral donor becomes positively charged (ionised).

Diffusion, implant etc. w.wang 84 Acceptor (P type semiconductors)

Extra hole In semiconductor physics, an acceptor is a dopant atom that when added to a semiconductor can form a p‐type region. For example, when silicon (Si), having four valence electrons, needs to be doped as a p‐type semiconductor, elements from group III like boron(B) or aluminium (Al), having three valence electrons, can be used. The latter elements are also called trivalent impurities. Other trivalent dopants include indium (In) and gallium (Ga). Group When substituting a Si atom in the crystal lattice, the three III valence electrons of boron form covalent bonds with three of the Si neighbors but the bond with the fourth neighbor remains unsatisfied. The unsatisfied bond attracts electrons from the neighbouring bonds. At room temperature, an electron from the neighboring bond will jump to repair the unsatisfied bond thus leaving a hole (a place where an electron is deficient). The hole will again attract an electron from the neighbouring bond to repair this unsatisfied bond. This chain‐like process results in the hole moving around the crystal and able to carry a current thus acting as a charge carrier. The initially electroneutral acceptor becomes negatively charged (ionised). w.wang Diffusion, implant etc. 85 Semiconductor

w.wang 86 PN junction In the previous tutorial we saw how to make an N-type semiconductor material by doping a silicon atom with small amounts of Antimony and also how to make a P-type semiconductor material by doping another silicon atom with Boron.

This is all well and good, but these newly doped N-type and P-type semiconductor materials do very little on their own as they are electrically neutral. However, if we join (or fuse) these two semiconductor materials together they behave in a very different way merging together and producing what is generally known as a “PN Junction“.

- When the N-type semiconductor and P-type semiconductor materials are first joined together a very large density gradient exists between both sides of the PN junction. The result is that some of the free electrons from the donor impurity atoms begin to migrate across this newly formed junction to fill up the holes in the P-type material producing negative ions.

w.wang 87 - However, because the electrons have moved across the PN junction from the N-type silicon to the P-type

silicon, they leave behind positively charged donor ions ( ND ) on the negative side and now the holes from the acceptor impurity migrate across the junction in the opposite direction into the region where there are large numbers of free electrons. - As a result, the charge density of the P-type along the junction is filled with negatively charged acceptor ions (

NA ), and the charge density of the N-type along the junction becomes positive. This charge transfer of electrons and holes across the PN junction is known as diffusion. The width of these P and N layers depends on how

heavily each side is doped with acceptor density NA, and donor density ND, respectively. - This process continues back and forth until the number of electrons which have crossed the junction have a large enough electrical charge to repel or prevent any more charge carriers from crossing over the junction. Eventually a state of equilibrium (electricallyneutral situation) will occur producing a “potential barrier” zone around the area of the junction as the donor atoms repel the holes and the acceptor atoms repel the electrons. - Since no free charge carriers can rest in a position where there is a potential barrier, the regions on either sides of the junction now become completely depleted of any more free carriers in comparison to the N and P type materials further away from the junction. This area around the PN Junction is now called the Depletion Layer. Too far away so no combining The total charge on each side of a PN Junction must be equal and opposite to maintain a neutral charge condition around the junction. If the depletion layer region has a distance D, it therefore must therefore

penetrate into the silicon by a distance of Dp for the positive side, and a distance of Dn for the negative side giving a relationship between the two of:

Dp*NA = Dn*ND in order to maintain charge neutrality also called equilibrium.

w.wang 88 Diode Fabrication Process

w.wang 89 By manipulating this non- Pictorial description conductive layer, p–n junctions are commonly used as diodes: circuit elements that allow a flow of electricity in one direction but not in the other (opposite) direction. In a p– n junction, without an external applied voltage, an No equilibrium condition is reached in which a potential difference forms across the charges junction. This potential difference is called built-in in this potential When put two materials together, free region electrons from the N-type material fill holes from the P-type material. This creates an insulating layer in the middle w.wang of the diode called the depletion zone. 90 D1 How Diodes Work ANODE CATHODE DIODE

Electrons can be excited across the energy band gap of a semiconductor by various means. These electrons can carry their excess energy over distance scales of microns before dissipating their energy into heat, significantly longer than is possible in metals. This effect is essential to the operation of bipolar junction transistors.

w.wang narrowed 91 D1 How Diodes Work ANODE CATHODE DIODE

When the positive end of the battery is hooked up to the N-type layer and the negative end is hooked up to the P-type layer, free electrons collect on one end of the diode and holes collect on the other. The depletion zone gets bigger and no current flows. w.wang 92 Introduction to Diodes D1

V D1N4002 V VAMPL = 10V V1 Only positive R1 FREQ = 1k 1k current flows

0 10V

-5V

-10V 0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms V(D1:1) V(D1:2) Time w.wang 93 Forward and Reverse Bias

w.wang 94 Semiconductor

w.wang 95 Energy levels

Helium (He) with its two electrons, has filled its s‐orbital and its K‐shell. Because its outer shell is filled, Helium (He) does not have any valance electrons. As a result, it tends to be nearly perfectly inert or non‐ reactive w.wang 96 Conductor Semiconductor and Insulator

The first crystal in the diagram has an odd number of electrons in its valenceband and no electrons in the next band, which makes for a single, loose electron inits highest energy level. This will readily flow to the conduction band when given a small kick or connected to a battery, providing a huge current. This crystalisa conductor; examples of conductors are metals, such as Copper and Iron.The electrons of the second crystal are not only very stable and tied to each other, but there is also a pair of electrons in its conduction band too, thus making the flow of electrons to the conduction band nearly impossible. This represents an insulator. Paper, rubber and glass are some common insulators. The third crystal has a loose electron, but not an empty conduction band. However, it does contain half-filled energy levels that can accommodate more electrons. This loose electron can be projected to the conduction band whengivena strong enough kick, rendering a small amount of current. This crystal is a semiconductor; major examples are Silicon and Germanium. w.wang 97 Band theory of Solid

A useful way to visualize the difference between conductors, insulators and semiconductors is to plot the available energies for electrons in the materials. Instead of having discrete energies as in the case of free atoms, the available energy states form bands. Crucial to the conduction process is whether or not there are electrons in the conduction band. In insulators the electrons in the valence band are separated by a large gap from the conduction band, in conductors like metals the valence band overlaps the conduction band, and in semiconductors there is a small enough gap between the valence and conduction bands that thermal or other excitations can bridge the gap. With such a small gap, the presence of a small percentage of a doping material can increase conductivity dramatically.

w.wang 98 Fermi Level

"Fermi level" is the term used to describe the top of the collection of electron energy levels at absolute zero temperature. This concept comes from Fermi-Dirac statistics. Electrons are fermions and by the Pauli exclusion principle cannot exist in identical energy states. So at absolute zero they pack into the lowest available energy states and build up a "Fermi sea" of electron energy states. The Fermi level is the surface of that sea at absolute zero where no electrons will have enough energy to rise above the surface. The concept of the Fermi energy is a crucially important concept for the understanding of the electrical and thermal properties of solids. Both ordinary electrical and thermal processes involve energies of a small fraction of an electron volt. But the Fermi An important parameter in the band theory is energies of metals are on the order of electron volts. This the Fermi level, the top of the available implies that the vast majority of the electrons cannot electron energy levels at low temperatures. The position of the Fermi level with the relation to receive energy from those processes because there are no the conduction band is a crucial factor in available energy states for them to go to within a fraction determining electrical properties. of an electron volt of their present energy. Limited to a tiny depth of energy, these interactions are limited to "ripples on the Fermi sea". w.wang 99 Fermi Level

In metals, the Fermi energy gives us information about the velocities of the electrons which participate in ordinary electrical conduction. The amount of energy which can be given to an electron in such conduction processes is on the order of micro-electron volts (see copper wire example), so only those electrons very close to the Fermi energy can participate. The Fermi velocity of these conduction electrons can be calculated from the Fermi energy.

The Fermi energy also plays an important role in understanding the mystery of why electrons do not contribute significantly to the specific heat of solids at ordinary temperatures, while they are dominant contributors to thermal conductivity and electrical conductivity. Since only a tiny fraction of the electrons in a metal are within the thermal energy kT of the Fermi energy, they are "frozen out" of the heat capacity by the Pauli principle. At very low temperatures, the electron specific heat becomes significant.

w.wang 100 • In a p-type material the fermi-level is closer to the valence band than to the conduction band and the opposite is true for n-type materials. When these are drawn in thermal equilibrium the result is an image like the one above. • The image above doesn't really give energy levels in the sense but rather show potential difference. Given the two different materials they accumulate positive and negative charges respectively. As we know, when we interface these two materials carriers drift into the other material in an attempt to achieve charge neutrality, this manifests itself as a diffusion current and a potential difference. • The actual energy levels of the materials is given by the vacuum-function which is essentially the energy-difference relative to an electron at rest unaffected by the material ( e.g an electron in vacuum ). • The semiconductor above is drawn in thermal equilibrium, as a result of this (the fermi level being constant throughout the material) the p-side appears to have bands at higher energy levels

w.wang 101 Spontaneous Emission

h hc

Whole bunch of interbands not shown

w.wang 102 The energy conversion takes place in two stages: first, the energy of carriers in the semiconductor is raised above their equilibrium value by electrical input energy, and second, most of these carriers, after having lived a mean lifetime in the higher energy state, give up their energy as spontaneous emission of photons with energy nearly equal to the bandgap Eg of the semiconductor:

Ego = h hc where h is plank constant and is frequency of emitting light.

w.wang 103 What is an i-v characteristic curve? • Recall that the i-v relationship for a resistor is given by Ohm’s Law: i=v/R • If we plot the voltage across the resistor vs. the current through the resistor, we obtain i The slope of the straight line is given by 1/R v

w.wang 104 i-v characteristic of a real diode The current‐voltage relation of a diode is derived based on the Boltzman’s and Maxwell’s equations. The equation of voltage and current is given:

 v D  nV T i D  I S  e  1    Ideal Diode

Threshold or bias voltage

• vD is the voltage across the diode and iD is the current through the diode. n and Is are constants. VT is a voltage proportional to the temperature, we use 0.0259V. • Note that for vD less than zero, the exponential term vanishes and the current w.wang iD is roughly equal to minus the saturation current. 105 • For vD greater than zero, the current increases exponentially. Real diode characteristics

• A very large current can flow when the diode is forward biased. For power diodes, currents of a few amps can flow with bias voltages of 0.6 to 1.5V. Note that the textbook generally uses 0.6V as the standard value, but 0.7V is more typical for the devices. • Reverse breakdown voltages can be as low as 50V and as large as 1000V.

• Reverse saturation currents Is are typically 1nA or less. w.wang 106 Facts about LEDs ~ cutoff frequency, – LEDs switch off and on rapidly, are very capacitance value rugged and efficient, have a very long lifetime, and are easy to use (~ns to s). – They are current-dependent sources, and their light output intensity is directly proportional to the forward current through the LED. – Always operate an LED within its ratings to prevent irreversible damage.

– Use a series resistor (Rs) to limit the current through the LED to a safe value. VLED is the LED voltage drop. It ranges from about 1.3V to about 3.6V. Intrinsic resistance of VVin LED LED is small (~ 10ohms) R s  ILED –ILED is the specified forward current. (Generally 20mA).

w.wang 107 The current‐voltage relation of a diode is derived based on the Boltzman’s and Maxwell’s equations. The equation of voltage and current is given:

/ 𝐼 𝐼 𝑒 1 where k=Boltzman’s constant= 8.62e‐5 eV/oK, T= temperature (oK) and q = single electron charge =1.6e‐19 coulombs and Is a function of energy gap. The later term means that different emitting light diodes give different Is. As energy gap increases the current Is increases. This mean the yellow light will increase a lot quicker in current than red when same voltage is applied

yellow current redblue

voltage

Vthreshold w.wang 108 Approximate LED threshold voltages

Diode VLED Diode VLED

infra-red 1.2 blue 3.6

red 2.2 purple 3.6

yellow 2.2 ultra-violet 3.7

green 3.5 white 3.6

w.wang 109 Device efficiency

A photodiode's capability to convert electrical energy to photon energy, expressed as a percentage, device efficiency

# of output photons/sec

rp/re = ------# of electrons (holes) excited as Iinj/sec

Depends on , through emission coefficient, thickness of layers, Doping, geometry, etc.

w.wang 110 Radiative Efficiency

Rdiative Efficiency R is defined as the ratio of radiant energy (in watts), P, radiate from the photodiode to the injection currnt in amperes Iink. It is expressed as the absolute responsivity in amps per watt. Please note that radiant energy is usually expressed as watts/cm^2 and that photodiode current as amps/cm^2. The cm^2 term cancels and we are left with watt/amps (W/A). 𝑃 𝑅 (W/A) 𝐼

Since h = energy of photon, P = rp h

where rp = photon flux = P/ h # photons/ sec w.wang 111 Since DE is rp/re and P = rp h

where rp = photon flux = P/ h # photons/ sec Electron rate then

re = rp/ = P/(h)

Therefore, the output photo current Iinj=ere is

Iinj = eP/(h)

P = 𝑜𝑢𝑡𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟 𝜂 𝐼

h= plank constant = 6.63x10-34 joule-sec w.wang 112 The output power vs. forward current for the LED appears as a linear function. The relation is given as

P = 𝑜𝑢𝑡𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟 𝜂 𝐼

Where 𝜂 is overall device efficiency=extraction efficient x the radiative efficiency x injection efficiency, h is plank constant and n is frequency of

emitting light, e = single electron charge and Iinj = Id . All but Iinj = Id are constant. This linear relation should be verified with your experiment

output power P (W)

𝐼 DC drive current (mA) The output power vs. forward current for the LED 𝐼 w.wang 113 Junction capacitance of the LED

The dominant factors for rapid LED switching are not just the LED's inherent emission transition times: • Inductance of the traces causes longer rise and fall times. Longer traces = slower transitions. • Junction capacitance of the LED itself is a factor(#2). For instance, 5mm through-hole LEDs have a junction capacitance of 50 pF nominal. Smaller junctions e.g. 0602 SMD LEDs have correspondingly lower junction capacitance, and are in any case more likely to be used for screen backlights. • Parasitic capacitance (traces and support circuitry) plays an important role in increasing the RC time constant and thus slowing transitions. • Typical LED driving topologies e.g. low-side MOSFET switching, do not actively pull the voltage across the LED down when turning off, hence turn-off times are typically slower than turn-on. • As a result of the inductive and capacitive factors above, the higher the forward voltage of the LED, the longer the rise and fall times, due to the power source having to drive current harder to overcome these factors. Thus IR LEDs, with typically the lowest forward voltages, transition fastest.

w.wang 114 C=A/W

1/2 W = [2rVbi-VA)(NA+ND)/(q NAND)]

w.wang 115 LED There are two basic types of LED structures: edge emitters and surface emitters.

P = P0 cos

simple structure, are relatively inexpensive, offer low‐ output power is high (emitting spot is very small, typically 30‐ to‐moderate output power levels, and are capable of 50 µm) low‐to‐moderate operating speeds narrow emission spectra (FWHM is typically about 7% of the output power is as high or higher than the edge‐ central wavelength) emitting LED, but the emitting area is large, causing poor coupling efficiency to the optical fiber Narrow beam pattern emit light in all directions

W. Wang 116 Using the setup as shown in Figure., where the current is provided by the voltage source V is limited by the series resistance R. Under operating conditions, the voltage drop across the LED is Vd , the operating voltage of the device. If operating current is I=Id , then the circuit can be described by

𝑉𝑉 𝐼𝑅

W. Wang 117 RGB LED

Specifications (most specs in R/G/B format):

Continuous Color (R/G/B) Red Green Blue 400/350/350 Forward Current Forward Voltage 3.4V/2.5V Viewing Angle 120º LED Package 14.5 mm x 7.5 mm Lumens 55/60/20 Maximum LED Maximum 2.8V/3.8V/3.8V Junction 120ºC Forward Voltage Temperature

Operating Millicandela 43000mcd ‐35~+50 Temperature(ºC) Standard Rated 350mA Reverse Voltage 5V Current Storage ‐35~+50 Power Output 3W (~1W per channel) Temperature(ºC) Peak Forward Wavelength 625nm/525nm/465nm 800mA Current

w.wang 118 https://solarbotics.com/product/led-rgb-star43cd/ OLED

• Organic light emitting Diode • a light‐emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound that emits light in response to an electric current. This layer of organic semiconductor is situated between two electrodes; typically, at least one of these electrodes is transparent. • There are two main families of OLED: those based on small molecules and those employing polymers. Adding mobile ions to an OLED creates a light‐emitting electrochemical cell (LEC) which has a slightly different mode of operation. OLED displays can use either passive‐matrix (PMOLED) or active‐matrix (AMOLED) addressing schemes.

w.wang 119 Fiber Optic Sources

Two basic light sources are used for fiber optics: lasers and light‐emitting diodes (LED). Each device has its own advantages and disadvantages as listed in Table Both LED and semiconductor laser are diodes! Characteristic LED Laser Output power Lower Higher Spectral width Wider Narrower Numerical aperture Larger Smaller

Speed Slower Faster Cost Less More Ease of operation Easier More difficult w.wang A. Guenther UCONN 120 Output Power Linearly proportional to drive Proportional to current above the threshold current Drive Current: 50 to 100 mA Current Threshold Current: 5 to 40 mA Peak Coupled Power Moderate High

Speed Slower Faster

Output Pattern Higher Lower

Bandwidth Moderate High Wavelengths 0.66 to 1.65 µm 0.78 to 1.65 µm Available Spectral Width Wider (40-190 nm FWHM) Narrower (0.00001 nm to 10 nm FWHM)

Fiber Type Multimode Only SM, MM Ease of Use Easier Harder Lifetime Longer Long Cost Low ($5-$300) High ($100-$10,000) A. Guenther UCONN w.wang 121 Laser

w.wang 122 LASER

LASAER = light amplification by stimulated emission of radiation

Invented dated to 1958 with the publication of the scientific paper, Infrared and Optical Masers (microwave amplification by stimulated emission of radiation), by Arthur L. Schawlow, Nobel laureate.(https://journals.aps.org/pr/abstract/10.1103/PhysRev.112.1 940).

The maser was the forerunner of the laser, inspiring theoretical work by Charles H. Townes, a consultant to Bell Labs and Nobel laureate in 64, and Arthur Leonard Schawlow that led to the invention of the laser.

Gordonw.wang Gould is credited with creating this acronym in 1957. 123 Property of Laser Light

• Nearly"monochromatic: consists of an extremely narrow range of wavelengths (etrem narrow band)

• Highly Directional: travel in a single direction within a narrow cone of divergence

• Highly Coherence: coherence is the most fundamental property of laser light and distinguishes it from the light from other sources

w.wang 124 Coherence

For longitudinal or temporal coherence, the coherence length C is related to the wavelength  and the total frequency bandwidth of the laser  by

Note:  is the actual bandwidth of the laser beam given in wavelength units.

For transverse or spatial coherence, the transverse coherence length t is related to the laser wavelength , the laser source diameter at its origin s, and the distance r the beam has propagated from its origin, by the following relationship.

w.wang 125 Coherence Two sources of light are said to be coherent if the waves emitted from them have the same frequency and are 'phase-linked'; that is, they have a zero or constant phase difference.

Out of phase of same wavelength or different wavelengths

w.wang 126 Coherence

Temporal incoherent

Spatial coherence describes the correlation (or predictable relationship) between waves at different points in space, either lateral or longitudinal. Temporal coherence describes the correlation between waves observed at different moments in time.

w.wang 127 Temporal Coherence Temporal coherence is a measure of the correlation between the phases of a light wave at different points along the direction of propagation. Temporal coherence tells us how monochromatic a source is.

Assume our source emits waves with wavelength λ ± Δλ. Waves with wavelength λ and λ + Δλ, which at some point in space constructively interfere, will no longer 2 constructively interfere after some optical path length lc = λ /(2πΔλ); lc is called the coherence length.

The phase of a wave propagating into the x-direction is given by φ = kx - ωt. Look at the wave pattern in space at some time t. At some distance l, the phase difference between two waves with wave vectors k1 and k2 which are in phase at x = o 0 becomes Δφ = (k1 -k2)l. When Δφ = 1 (rad), or Δφ ~ 60 , the light is no longer considered coherent which occurs some time later. Interference and diffraction patterns severely loose contrast. We therefore have sinA+sinB = 2sin(A+B)/2 *cos(A-B)/2 1= (k1 -k2)lc = (2π/λ - 2π/(λ + Δλ))lc. 2 Let A = k1x+1tk1 = n1 (λ + Δλ - λ)lc/(λ(λ + Δλ)) ~ Δλlc/λ = 1/2π. B = k2x+2tk2 = n2

2 lc = λ /(2πΔλ) z w.wang 128 lc Coherence Time

• The wave pattern travels through space with speed c.

The coherence time tc is tc = lc/c. Since λf = c, we have Δf/f = Δω/ω = Δλ/λ. We can write 2 • lc = λ /(2πΔλ) = λf/(2πΔf) = c/Δω,

• tc = 1/Δω. (how long before wave becomes incoherent) • If we know the wavelength or frequency spread of a light source,

we can calculate lc and tc. We cannot observe interference patterns produced by division of amplitude, such as thin‐film

interference, if the optical path difference greatly exceeds lc.

w.wang 129 Temporal coherent ~ fix wave front distance (longitudinal) Spatial Coherence

Spatial coherence is a measure of the correlation between the phases of a light wave at different points transverse to the direction of propagation.Spatial coherence tells us how uniform the phase of the wave front is.AdistanceL from a thermal monochromatic (line) source whose linear dimensions are on the

order of δ, two slits separated by a distance greater than dc = 0.16λL/δ will no 2 longer produce a recognizable interference pattern. We call πdc /4 the coherence

area of the source. Lateral in coherent Lateral correction longitudinal

A wave with finite coherence area is incident on a pinhole (small A plane wave A wave with a varying A wave with a varying aperture). The wave will diffract out of the pinhole. Far from the with an infinite profile (wavefront) profile (wavefront) and pinhole the emerging spherical wavefronts are approximately coherence length. and finite coherence infinite coherence length flat. The coherence area is now infinite while the coherence length. length is unchanged.

w.wang 130 Temporal coherent ~ varying coherent length (longitudinal) Recall

wavelength Wave front = wave crest

To describe the light as ray, we need to borrow some of the properties from wave  equation:

Wavefronts: a surface passing trough points of a wave that have the same phase and amplitude

Rays (wave vector): a ray describe the direction of the wave propagation. A ray is a vector perpendicular to the wavefront

W. Wang 131 Spatial Coherence

At time t look at a source of width δ a perpendicular distance L from a screen. Look at two points (P1 and P2) on the screen separated by a distance d. The electric field at P1 and P2 is a superposition of the electric fields of the waves emitted by all source points, whose emissions are not correlated. In order for EM waves leaving P1 and P2 to produce a recognizable interference pattern, the superpositions at P1 and P2 must stay in phase. Light waves emitted from the two edges of the source have a some definite phase difference right in the center between the two points at some time t. A ray traveling from the left edge of δ to point P2 must travel a distance ~d(sinθ)/2 farther then a ray traveling to the center. The path of a ray traveling from the right edge of δ to point P2 travel is ~d(sinθ)/2 shorter then the path to the center. The path difference for the two rays therefore is dsinθ, which introduces a phase difference Δφ' = 2πdsinθ/λ. For the distance from P1 to P2 along the wave front we therefore get a phase difference Δφ = 2Δφ' = 4πdsinθ/λ. Wavelets emitted from the two edges of the source are that are in phase at P1 at time t are are out of phase by 4πdsinθ/λ at P2 at the same time t. We have sinθ ~ δ/(2L), so Δφ = 2πdδ/(Lλ). When Δφ = 1 or Δφ ~ 60o, the light is no longer considered coherent. Δφ = 1 (rad)--> d = Lλ/(2πδ) = 0.16 Lλ/δ.] 132 w.wang Recall Double Slits Interference



hyperphysics

=dsin = m Change to because we2 are talking about coherent length   Nkd    sin  sin   y ~mDd  2  I  I 0     kd   sin  sin   Where N = 2   2  

W. Wang d >> a 133 Solution for spatial incoherent

UTK

We can produce coherent light from an incoherent source if we are willing to throw away a lot of the light. We do this by first spatially filtering the light from the incoherent source to increase the spatial coherence, and then spectrally filtering the light to increase the temporal coherence.

w.wang UTK 134 Why Coherent Length is important?

You will not see above unless you have infinite coherent length. This is what you see with finite coherent length

Additional modulation or interference due to incoherent light behave due to phase difference between phase difference between wavelengths (in this case due to wavelength difference 2nd/ w.wang 135 Example of Coherence length ~ Beats (2 wavelengths) The interferogram shows beats. They are represented in figure by two slightly different wavelengths. Beat or envelop formed by coherent length is like another inference due to phase difference between wavelengths

Coherent length ~ beat length

sinA+sinB = 2sin(A+B)/2 *cos(A-B)/2

Let A = k1x+1tk1 = n1

B = k2x+2tk2 = n2 w.wang 136 lc Coherence and Beats (example) If there are more than two wavelength in case of a broadband light source, then the sum Of all wavelengths in time domain is shown as

frequency time frequency time

lc l Only within coherent length you can see interference! c 2 2 lc = λo /(2πnΔλ) lc = (2ln2/)(λo /(2πnΔλ)) Plane wave input Gaussian input Recall

+ + + 137 W. Wang This is multiwavelength interference!!!! Optical Coherence Tomography (example of low coherent light source)

Only within coherent length l = λ 2/(2πnΔλ) you can see interference! c o

Different coherent lengths with different wavelengths w.wangResolution proportional to coherent length 138 Time Domain Optical Coherence Tomography

The resolution of OCT in the A-line direction is determined by the bandwidth of the laser source and is typically on the order of 10 µm, sufficient for resolving tissue layers and larger morphological structures. Imaging depth in OCT is limited by the optical transparency (absorption and scattering) of the tissue but is typically on the order of 1 to 2 mm depending on operating . w.wang 139 Double interference from double mirrors

Temporal interferometer Recall

+ + + Blue = 2 arm interferences Fix Envelop = brroad band source mirror Look at matlab program

The combination of reflected light from the sample arm and reference light from the reference arm gives rise to an interference pattern, but only if light from both arms have traveled the "same" optical distance ("same" meaning a difference of less than a coherence length). Any light that is outside the short coherence length will not interfere.

look like this cos 𝑥cos ) = output but correct equation should looks like ∆ ) = output ∆ x = Interference between Broadband source Output signal W. Wang moving mirror+ fix mirror Recall diffraction modulated interference

d a

Slit separation d <~ slit width a W. Wang 141 Grating Intensity The intensity is given by the interference intensity expression 2   Nkd    sin  sin    2  I  I 0     kd   sin  sin     2   Modulated by the single slit diffraction envelope for the slits which 2 make up the grating:   ka  sin sin   2  I  I    0 ka  sin   2  The given total intensity expression, 2 2   ka    Nkd   sin sin  sin sin   2   2  I  I0      ka    kd   sin sin sin  W. Wang  2    2   142 • The subsequent slides suppose to help student understand first broad band light source create the low frequency modulation and moving mirror creates the high frequency interference. When each layer of sample is scan by moving mirror, we generate a series of pockets of interference and also refractive index impulse function and from the refraction we can generate 1D image

• Previous slide only show one modulated pocket because only one layer!!!!!!!!!!!!!

w.wang 143 Double interference

Z Recall

+ + +

mirror

R1,R2,R3 R1, R2, R3 The combination of reflected light from the sample arm and reference light from the reference arm gives rise to an interference pattern, but only if light from both arms have traveled the "same" optical distance ("same" meaning a difference of less than a coherence length). Any light that is outside the short coherence length will not interfere. Each sample location will create its own interference within coherent length due to moving mirror ) = output x =

Broadband source Moving mirror + sample Output signal 144 wwang If we assume a wavelength-independent splitting ratio for the coupler or beam-splitter, then the broadband source light output propagating into each arm of the interferometer can be written as:

The reference arm typically has a variable attenuator or neutral density filter to adjust the reference light power level. The attenuated electric field reflected by the reference mirror is given as

The light incident on the tissue will undergo backreflection and backscattering from multiple sites due the presence of scattering particles and refractive index variations within the tissue. The backscattered photons returning from the sample arm can be described as the convolution of the incident field and the backscattering function and can be written as

where rS(z) is the depth-dependent amplitude reflectivity function of the sample. The sample and reference electric fields are recombined at the beamsplitter and are given by

w.wang 145 The incident light is then converted into photo-current by optical detectors, which are square law intensity detection devices. The generated photo-current is proportional to the time average of the incident electric field multiplied by its complex conjugate and is given by:

where ρ is the detector responsivity (Ampere/Watt) and S(k) = |s(k)|2 is the normalized spectral power density of the source. The first two terms on the right hand side of the equation represent the DC component of the current and self interference. The final term in this equation accounts for the interference between the reference and sample electric fields and is used to extract the axial depth profile or structural information in OCT. When simplified, the AC component of the photocurrent can be written as follows:

The goal of various OCT signal processing techniques is to extract the depth-dependent reflectivity function (RS (Δz)) of the sample in order to obtain its axial structural profile information. w.wang 146 Fix sample location and scan mirror

Recall

+ + +

high frequency interference is due to optical path length difference between sample and mirror Envelope is the beat length! Multiwavelength interferrence Moving mirror help generate the output interference but interference only appear at output from different sample location when mirror and sample location traveled the • Each envelope "same" optical distance represents different sample location within coherent length • Mirror helps produce a time varying time sample Lc Lc delay between each wave pocket Lc Depth resolution = coherent length!! Peak Reflected intensity of each enveloped signal represent intensity profile at specific depth based on its refractivew.wang index at that point 147 Fix mirror location Moving mirror help generate the and scan different output interference but interference only appear at output from different sample location sample location when mirror and sample location traveled the "same" optical distance Point on interference curve is combination of many reflections from sample and mirror. Wave packet is also formed by light source multiwavelengths

Lc/2 before blue line reflection but in phase

sample Lc Lc

Lc/2 after bue line reflection but in phase Lc

• Each envelope represents different sample location within coherent length w.wang • Mirror helps produce a time varying time delay between each pocket 148 How Beat amplitude is modulated

n2 n1 sample

R1, R2, R3 𝑅𝑅𝛿𝑧 𝑧 Lc R1, R2, R3 Refractive index n2 is different at different depth Depth resolution ~ coherent length!! Reflection coefficient: Note that for most samples such as biological tissues imaged with OCT, sample reflectivities RS1, RS2 . . . are typically very small (on the order of ∼10−4 to 10−5); thus the returned reference field typically dominates the reflected sample field. Indeed, published studies have shown that selection of the appropriate reference Rsn= (n2-n1/ n2- +n1 ) reflectivity is an important design criterion in OCT system design Peak Reflected intensity of each enveloped signal represent intensity profile at specific depth based on its refractive index at that point

A one-dimensional OCT image (axial line or “A-line”) can be conceptually thought of as illuminating tissue with pulses of light and measuring the time required to detected reflected pulses from sub- surface surfaces within the tissue. The time-of-flight for each detected pulse is used to construct the A-line.

w.wang 149 Using phase matching condition, we get,  k   k R  2 z 1 tz l k   k 1 Rl  Tl 2 z 1 tz 1ktz  22kz 1 Rl  Tl Tl  2kz 2kz  1ktz

Substitute solution for Ei, Er, Et, into wave equation

 k  k R   2 z 1 tz We get, 1 R  T ll ll ll 2kz  1ktz 1ktz 2 k 1 Rll  Tll T   2 z 2kz ll 2kz  1ktz

w wang 150 R1,R2,R3 R , R , R 1 2 3 Beat or envelop formed by coherent length is like Envelope is the beat length! another inference due to phase difference between 2π𝑛Δ𝑑 wavelengths 𝐼𝐼 𝑅𝑅𝛿𝑧 𝑧 cos ) 2 𝜆 ∆𝜆 lc = (2ln2/)(λo /(2πnΔλ)) ∆

x = moving mirror + sample interference!

sample Depth resolution = coherent length!! Peak Reflected intensity of each enveloped signal represent intensity profile at specific depth based on its refractivew.wang index at that point 151 R1,R2,R3

R1, R2, R3 Beat or envelop formed by coherent length is like Envelope is the beat length! another inference due to phase difference between wavelengths 2π𝑛Δ𝑑 𝐼𝐼 𝑅𝑅𝛿𝑧 𝑧 cos ) 2 𝜆 ∆𝜆 lc = (2ln2/)(λo /(2πnΔλ)) ∆

x = Moving mirror +sample interference

Depth resolution = coherent length!! Peak Reflected intensity of each enveloped signal represent intensity profile at specific depth based on

w.wang its refractive index at that point 152 Construction of image

w.wang 153 Requirements for a laser

There are three types of processes involving the interaction of light beams with atoms that have electrons residing in various energy levels:

SPONTANEOUS EMISSION

ABSORPTION

STIMULATED EMISSION

A. Guenther UCONN w.wang 154 SPONTANEOUS EMISSION

An atom in an excited state is unstable and will release spontaneously its excess energy and return to the ground state. This energy release may occur in a single transition or in a series of transitions that involve intermediate energy levels. For example, an atom in state E3 could reach the ground state by means of a single transition from E3 to El, or by two transitions, first from E3 to E2 and then from E2 to E1. In any downward atomic transition, an amount of energy equal to the difference in energy content of the two levels must be released by the atom.

A. Guenther UCONN w.wang 155 Most excited energy levels undergo spontaneous emission. Each level has a specific lifetime  over which it will remain in that level before decaying to a lower-lying level. That lifetime is determined by the interactions of the electron with the other electrons and nuclei of that atom. Typical lifetimes of electrons residing in specific levels that decay by radiating in the visible portion of the spectrum are of the order of 10–100 nsec. The photon radiated during spontaneous emission has the exact wavelength 21 and frequency 21 corresponding to the difference in energy E21 of the two involved energy levels (1 and 2 in this case) according to the relationship

E21 = h21 = in which h is Planck’s constant such that h = 6.63 × 10–34 joule-sec and c is the speed of light, c = 3 × 108 m/sec. Because different materials have different energy-level arrangements, they radiate at different wavelengths and thus emit different colors or frequencies of light that are specific to the material.

w.wang 156 ABSORPTION OF LIGHT

The second process is absorption, shown in Figure b, which occurs if the atom has its electron in level 1 and a photon of light of E21 wavelength 21 collides with the atom. During the collision, the photon is absorbed by the atom and the electron is moved up to the higher energy level 2. This process is the way light interacts with practically all of matter. It can E12 happen from any energy level that is occupied (generally the ground state) and always boosts the atom E21 to a higher-lying level while eliminating the photon. This often E results in heating of the absorbing 21 material E21 A. GuentherA. Guenther UCONN UCONN w.wang 157 ABSORPTION

When a large group of atoms is assembled and irradiated with light, most of those atoms are in the ground-state energy level. If the photons of the impinging light have the appropriate energy

E20 for example, the light will be absorbed according to the following expression for the variation of intensity I with the distance L into the material (known as (Lambert Law):

Where I0 = intensity of the beam when it first reaches the atoms

20 = cross section for absorption or emission of those two levels (cm2),

N0 = population density of atoms residing in 3 level 0 (atoms/cm ), A. Guenther UCONN

20 N0 = absorption coefficient Intensity variation versus depth z into an absorbing sample w.wang 158 Stimulated Emission The third process, shown in Figure is referred to as stimulated emission. It results when an electron is in a higher- lying level, such as level 2, and a E21 photon of light of wavelength 21 collides with the atom. During the collision the photon stimulates the atom to radiate a second photon having exactly the same energy E21 as that of the incident photon and traveling in exactly the same direction in order to E satisfy the laws of conservation of 12 energy and momentum. Hence, one photon leads to two identical photons, E which, in effect, leads to an 21 amplification process. A photon has been gained at the expense of the loss of E21 energy stored within the atom. E21 A. Guenther UCONN

w.wang 159 Stimulated Emission

Suppose that we were able to “pump” (excite) a significant amount of population of the medium from level 0 to level 2. Also, for the time being let us assume that there is no population in level 1. (This is an unlikely scenario but we will do this as a “thought” experiment for illustrative purposes.) Then again, let us consider having a beam of photons of energy E21 and wavelength 21 enter the medium. According to the earlier discussion, and considering the process that can occur is stimulated emission, and we would expect more photons to be generated as the beam progresses. This can be described mathematically in the equation below

in which we now have the population density N2 in the expression along with the appropriate cross section of emission 21. In case the atom, still in an excited state, is struck by an outside photon having precisely the energy necessary for spontaneous emission, the outside photon is increased by the one given up by the excited atom, Moreover, both the photons are released from the same excited state in the same phase, This process, called stimulated emission, is fundamental for laser action (shown in 160 w.wang above figure). In this process, the key is the photon having exactly the same wavelength as that of the light to be emitted. POPULATION INVERSION

A population inversion exists whenever more atoms are in an excited atomic state than in some lower energy state. The lower state may be the ground state, but in most cases it is an excited state of lower energy. Lasers can produce coherent light by stimulated emission only if a population inversion is present. And a population inversion can be Rami Arieli: "The Laser Adventure" Chapter 2.6 page achieved only through external excitation of the atomic population. w.wang 161 POPULATION INVERSION

Now, if population is allowed to be in both level 1 and level 2, both absorption and stimulated emission will occur within the medium and therefore

Hence, if more population exists in level 2

than in level 1, N2 will be greater than N1 and the exponent of above equation will be positive. The beam will grow and emerge from the medium with a greater intensity than when it entered. In other words, for amplification or gain to occur, the condition must be

Having N2 be larger than N1 is known as having a population inversion, which is not a A. Guenther UCONN normal, naturally occurring relationship. w.wang 162 Small‐signal‐gain coefficient

It is useful to describe the product of 21 and N21 as the small-signal- g =  N gain coefficient g21 or 21 21 21

2 3 By considering the units of both 21 (length ) and N21 (l/length ) we can see that g21 has the units of 1/length. Hence, if 21 is given in 2 3 units of cm and N21 is given in units of (1/cm ), g21 will be given in (1/cm), more commonly expressed as cm–1. Values of the cross

sections 21 and N21, and the small-signal gain g21

w.wang 163 Examples of Current Laser system

2 –3 –1 2 Type of Laser 21(nm) 21(Hz) 21(cm ) N21(cm ) g21(cm ) Isat(W/cm ) HeNe 632.8 2 × 109 3 × 10–13 7 × 109 2 × 10–3 6.2

Argon 488.0 2 × 109 2.5 × 10–12 1 × 1015 5 × 10–3 16.3

HeCd 441.6 2 × 109 9 × 10–14 4 × 1012 3 × 10–3 7.1

Copper Vapor 510.5 2 × 109 8 × 10–14 6 × 1013 5 × 10–2 9.0

7 –18 15 –3 –2 CO2 10,600 6 × 10 3 × 10 5 × 10 8 × 10 1.6 × 10 Excimer 248.0 1 × 1013 2.6 × 10–16 1 × 1016 2.6 × 10–2 3.4 × 105

Dye (Rh6-G) 577 5 × 1013 2 × 10–16 2 × 1018 2.4 3.4 × 109

Ruby 694.3 3 × 1011 2.5 × 10–20 4 × 1019 1.0 3.8 × 107

Nd:YAG 1064.1 1.2 × 1011 6.5 × 10–19 3 × 1019 2.0 1.2 × 107

14 –19 18 9 Ti:Al2O3 760 1.5 × 10 3.4 × 10 3 × 10 1.0 2.0 × 10 Semiconductor 800 1 × 1014 1 × 10–15 1 × 1018 103 2.5 × 109 wavelength 21 and frequency 21 ,the small-signal gain g21 cross section for absorption or emission of, 20 population density of atoms N21 residing in two involved energy levels (1 and 2 in this case) A. Guenther UCONN w.wang 164 Laser gain medium Atoms such as in the red helium-neon (HeNe) laser, the visible and ultraviolet argon ion and helium-cadmium (HeCd) lasers, and the green and yellow copper vapor lasers (CVL)

Molecules such as in the infrared carbon dioxide (CO2) laser, the ultraviolet excimer lasers such as ArF and KrF, and the pulsed N2 laser

Liquids such as those involving various organic dye molecules dilute dissolved in various solvent solutions

Dielectric solids such as those involving neodymium atoms doped in YAG or glass to make the crystalline Nd:YAG or Nd:glass lasers

Semiconductor materials such as gallium arsenide or indium phosphide crystals or various mixtures of impurities blended with those and other semiconductor species

w.wang 165 Population Inversion in lasing mediums excitation is a two-step process

- Gas (inert gas) A. Guenther UCONN - Liquid (pump source) - Solid state (pump source) w.wang - Semiconductor (PN junction) 166 Gas Laser Within gas lasers, they are divided into 3 different types:

ATOM MOLECULE ION •He-Ne (Helium- •CO (Carbon Neon) 2 •Ar+ (Argon ion) Dioxide) •Kr+ (Krypton ion) Metal Vapor Lasers •N2 (Nitrogen) •Cu (Copper) Vapor •Chemical (HF-DF) •Au (Gold) Vapor •FIR - Far Infrared •Excimer Laser Ionized vapor Lasers *He-Cd (Helium- Cadmium)

w.wang 167 Population Inversion in gas laser

- Applied voltage produces an electric field accelerates the electrons within the gas.

- Excited electrons collide with the gas atoms and excite the atoms to excited energy levels, some of which serve as upper laser levels.

- Lower-lying levels, those to which higher-lying levels can transition, typically decay to the ground state faster than the higher-lying levels, thereby establishing a population inversion between some of the higher and lower levels.

- The laser light then occurs when the higher-lying levels decay to the lower levels while radiating photons at the wavelengths corresponding to the energy separation between the levels.

- In many instances the excitation is a two-step process in which the electrons * first excite a long-lived or metastable (storage) level or they ionize the atom, leaving an ion of that species and another electron. In either case, that level * then transfers its stored energy to the upper laser level via a subsequent collision with the laser species.

w.wang 168 Gas laser

A. Guenther UCONN

One popular type of gas laser contains a mixture of helium (He) and neon (Ne) gases and is illustrated in Figure. The gas mixture is contained at a low pressure within a sealed glass tube called the "plasma tube. The feedback mechanism consists of a pair of mirrors sealed to the ends of the plasma tube. One of these mirrors, the output coupler, transmits 1-2 percent of the light to form a continuous (CW) output beam. w.wang 169 Why the laser light is monochromatic?

• Laser light consists of essentially one wavelength, having its origin in stimulated emission from one set of atomic energy levels. This is possible because laser transition, in principle, involves well‐defined energy levels. • EM wave of frequency n = (E2 ‐ E1) only can be amplified, n has a certain range which is called line width. This line width is decided by various broadening factors such as Doppler effect of moving atoms and molecules. • The generation of laser is such that the laser cavity forms a resonant system and laser oscillation is sustained only at the resonant frequencies of the cavity. This leads to the further narrowing of the laser line width. So laser light is usually very pure in wavelength, we say it has the property of monochromatic

w.wang 170 Laser Cavity

w.wang 171 How spectral filter created by mirror cavity

w.wang 172 Fabry‐Perot Interferometer

Interference of an infinite number of waves progressively smaller amplitude and equal phase difference.

w.wang 173 . . . .

r1, t1 r2, t2

w.wang 174 ER

w.wang 175 Trigonometric Functions in Exponential Function vs. Terms of Exponential Functions Trigonometric and Hyperbolic Functions

Hyperbolic Functions in Terms of Exponential Functions

w wang 176  Fabry Perot Interferometer

T1T2 It ( )  1 R1xR2  2x R1xR2 x cos() 2 2  y  n   cos( )  where cos(normal incident; y = distance separation of two mirrors n = index of refraction of the air gap; = wavelength of the incoming He-Ne laser = 632.8 nm;

st T1 = intensity transmission coefficient of 1 mmirror; nd w.wang T2 = intensity transmission coefficient of 2 mirror; 177 Refection Intensity

(R1  R2  2x R1xR2 cos()) Ir ()  1 R1xR2  2x R1xR2 xcos() 2 2  y  n   cos( )  where cos(normal incident; y = distance separation of two mirrors; n = index of refraction of the air gap; = wavelength of the incoming He-Ne laser = 632.8 nm;

st R1 = intensity reflection coefficient of 1 mirror; nd w.wang R2 = intensity reflection coefficient of 2 mirror; 178 Finesse 

2 f   2

4 R1  R2 f  2 (1 R1  R2 ) 2 f  Where = half power bandwidth  This parameter is defined as the ratio of the half power bandwidth over the peak to peak full bandwidth. It’s a way to measure the sharpness of the curve. w.wang 179 Transmission Spectrum

The frequency of each line is given by

f = m Co/(2nycos) where m = +1, +2, +3,… (1)

The lines are separated in frequencies by

f = Co/(2nycos) (2)

The mode number of the etalon is m = f/f (3)

The spacing between etalon modes is

2  = f  /Co (4) w.wang 180 2m ycosnycosm = Co/f

=> f = mCo/nycoswhere m = +1, +2, +3,… (1)

f= fm+1 –fm = Co/nycos

=> mff (3)

 2 f Cdf/ dC df Co/ d

2 2  ddf  /Co  f  /Co 

w.wang 181 Bandwidth of laser gain medium

The bandwidth of the laser gain medium determines the range of wavelengths over which amplification can occur for any specific laser. This bandwidth is expressed in either a wavelength range

G or a frequency range fG. These two expressions are related by

fG in which  is the laser wavelength and c is Laser gain bandwidths for the HeNe, the speed of light. The bandwidth of the Nd:YAG, and Ti:Al2O3 lasers gain medium is usually determined by the bandwidth over which the spontaneous A. Guenther UCONN emission occurs for a given laser Full width at half maximum (FWHM) transition. w.wang 182 Gain bandwidth

Full width at half maximum (FWHM‐ bandwidth) of the Doppler broadened gain curve

Doppler-broadened gain curve supporting 6 longitudinal modes or cavity resonances. The gain curve has a peak at 632.8 nm. The width of the gain curve is indicated as the Doppler FWHM. The mode spacing is given by f = Co/(2nLcos) . The linewidth of a single laser mode is shown as the cavity FWHM w.wang 183 Free Spectral Range and Instrument bandwidth What is the width of the spectrum ?, f? fc/  df(c / 2)d  f (c /  f coherence‐ length

w.wang 184 Laser cavity modes

In practice, L is usually much greater than λ, so the relevant values of q are large (around 105 to 106). Of more interest is the frequency separation between any two adjacent modes q and q+1; this is given (for an empty linear resonator of length L) by Δνor (f): f = c/ 2L where c is the speed of light (≈3×108 m∙s−1). Using the above equation, a small laser with a mirror separation of 30 cm has a frequency separation between longitudinal modes of 0.5 GHz. Thus for the two lasers referenced above, with a 30‐cm cavity, the 1.5 GHz bandwidth of the HeNe laser would support up to 3 longitudinal modes, whereas the 128 THz bandwidth of the Ti:sapphire laser could support approximately 250,000 modes. When more than one longitudinal mode is excited, the laser is said to be in "multi‐mode" operation. When only one longitudinal mode is excited, the laser is said to be in "single‐mode" operation. Each individual longitudinal mode has some bandwidth or narrow range of frequencies over which it operates, but typically this bandwidth, determined by the Q factor (see Inductor) of the cavity (see Fabry–Pérot interferometer), is much smaller than the intermode frequency separation

w.wang 185 Different Cavity Length

Free spectral range (FSR) is the spacing in optical frequency or wavelength between two successive reflected or transmitted optical intensity maxima or minima of an interferometer

A function of the cavity length w.wang 2 186 f = Co/(2nycos)  = f  /Co Coherence

For longitudinal or temporal coherence, the coherence length C is related to the wavelength  and the total frequency bandwidth of the laser  by

Note:  is the actual bandwidth of the laser beam given in wavelength units.

w.wang 187 Recall

UTK

w.wang UTK 188 Laser Cavity

w.wang 189 How Laser works

Mirror cavity

Gain medium = (e.g.Arc lamp) Laser output w.wang 190 Laser structure

w.wang A. Guenther UCONN 191 w.wang A. Guenther UCONN 192 Most Common Laser Cavity

At both ends there are two plan mirrors (R1=∞, R2= ∞ ), parallel to each other, and perpendicular to the laser optical axis. Advantages: •Optimal use of all the volume of the active medium. Thus, used in pulsed lasers which need the maximum energy. •No focusing of the laser radiation inside the optical cavity. In high power lasers such focusing can cause electric breakdown, or damage to the optical elements. Disadvantages: •High diffraction losses. •Very high sensitivity to misalignment. Thus, very difficult to operate

Two spherical mirrors with the same radiuses.The distance between the center of the mirrors is equal to twice the radius of curvature of each of them (R1= R2= L/2).focusing of the beam at the center of the cavity. Advantages: •Very low sensitivity to misalignment. •Low diffraction losses. Disadvantages: •Limited use of the volume of the active medium. Used in optical pumping of continuous Dye lasers •In these lasers the liquid dye is flowing in the region of the beam focusing (The flow direction is perpendicular to the optical axis of the laser). Thus very high power density is used to pump the dye. •Maximum focusing of the radiation inside the optical cavity.

At both ends there are two spherical mirrors with the same radiuses.(R1= R2= L).This arrangement cause much less focusing of the beam at the center of the cavity.Advantages: •Little sensitivity to misalignment. •Low diffraction losses. •No high focusing inside the cavity. •Medium use of the volume of the active medium. The main difference between the Confocal cavity and the spherical cavity is that in the Confocal cavity the focal point of each mirroris at the center of the cavity, while in spherical cavity the center of curvature of the mirrorsis in the center of the cavity. w.wang 193 This cavity is a better compromise than Confocal cavity between plan parallel and circular optical cavities. At both ends there are two spherical mirrors with big radiuses of curvature(does not need to be the same). The distance between the center of the mirrors is much less then the radius of curvature of each of them (R1, R2>> L).This arrangement cause much less focusing of the beam at the center of the cavity. Advantages: •Medium sensitivity to misalignment. •Medium diffraction losses. •No high focusing of the beam inside the cavity. •Good use of the volume of the active medium

The cavity is created by one plan mirror, and one spherical mirror with radius of curvature equal to the length of the cavity.This cavity is similar in properties to circular optical cavity, with the advantage of the low price of the plan mirror.Most Helium-Neon lasers use this cavity which have low diffraction losses, and is relatively easy to align. Advantages: •Low sensitivity to misalignment. •Low diffraction losses. an example of Unstable Cavity.The concave mirroris big and its radius of curvature is longer than the length of the cavity. The convex mirroris small and its radius of curvature is small. In such cavity no standing wave pattern is created inside the cavity.The radiation does not move in the same path between the mirrors. The radius of curvature of both mirrors meet at the same point. Advantages: •High volume of the modes inside the active medium (The entire volume). •All the power inside the cavity is emitted out of the laser, not just a small fraction of it. The laser radiation is emitted out of the laser around the edges of the small mirror. This cavity is used in high power lasers, which can not use standard output coupler.Disadvantages: •The beam shape has a hole in the middle

w.wang 194 An example of the pattern of circular fringes formed in the central plane of a a) Schematic diagram of a spherical Fabry-Perot Fabry-Perot interferometer. The pattern resonator. Each of the mirrors has a radius of can be observed when the Fabry- Perot curvature r, and they are separated by r+ . is set in the “fringe display” mode of b) A paraxial ray entering the resonator at point P1 operation is reflected four times off the surfaces of the mirrors and it falls upon itself after this (i.e., is reentrant). c) Due to aberration, a general ray is not reentrant, but follows a path such as the one shown in Figure c). The rays will continue to intersect themselves in the vicinity of the central plane of the spectrometer, w.wang 195 where a circular fringe pattern is produced. Calculating gain including the reflection gain from cavity

BC Choudhary

w.wang 196 BC Choudhary

w.wang 197 BC Choudhary w.wang 198 By BC Choudhary

w.wang 199 Gain coefficient

For a laser in which the amplifier length has a value of L and the mirrors have identical reflectivities R, with no other losses in the cavity, the threshold condition for the gain coefficient g is given as

which has dimensions of 1/length. Any value of g higher than above equation will produce a laser beam within the cavity. For a more complex laser cavity in which the mirrors have

different reflectivities R1 and R2, and a1 and a2 represent other losses within the cavity (beyond the amplifier), the expression for the threshold gain g is given as

The term  represents a potential absorption loss within the amplifier itself, which is present in only a few types of lasers. It is a distributed loss expressed in the same units as g or (1/length). For example, in solid-state lasers it is termed excited state absorption.

w.wang 200 Gas Laser Within gas lasers, they are divided into 3 different types:

w.wang 201 Atom Lasers

w.wang 202 HeNe laser A helium–neon laser or HeNe laser, is a type of gas laser whose gain medium consists of a mixture of helium and neon(10:1) inside of a small bore capillary tube, usually excited by a DC electrical discharge. The best-known and most widely used HeNe laser operates at a wavelength of 632.8 nm, in the red part of the visible spectrum. The current pumps the helium atoms to an excited atomic state. The Discharge electrons flowing energy of the excited helium atoms from the cathode is transferred to neon atoms through toward the anode. collisions, and the neon atoms then undergo a transition to a lower energy state that results in lasing.

w.wang A. Guenther UCONN 203 HeNe laser It was first built in 1961 by Ali Javan. The active medium is a noble gas Neon (Ne), and it is a 4 level laser. Two meta-stable energy levels act as upper laser levels. The He-Ne laser have two lower laser levels, so quite a few wavelengths can come out of the transitions between these levels. The important wavelengths are:

1=0.6328 [m] (632.8 [nm]), 2=1.152 [m], 3=3.3913 [m], 2=0.5435 [m]

w.wang Rami Arieli: "The Laser Adventure" Chapter 6, Helium-Neon Lasers, page204 Metal Vapor Lasers

w.wang 205 Helium‐Cadmium Laser

Helium-Cadmium lasers can be categorized among either: •Metal vapor lasers - Cadmium is a metal, the lasing action in Helium Cadmium laser occurs between energy levels of Cadmium ions, so the lasing medium is ionized metal vapor. •Ion gas lasers - The properties of Helium-Cadmium laser are similar to those of Helium-Neon laser which is a neutral atom gas laser. The He-Cd laser is a gas laser, and the metal Cadmium can be transform into the gas phase by heat. The excitation to the upper laser level of the Cadmium atoms in the gas is similar to the excitation process of the Neon gas in a Helium-Neon laser: Helium atoms are excited by collisions with accelerated electrons, and than they pass their energies to Cadmium atoms by collisions. Thus, the main application of the He-Cd laser is in the optics laboratory, for fabricating holographic gratings. (UV, 351nm)

Rami Arieli: "The Laser Adventure" Chapter 6, Helium-Cadmium Lasers, page 1 w.wang 206 Copper vapor laser (CVL) This laser was attractive because of its relative high efficiency (up to 1%) for lasers in the visible spectrum range, and the high pulse power achieved.

Copper Vapor Laser Structure

Copper vapor laser is a gas laser, build as a tube with windows at both ends. The tube is filled with an inert gas and a small quantity of pure copper.

In order to have copper vapor, the metal needs to be at very high temperatures, so the tube is build from Alumina or Zirkonia, which are high temperature resistant materials. The tube diameter is 10-80 [mm], and it contain Neon gas at a pressure of 25-50 [Torr].

w.wang Rami Arieli: "The Laser Adventure" Chapter 6, Metal Vapor Lasers, page207 2 Rrecallhigh intensity discharge lamp (e.g. metal halide lamp)

High pressure gas discharge lamps emit radiation directly as visible light. In this type of lamp the combination of different element atoms in the hot gas plasma, each emitting at specific wavelengths, determines the color characteristics of the lamp as a whole, as well as the quality of color rendition properties.

Most gas discharge lamps need at least one free electron combined with a high pulse to start the lamp operation and to produce light. Usually minute quantities of materials like tritium or krypton-85 are applied either in the lamps themselves or in starter devices as a source for electrons.

w.wang 208 Ion Laser

w.wang 209 Gas Laser Within gas lasers, they are divided into 3 different types:

w.wang 210 Argon Ion Laser The Argon laser was invented in 1964 by William Bridges at Hughes. Argon ion laser contains a tube filled with Argon gas which transforms into plasma in an excited state. (Plasma is a state of matter in which the electrons are separated from the atoms and molecules, which means that it contains free electrons and ions).

The two main laser transitions are at visible wavelengths: Blue 0.488 [m] green 0.5145 [m], but the Argon ion laser emits also in the UV spectrum: Energy Level Diagram of Ion Argon 0.3511 [m] Laser. w.wang 0.3638 [m]. Rami Arieli: "The Laser Adventure" Chapter 6, Ion Gas Lasers,211 page 1 Krypton Laser The Krypton laser is very similar to the Argon laser, but its efficiency is lower.

This laser has many lines in the visible spectrum, especially in the yellow to red part of the spectrum.

The maximum output power in each line is about 100 [mW].

The main applications of this laser are in the art and entertainment business, to create fantastic visual effects.

w.wang 212 Molecular laser

w.wang 213 Gas Laser Within gas lasers, they are divided into 3 different types:

w.wang 214 Nitrogen Laser The Nitrogen laser was first developed in 1963 and has been sold as a commercial product since 1972. Laser Action: The active medium in Nitrogen lasers is Nitrogen gas at pressures of 20 [torr] up to 1 [At]. In some Nitrogen lasers the gas flows in the tube, while others have a sealed tube. Like most gas lasers, the Nitrogen laser is based on transitions between vibration energy levels, and is electrically excited.

w.wang 215 CO2 laser

Lasing action in a CO2 molecule was first demonstrated by C. Patel in 1964. He transmitted an electric discharge pulse through pure CO2 gas in a laser tube, and got a small laser output.

CO2 is the gas in which the lasing process occurs, but other gas additives to the laser tube improve the total efficiency of the laser.

The standard CO2 laser includes in the active medium a mixture of CO2 with N2 and He. The optimal proportion of these 3 gases in the mixture depends on the laser system and the excitation mechanism. In general, for a continuous wave laser the proportions are:

CO2:N2:He - 1:1:8

w.wang 216 CO2 laser

CO2 is a linear molecule, and the three atoms are situated on a straight line with the Carbon atom in the middle. Three vibrational modes of CO2 molecule are illustrated:

1. Symmetric stretch mode (1).

2. Bending mode (2).

3. Asymmetric stretch mode (3).

Rami Arieli: "The Laser Adventure" Chapter 6, Carbon Dioxide Lasers page 1 w.wang 217 CO2 laser Transitions between vibrational energy states/levels results in photon emission in the infrared, while transitions between rotational states emit photons in the microwave region.

Necessary mechanisms for operation of the CO2 laser are, 1. Excitation of N2 vibration by electron impact 2. Transfer of vibrational energy from N2 to the nearly resonant v3 mode of CO2

3. Laser transition from v3 to v1 mode. 4. Sharing of population between v1 and 2v2l modes and relaxation within the v2 manifold

5. The vibrational energy in the v2 manifold converted into translational energy by collisions with He.

http://www.phy.davidson.edu/StuHome/sethvc/Laser-Final/co2.htm

w.wang 218 Excimer Laser

• An excimer laser (sometimes more correctly called an exciplex laser) is a form of ultraviolet laser which is commonly used in the production of microelectronic devices (semiconductor integrated circuits or “chips”), eye surgery, psoriasis treatment, and micromachining. • An excimer laser typically uses a combination of a noble gas (argon, krypton, or xenon) and a reactive gas (fluorine or chlorine). Under the appropriate conditions of electrical stimulation and high pressure, a pseudo‐molecule called an excimer (or in the case of noble gas halides, exciplex) is created, which can only exist in an energized state and can give rise to laser light in the ultraviolet range.

w.wang 219 Excimer Laser There are lasers in which the required conditions for lasing are achieved in exotic ways. As an example, we shall examine a family of lasers in which the radiation is emitted from a molecule which only exists for a very short time. This molecule is composed of an atom of noble gas: Argon, Krypton or Xenon, and an atom of halogen: Fluorine, Chlorine, Bromine or Iodine. An Excimer is a molecule which has a bound state (existence) only in an excited state.

In the ground state this molecule does not exist, and the atoms are separated. The excited state exists for a very short time, less than 10 nanoseconds. The name Excimer comes from the combination of the two words: exited dimer, which means that the molecule is composed of two atoms, and exists only in an excited state. (Some scientists consider this molecule to be a complex, and they call the laser "Exiplex").

Excimer lasers are usually operated with a pulse repetition rate of around 100 Hz and a pulse duration of ~10 ns, although some operate at pulse repetition rates as high as 8 kHz and some have pulsewidths as large as 30 ns. w.wang 220 Excimer Laser Lithography

Excimer lasers are widely used in high‐resolution photolithography machines, one of the critical technologies required for microelectronic chip manufacturing. Current state‐of‐ the‐art lithography tools use deep ultraviolet (DUV) light from the KrF and ArF excimer lasers with wavelengths of 248 and 193 nanometers (the dominant lithography technology today is thus also called “excimer laser lithography”[12][13][14][15]), which has enabled transistor feature sizes to shrink below 45 nanometers. Excimer laser lithography has thus played a critical role in the continued advance of the so‐called Moore’s law for the last 20 years.[16]

The most widespread industrial application of excimer lasers has been in deep‐ultraviolet photolithography,[12][14] a critical technology used in the manufacturing of microelectronic devices (i.e., semiconductor integrated circuits or “chips”). Historically, from the early 1960s through the mid‐1980s, mercury‐xenon lamps had been used in lithography for their spectral lines at 436, 405 and 365 nm wavelengths. However, with the semiconductor industry’s need for both higher resolution (to produce denser and faster chips) and higher throughput (for lower costs), the lamp‐based lithography tools were no longer able to meet the industry’s requirements. This challenge was overcome when in a pioneering development in 1982, deep‐UV excimer laser lithography was proposed and demonstrated at IBM by Kanti Jain.[12][13][14][17] With phenomenal advances made in equipment technology in the last two decades, and today microelectronic devices fabricated using excimer laser lithography totaling $400 billion in annual production, it is the semiconductor industry view[16] that excimer laser lithography has been a crucial factor in the continued advance of Moore’s law, enabling minimum features sizes in chip manufacturing to shrink from 800 nanometers in 1990 to 22 nanometers in 2012. This trend is expected to continue into this decade for even denser chips, with minimum features approaching 10 nanometers. From an even The Electra Laser Facility broader scientific and technological perspective, since the invention of the laser in 1960, the development of excimer laser lithography has been highlighted as one of the major NRL milestones in the 50‐year history of the laser.[18][19][20]w.wang 221 Medical Application The high‐power ultraviolet output of excimer lasers also makes them useful for surgery (particularly eye surgery) and for dermatological treatment. In 1980–1983, Rangaswamy Srinivasan, Samuel Blum and James J. Wynne at IBM’s T. J. Watson Research Center observed the effect of the ultraviolet excimer laser on biological materials. Intrigued, they investigated further, finding that the laser made clean, precise cuts that would be ideal for delicate surgeries. This resulted in a fundamental patent[21] and Srinivasan, Blum and Wynne were elected to the National Inventors Hall of Fame in 2002. In 2012, the team members were honored with National Medal of Technology and Innovation by the President of The United States Barack Obama for their work related to the excimer laser.[22] Subsequent work introduced the excimer laser for use in angioplasty.[23] Xenon chloride (308 nm) excimer lasers can also treat a variety of dermatological conditions including psoriasis, vitiligo, atopic dermatitis, alopecia areata and leukoderma.

As light sources, excimer lasers are generally large in size, which is a disadvantage in their medical applications, although their sizes are rapidly decreasing with ongoing development.

w.wang 222 Far Infra‐Red (FIR) Lasers

Far Infra-Red (FIR) lasers emit radiation in the Far-Infra-Red spectrum (wavelength range 12-1000 [m]. The wavelength range greater than 100 [m] is sometimes called sub-millimeter wave.

Far Infra-Red (FIR) lasers are gas lasers, and their lasing action occur between rotational levels of the gas molecules of the active medium. Usually these transitions are within the same vibrational level. The active medium in FIR lasers is usually a gas of simple organic molecule such as: C2H4, CF4, NH3,

Because of the very narrow width of each energy level of these materials, it is inefficient to optically pump them with ordinary light sources. The best way to achieve population inversion in these lasers is to pump them with another laser at shorter wavelength. Usually CO2 laser is used for pumping.

w.wang 223 Chemical Laser

The chemical laser is an example of a laser where the pump energy comes from a chemical reaction between two atoms. The chemical laser is a member of the family of Gas Dynamic Lasers: Gas dynamic lasers are based on rapid expansion of hot, high pressure gas, through nozzles into a near vacuum. This rapid expansion reduce the gas temperature. As a result, since the transfer of the molecules to the ground state takes more than the time of rapid expansion, we get at low temperature many molecules at excited levels. Thus, "population inversion". The gas usually flow through the nozzles in a transverse flow (perpendicular to the optical axis of the laser), so many nozzles can operate at the same time, yielding high power from the laser. The first chemical laser, which was operated in the pulsed mode, was developed in 1965 by J. V. V. Kasper, and G. C. Pimental. The lasing action of the chemical laser is usually based on vibrational transitions of diatomic molecule. w.wang 224 Liquid Laser

w.wang 225 Population Inversion in lasing mediums

- Gas (inert gas) A. Guenther UCONN - Liquid (pump source) - Solid state (pump source) w.wang - Semiconductor (PN junction) 226 Fluorescence

Fluorescence is a kind of luminescence, excited by irradiation of a substance with light. The light hitting a sample puts atoms, ions or molecules in the sample into excited states (by absorption of photons), from where they decay into lower‐lying states (e.g. their ground recall Phosphor coating states) through spontaneous emission of fluorescence photons. This phenomenon occurs in various kinds of optically pumped lasers and amplifiers, e.g. in solid‐state doped‐insulator lasers and amplifiers (including fiber lasers and fiber amplifiers), in optically pumped semiconductor lasers, and in dye lasers. The resulting radiation is called fluorescent light.

w.wang 227 Fluorescent Emission

First, the energy of S1' is partially dissipated, yielding a relaxed singlet excited state (S1) from which fluorescence emission originates. Second, not all the molecules initially excited by absorption (Stage 1) return to the ground state (S0) by fluorescence emission. Other processes such as collisional quenching, fluorescence resonance energy transfer (or intersystem crossing may also depopulate S1. The fluorescence quantum yield, which is the ratio of the spontaneous emission number of fluorescence photons emitted (Stage 3) to the number of photons absorbed (Stage 1), is a measure of the relative extent to which these processes occur. w.wang 228 Population inversions in liquids There are some molecules however, namely organic dye molecules, that do have a sufficiently long lifetime in an upper energy level (of the order of 1–5 nsec) so they can participate in the laser process by being excited to an upper laser level. These molecules also have the ability to radiate the energy from that level rather than lose the energy due to decay by collisions. Those molecules are the dyes that are used to color cloth and other objects that we use in our everyday life

When dissolved in a solvent such as alcohol or water, they can be concentrated in sufficient quantity to be used as a laser gain medium. When the light is applied to the dye solution, it is absorbed at certain wavelengths by the dye as described by absorption equation, placing the dye molecules in highly excited upper laser levels. A population inversion is then produced between those levels and a very broad range of lower-lying energy levels, thereby allowing the possibility for a wide range of laser wavelengths to be produced within the gain medium. Those lower levels are not initially pumped by the light and therefore are sufficiently empty to produce the inversion. Dye lasers thus allow the possibility of wide wavelength tunability and have been used extensively in doing a variety of spectroscopic studies in which very specific laser wavelengths are desired.

w.wang 229 Population inversions in liquids

Radiating light exited by argon laer

A. Guenther UCONN The pump source is an argon laser, whose beam is focused to a small spot. The argon laser is a gas laser which emits blue and green light. The dye flows in a high velocity jet with the argon laser beam focused on the jet. The wavelength of the output is adjusted by the tuning element. One of the most important features that dye lasers offer is tunability, that is, the color of the output beam can be varied by adjusting the intercavity tuning element and also by changing the type of dye that is used. The monochromatic output of available dye lasers can be tuned over a broad range, from the ultraviolet, to the near infrared.Liquid dye lasers that can be tuned to any visible wavelength, and to portions of the infrared and ultraviolet, are commercially available in both pulsed and continuous models. Dye lasers are chosen for applications, like spectroscopy, in which tunability is important.

w.wang 230 Dye Laser A dye laser can be considered as a special device to convert electromagnetic radiation from one wavelength, to another wavelength which can be tuned. The output of a dye laser is always a coherent radiation tunable over a specific spectrum region, determined by the Dye material. A dye laser is a laser which uses an organic dye as the lasing medium, usually as a liquid solution. Molecular fluorescence is responsible for dye laser emission.

diagram illustrating the processes involved in the creation of an excited electronic singlet state by optical absorption and subsequent emission of fluorescence History: Dye laser was first demonstrated in 1965 at IBM laboratories in the US, by Peter P. Sorokin and J. R. Lankard. They discovered the dye laser action during a fluorescence research of organic dye molecules, which were excited by Ruby laser. In 1967 scientists discovered the possibility to tune the emitted wavelength,w.wang using a grating at the end of the optical cavity 231 Solid state Lasers

w.wang 232 Population Inversion in lasing mediums

- Gas (inert gas) A. Guenther UCONN - Liquid (pump source) - Solid state (pump source) w.wang - Semiconductor (PN junction) 233 Solid State Lasers

A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid such as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class from solid-state lasers

Insulator Semiconductor

•Ruby Laser •Laser Diodes •Nd-Yag and Nd-Glass Lasers •Color Center Laser •Alexandrite Laser •Ti - Saphire Laser w.wang 234 Population inversions in crystalline solids and glasses As in the case of liquids, when energy levels in solids are excited, typically by irradiating those solids with light, the levels tend to decay much more rapidly via collisions with their surrounding neighbors rather than by radiating their energy in the form of light. In a few cases, however, specific types of atoms are embedded into a transparent host material (such as a specific crystalline solid or a glass) at concentrations of up to 1 part in 100, and the atoms radiate their energy rather than decay by collisions. These specific types of atoms, such as chromium or neodymium, consist of a radiating electron surrounded by a “screen” of other electrons that protect that radiating electron from being bombarded by collisions from neighboring atoms. The consequence is that the atoms can absorb pump light that passes through the transparent host medium and can then subsequently radiate that energy. Gemstones such as rubies fall into that category. Ruby, a desired gemstone and also the material that comprised the gain medium for the first laser, consists of chromium atoms doped into a transparent sapphire (Al2O3) host crystal. The color of the ruby crystal is determined by the chromium atoms, which absorb light in the blue and green regions of the spectrum andw.wang radiate in the red. 235 Population inversions in crystalline solids and glasses

Solid crystals of rare-earth salts might emit laser light if they were first irradiated with intense light of just the right wavelength, a scheme now known as optical pumping. (only in liquid or solids state laser)

Solid-state lasers include all optically pumped lasers in which the gain medium is a solid at room temperature

In solid-state laser materials, the atoms responsible for generating laser light are first excited to higher energy states through the absorption of photons, and the way in which these atoms relax from their excited states determines the quality and quantity of laser light produced . w.wang 236 SOLID CRYSTALLINE AND GLASS LASERS

A. Guenther UCONN The active medium is a cylinder of laser crystal whose ends have been cut parallel and polished. Antireflection coatings have been applied to the rod ends to reduce losses. The excitation mechanism for this particular laser is a tungsten filament lamp attached to an ac power source. Larger models utilize do krypton arc (gas discharge) lamps as pumping sources. Both types of lamps provide continuous optical pumping to the laser crystal. The mirrors of the Nd:YAG (yttrium-aluminum-garnet) laser usually are mounted separately from the active medium as shown, but one of the mirror coatings sometimes is applied directly to one end of the rod. w.wang 237 Pump Sources

The most common types of flashlamps used for pumping lasers are narrow, cylindrical quartz tubes with metal electrodes mounted on the ends, filled with a gaseous species such as A. Guenther UCONN xenon that serves as the radiating material within the lamp. Xenon is the most common species because of both its radiating efficiency and its emission of a broad spectrum of wavelengths from which to choose in matching the lamp emission to the Pumping lasers include the argon ion or doubled Nd:YAG cw pumping absorption bands of the laser. lasers for pumping titanium-sapphire lasers, excimer lasers for pumping dye lasers, and gallium arsenide semiconductor w.wang lasers for pumping Nd:YAG lasers. 238 Ruby Laser

Ruby laser was the first man made laser, which was build by Theodore Maiman in 1960.

Ruby is a synthetic crystal of Aluminum Oxide (Al2O3), and is more familiar in daily life as a precious stone for jewel.

The chemical structure of Ruby is of Al2O3 (which is called Sapphire), with impurity of about 0.05% (by weight) of Chromium Ions (Cr+3). The active ion is Cr+3, which replace Al atom in the crystal. This ion causes the red color of the crystal. The impurity ion of Cr+3 is responsible for the energy levels which participate in the process of lasing.

w.wang 239 Nd Laser In Nd laser Nd+3 ions (as impurities of up to a few percent by weight) are replacing the atoms of the solid host in the active medium. Three known solid hosts are used for Nd-YAG laser where Nd+3 ions are added as impurities: •Glass. •YAG (Yttrium Aluminum Garnet) Crystal.

•YLF (LiYF4) Crystal. The choice between the three possible hosts is according to the intended use of the laser: •Glass is used as the host material when a pulsed laser is needed, with each pulse at high power, and the pulse repetition rate is slow. The active medium of Nd-Glass Laser can be manufactured in a shape of disk or rod, with diameters of up to 0.5 meter (!) and length of up to several meters (!). Such dimensions are possible because glass is isotropic material, cheap, and can be easily worked to the right shape. High percentage (up to about 6%) of Nd ions can be added to glass as impurity. The problem with glass as a host is its poor thermal conductivity. Thus cooling the laser when it operates continuously or at high repetition rate is difficult. •YAG crystal is used for high repetition rate pulses (more than one pulse per second). In this case a large amount of heat need to be transferred away from the laser, and the thermal conductivity of the YAG crystal is much higher than that of glass. •YAG crystal with the high quality needed for lasers can be made with diameters of 2-15 [mm] and at lengths of 2-30 [cm]. •The price of a YAG laser rod is high, since growing crystals is a slow and complicated process. w.wang•The percentage of Nd ions in the YAG host is 1-4% by weight. 240 Bandwidth of laser gain medium

The bandwidth of the laser gain medium determines the range of wavelengths over which amplification can occur for any specific laser. This bandwidth is expressed in either a wavelength range

G or a frequency range vG. These two expressions are related by

in which  is the laser wavelength and c is Laser gain bandwidths for the HeNe, the speed of light. The bandwidth of the Nd:YAG, and Ti:Al2O3 lasers gain medium is usually determined by the bandwidth over which the spontaneous A. Guenther UCONN emission occurs for a given laser Full width at half maximum (FWHM) transition. w.wang 241 Erbium fiber

Fiber lasers were first operated in Nd-doped glass fibers, but the fiber laser of major current interest is the erbium-doped fiber laser operating at 1.4 to 1.6 m. This fiber laser can be spliced into a normal transmitting optical fiber, and, when optically pumped with a semiconductor laser at either 980 nm or 1.48 m, it provides amplification to a communication signal that is being transmitted through the fiber. The pump light is fed into the fiber line—with a beam-combining device—thereby merging with the signal. This laser amplifier is especially useful in undersea optical fiber cables transmitting phone and data information between continents.

Erbium Fibre Amplifier Courtesy E-Bay w.wang 242 Semiconductor Lasers

w.wang 243 Population Inversion in lasing mediums

- Gas (inert gas) A. Guenther UCONN - Liquid (pump source) - Solid state (pump source) w.wang - Semiconductor (PN junction) 244 Semiconductor Lasers

Semiconductor lasers are quite different from conventional lasers. In particular:

1. The gain of the laser material is very high and is generated by a population inversion between the conduction and valence bands of semiconductors. In some sense, a semiconductor laser is a two-state laser system. 2. Since the electromagnetic mode is on the order of the size of the laser device, then the transverse mode of the semiconductor laser is quite different from that of a conventional laser. In particular, the beam is not Gaussian, the beam profile tends to be elliptical, and the beam divergence tend to be large. 3. The gain spectrum is quite large (many THz or hundreds of angstroms). 4. The short cavity (several hundred microns) means that the longitudinal mode spacing is much larger than that of a conventional gas or solid state laser (on the order of GHz or angstroms).

Holes and electrons compare to only electrons in other laser system w.wang 245 Bandwidth of laser gain medium

The bandwidth of the laser gain medium determines the range of wavelengths over which amplification can occur for any specific laser. This bandwidth is expressed in either a wavelength range

G or a frequency range vG. These two expressions are related by

Free spectral range (FSR) is the spacing in optical frequency or wavelength between two successive reflected or transmitted optical intensity maxima or minima of an interferometer

A function of the cavity length w.wang 2 247 f = Co/(2nycos)  = f  /Co Population inversions in semiconductors Inversions in semiconductors are produced when joining a p-doped semiconductor material with an n-doped semiconductor material in a similar way to that of producing a transistor to create a pn junction. The n-doped material contains an excess of electrons and the p-doped material has an excess of holes (a material with excess positive charge). When a voltage is applied across the junction, with the positive voltage on the p side, the electrons are pulled through the junction toward the positive electrode and the holes are attracted to the negative side, producing an electrical current flow across the junction. The electrons and holes meet within the junction and are attracted to each other because of opposite charges. When they meet, they recombine and emit radiation and also can produce a population inversion. This inversion occurs between energy levels located above and below the semiconductor bandgap, the gap in energy below which the material is transparent. This energy typically corresponds to a wavelength in the infrared, and hence most semiconductors radiate in the infrared and are not transparent in the visible spectral region like glass is. However, semiconductor lasers are under development to operate in the green and blue regions of the spectrum. At very low currents, a population inversion does not occur even though recombination radiation is emitted. In fact, such nonlaser-like emission is the source of radiation from a light-emitting diode (LED). In comparision, to produce a population inversion, a very high current density is applied within the junction region. However, this high current density leads to excessive heat deposition in the material; therefore a significant part of the development of semiconductor lasers involves how to remove the heat, or to make smaller junctions so that less current is required. The material and itsw.wang corresponding energy bandgap determine the laser wavelength. 248 Semiconductor Lasers

Semiconductor lasers are quite different from conventional lasers. In particular:

w.wang 249 Difference between LED and Laser Diode

specification Light Emitting Diode Laser Diode Working operation It emits light by spontaneous emission. It emits light by stimulated emission. The emitted light is incoherent i.e. It possesses a coherent beam with Coherent/Incoherent photons are in random phase among identical phase relation of emitted themselves. photons. Output power is high (Few mW to GW) , Emitted light power is relatively low, Output power Proportional to current above the Linearly proportional to drive current threshold It requires small applied bias and It requires high driving power and high Bias/Current operates under relatively low current injected current density is needed. densities. Coupled power Moderate High Speed Slower Faster Output pattern Higher Lower Fiber Type Multimode only Singlemode and multimode Ease of use Easier Harder Lifetime Longer Long Wider, 25 to 100 nm Narrower, <10‐5 to 5 nm Spectral width (10 to 50 THz) (<1 MHz to 2 MHz) Modulation Bandwidth Moderate, Tens of KHz to tens of MHz High, Tens of MHz to tens of GHz Available Wavelength 0.66 to 1.65 mm 0.41 to 1.65 mm E/O Conversion Efficiency 10 to 20 % 30 to 70 % Must be rendered eye‐safe, especially for Eye Safety Generally considered eye‐safe λ < 1400 nm Cost Low Moderate to High w.wang 250 LED versus Laser diode

Lasers emit light from exciting a population of particles to an excited Spontaneous emission stem from holes state, photons generated then striking the atom and compels them to electrons recombination. Occurs at release the similar photon which then causes emission of another photon. Thus, in LASER every released photon strike another atom relatively low current and voltage to release similar photon and therefore, the beam of light thus produced is coherent in nature.(stimulated emission)

Mirrors also help to maintain the excitation of a population inversion. The gain of the lasing is increased with mirrors, photons are more likely to w.wang hit an excited atom causing the emission process to begin again. 251 Spontaneous Emission

h hc

h

w.wang 252 Stimulated Emission The third process, shown in Figure is referred to as stimulated emission. It results when an electron is in a higher- lying level, such as level 2, and a E21 photon of light of wavelength 21 collides with the atom. During the collision the photon stimulates the atom to radiate a second photon having exactly the same energy E21 as that of the incident photon and traveling in exactly the same direction in order to E satisfy the laws of conservation of 12 energy and momentum. Hence, one photon leads to two identical photons, E which, in effect, leads to an 21 amplification process. A photon has been gained at the expense of the loss of E21 energy stored within the atom. E21 A. Guenther UCONN

w.wang 253 BC Choudhary

w.wang 254 BC Choudhary w.wang 255 Output Characteristics

At low values of the input, the device acts as a light-emitting diode (LED), producing a relatively small amount of incoherent light. At a threshold value, where the population inversion is large enough so that gain by stimulated emission can overcome the losses, the laser threshold is reached. As current increases above the threshold value, the light output increases much more rapidly than in the LED region. w.wang 256 Spectral Characteristics

When the threshold current density is exceeded, the emission spectrum narrows dramatically and the intensity of the emission increases considerably. Figure shows the emission spectrum of a laser diode below and also above threshold. At higher currents the linewidth of the laser output decreases. The width of the spectral band represented by the spontaneous emission is much greater than that of the stimulated emission. However, stimulated emission produced by the laser is still much broader than that of conventional gas and crystalline lasers. It is of the order of two or three nanometers, as compared to a typical spectral width around 10–3 nanometers for a HeNe laser. The emission spectrum is relatively complex and typically contains a number of longitudinal modes of the optical cavity. The spacing between longitudinal modes is relatively large, because of the short length of the optical cavity. However, the relatively large spectral width of the GaAs laser allows several modes to be present.

w.wang 257 Laser Diode

Simply a diode with mirrors (and some other atomical arrangements to keep the emission on a narrower bandwidth)

It uses gallium arsenide doped with elements such as selenium, aluminium, or silicon to produce P type and N type semiconductor materials. While a laser diode has an additional active layer of undoped (intrinsic) gallium arsenide have the thickness only a few nanometers, sandwiched between the P and N layers, effectively creating a PIN diode (P type-Intrinsic-N type). It is in this layer that the laser light is produced.

w.wang 258 Laser with built in photodiode as feedback control

As the active layer is filled with oscillating photons, some (typically about 60%) of the light escapes in a narrow, flat beam from the edge of the diode chip. As shown below figure, some residual light also escapes at the opposite edge and is used to activate a photodiode, which converts the light back into the electric current. This current is used as a feedback to the automatic diode driver circuit, to measure the activity in the laser diode and so make sure by controlling the current through the laser diode, that the current and light output remain at a constant and safe level. w.wang 259 Semiconductor Lasers

Band structure near a semiconductor p-n junction. Left: No forward-bias voltage. Right: Forward-bias voltage present shows the relative populations of the energy bands or both sides of a p-n junction with no voltage applied to the diode. The n-type material contains electrons which behave as the current carriers in its conduction band, whereas the p-type material has holes for carriers in its valence band. When a forward voltage is applied to the diode, the energy levels are caused to shift. Under these conditions there is a significant increase in the concentration of electrons in the conduction band near the junction on the n-side and the concentration of holes in the valence band near the junction on the p-side.

The electrons and holes recombine and energy is given off in the form of photons. The energy of the photon resulting from this recombination is equal to that associated with the energy gap. In light-emitting diodes (LED) this light energy is transmitted out through the sides of the junction region. For the LED, all of the light is created by spontaneous emission due to electron and hole recombination. In semiconductor lasers the junction forms the active medium, and the reflective ends of the laser material provide feedback. Because of this feedback in diode lasers, most of the light is created by stimulated emission. w.wang 260 Semiconductor Lasers

Semiconductor lasers are light-emitting diodes within a resonator cavity that is formed either on the surfaces of the diode or externally. An electric current passing through the diode produces light emission when electrons and holes recombine at the p-n junction. Because of the small size of the active medium, the laser output is very divergent and requires special optics to produce a good beam shape. These lasers are used in optical- fiber communications, CD players, and in high-resolution molecular spectroscopy in the near-infrared. Diode laser arrays can replace flashlamps to efficiently pump solid-state lasers. Diode lasers are tunable over a narrow range and different semiconductor materials are used to make lasers at 680, 800, 1300, and 1500 nm. w.wang 261 a simplified classification scheme showing some of the major subdivisions of diode lasers and their relationship

w.wang 262 Simplest (and earliest) type of gallium arsenide laser (Edge Emission)

w.wang 263 Homojunction and Heterojunction

Structure and index of refraction for various types of junction in the aluminum gallium arsenide system. Top: Homojunction.

Middle: Single heterojunction.

Bottom: Double heterojunction

The variation in doping provided a small step in the index of refraction, as indicated on the right side of the figure. This tended to provide some confinement of light in the region of the junction, because of total Energy levels at a heterojuction internal reflection. allow conduction electronics to drop to the low band gap side, but w.wang not t return to the high-band gap 264 side Double Heterostructure ( edge emission)

Modern diode lasers are formed of structures that contain several thin layers of material of varying composition.

w.wang 265 Amplification

A semiconductor laser amplifier is a forward bias heavily doped p-n junction fabricated From a direct gap semiconductor materials. The injected current is sufficiently large To provide optical gain. The gain coefficient ro(v) of the laser amplifier has a peak value

rp that is approximately proportional to the injected carrier concentration, which In turn, proportional to the injected current density J Cleave surface

rp ~(J/Jt –1), Jt= nTel/intr w Where  = electron-hole recombination lifetime r d P n in= /r is the internal quantum efficiency l i l = thickness of the active region  = thermal equilibrium absorption coefficient

nT and Jt are injected-carrier concentration and current density just to make the semiconductor transparent.

w.wang 266 Feedback

The feedback is usually obtained by cleaving the crystal planes normal to the Plane of the junction, or by polishing two parallel surfaces of the crystal. The active region serves as a planar-mirror optical resonator of length d and cross-sectional area lw. The reflectance at the Semiconductor-air interface is Cleave surface

w R = (n-1/n+1)2

For GaAs, n=3.6 and R=0.32 d P n l i

w.wang 267 Resonator Losses

The principle source of resonator loss arises from the partial reflection at the surfaces of the crystal. For a resonator of length d the reflection loss coefficient is

m = m1 +m2 = (1/2d)ln(1/R1R2)

If two surfaces have the same reflectance R1=R2 =R, then am= (1/d)ln(1/R).

Include confinement factor which represents the fraction of the optical Energy lying within the active region (l), the total loss

r = 1/  (s +m)

Where s represents other sources of loss including free carrier absorption in and scattering from optical inhomogeneities. w.wang 268 Gain Condition: Laser threshold

The laser oscillation condition is the gain exceed the loss, rp > r, as discussed in the earlier section. The threshold gain coefficient is therefore

r. Set J = Jt and rp= ar, the threshold injected current density Jt is

Jt = JT(r + )/ or I = JA

Where the transparency current density,

JT = elnT/(r) or iT = J T A; A = wd

Smaller Jt indicates superior performance, maximize and minimize r, minimize transparent injected-carrier concentration nT and l. however, l is reduced beyonda point the r, increases because the confinement factor  w.wang 269 Gain Condition (Laser Threshold)

homostructure The confinement factor remains near 1 for lower values l because Jt the active layer behaves as an optical waveguide The result is a lower J Double heterostructure t

l w.wang 270 Internal Photon Flux

When laser current density is increased above its threshold value (J > Jt), the amplifier peak gain p exceeds loss coefficient r. Stimulated emission then outweights absorption and other Resonator losses so oscillation begin and the photon flux increases

int i-it)/e , i > it (steady-state laser internal photon flux) = 0 , i < it Photons per second generated within the active region

The internal laser power above threshold is simply related to the internal photon flux by P=hv  So we have

P = 1.24 int i-it)/o

w.wang 271 Output Flux and Efficiency

The output flux is the product of internal flux and emission efficiency , which is the ratio of the loss associated with the useful light transmitted through the

mirrors to the total resonator loss e = m / r = m1 +m2 = [(1/2d)ln(1/R1R2)]/

 eint i-it)/e

The laser output power above threshold is

Pout = 1.24d i-it)/o , d = ein (differential quantum efficiency)

Pout Slope = dPout/di = 1.24d/o (differential responsivity)

w.wang it i 272 Output Characteristics

At low values of the input, the device acts as a light-emitting diode (LED), producing a relatively small amount of incoherent light. At a threshold value, where the population inversion is large enough so that gain by stimulated emission can overcome the losses, the laser threshold is reached. As current increases above the threshold value, the light output w.wangincreases much more rapidly than in the LED region. 273 Overall Efficiency

The overall efficiency (Power-conversion efficiency) is defined as the ratio of the emitted laser light power to the power to the electrical input power iV

d (1-(it/I))hv/eV

w.wang 274 Device efficiency

A photodiode's capability to convert electrical energy to photon energy, expressed as a percentage, device efficiency

# of output photons/sec

rp/re = ------# of electrons (holes) excited as Iinj/sec

Depends on , through emission coefficient, thickness of layers, Doping, geometry, etc.

w.wang 275 Since DE is rp/re and P = rp h

where rp = photon flux = P/ h # photons/ sec Electron rate then

re = rp/ = P/(h)

Therefore, the output photo current Iinj=ere is

Iinj = eP/(h)

P = 𝑜𝑢𝑡𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟 𝜂 𝐼

h= plank constant = 6.63x10-34 joule-sec

w.wang 276 The output power vs. forward current for the LED appears as a linear function. The relation is given as

P = 𝑜𝑢𝑡𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟 𝜂 𝐼

Where 𝜂 is overall device efficiency=extraction efficient x the radiative efficiency x injection efficiency, h is plank constant and n is frequency of

emitting light, e = single electron charge and Iinj = Id . All but Iinj = Id are constant. This linear relation should be verified with your experiment

ℎ𝜐 slope 𝜂 𝑒 output power (W)

𝐼 DC drive current (mA) The output power vs. forward current for the LED 𝐼

w.wang 277 Temperature Dependence of Laser Output

a) Schematic sketch of the output of a typical laser diode as a function of drive current for three different operating temperatures. b) Temperature dependence of threshold current. w.wang 278 Wavelength as a function of Temperature

Gallium arsenide lasers emit radiation in the near infrared portion of the spectrum. The exact wavelength depends on the temperature at which the laser is operated.

w.wang 279 Cooling of Laser Diodes

w.wang 280 Infineon Technologies, Agilent Technologies, Hitachi, Intelite, Laser Components Instrument Group, Alcatel, Furukawa Electrics, Mitshubishi

w.wang 281 Spectral Characteristics

The spectral distribution is governed by three factors

1. Spectral width B within which the active medium small signal gain is

greater the loss coefficient r 2. Homogeneous and inhomogeneous nature of the line-broadening mechanism

3. Resonant modes vf = C / 2nd

w.wang 282 Spectral Characteristics

w.wang 283 Far‐Field Radiation Pattern

o/l A laser diode with an active dimnesion l and w emits o/w Light with far field angular divergence ~ o/l in the Plane perpendicular to the junction and o/w in the Plane parallel to the junction. Cleave surface Assume Gassian beam of diameter 2Wo, w the divergence angle is W

d P n l i

Active layer w.wang 284 Spatial Characteristics

A semiconductor laser typically has an elliptical spatial profile The profile is caused by diffraction. Light is emitted through the aperture defined by the small junction. Diffraction through the narrow dimensions of the junction causes the beam to spread into a broader angle than is observed with other types of lasers. w.wang 285 Laser Lifetime

Two different types of failure mechanisms have been identified in gallium arsenide lasers. One is a catastrophic decrease in the power output. This catastrophic damage may occur within a single pulse of the laser, and it is associated with damage of the end surfaces of the laser. The damage is produced by the light output of the laser itself. Tiny cracks or grooves in the junction are produced. To avoid this type of damage, peak power output of the laser must be limited.

There is also a gradual increase in power, which is manifested by increasing threshold current. This damage is produced by the current flowing through the junction. This is a complex phenomenon that is complicated by random variations in the laser life. To extend the life of the laser diode, current density through the junction should be limited. w.wang 286 Other Types of Semiconductor Lasers

Material Wavelength (µm) Material Wavelength (µm) ZnS 0.33 GaAs 0.84-0.95 ZnO 0.37 InP 0.91 Gan 0.40 GaSb 1.55 ZnSe 0.46 InAs 3.1 CdS 0.49 Te 3.72 ZnTe 0.53 PbS 4.3 GaSe 0.59 InSb 5.2 CdSe 0.675 PbTe 6.5 CdTe 0.785 PbSe 8.5

w.wang 287 Wavelength ranges covered by a number of semiconductor lasers of mixed composition w.wang 288 Distributed‐feedback (DFB) laser

With introduction of a corrugated structure into the cavity of the laser, only light of a very specific wavelength is diffracted and allowed to oscillate. w.wang 289 Bragg Grating

B Λ mm2;  1,2,3.. neff

Input light

Transmitted Reflected Period: Λ light light P

l Input light Reflected light Transmitted light 290 Packaging

14-pin DIL package (Courtesy of Lasertron)

1550-nm DFB laser in butterfly package (Courtesy of Lasertron)

w.wang 291 Laser fiber coupling

w.wang 292 Laser Applications

w.wang 293 Laser applications

Industrial applications:engraving, cutting, scribing, drilling, tooling, welding, etc.

Medical applications: cutting, soldering, correct vision diagnostics of cancer cells using fluorescence, and photo dynamic therapy , remove unwanted tissue, photothermal, etc.

Micromachining: 3-D micromaching, etching, engraving, etc.

w.wang 294 w.wang "Laser Applications and Processing in Precision Manufacturing" was help at the EOC295 on December 7, 2000 w.wang "Laser Applications and Processing in Precision Manufacturing" was help at the EOC296 on December 7, 2000 w.wang "Laser Applications and Processing in Precision Manufacturing" was help at the EOC297 on December 7, 2000 There are major performance differences between Nd:YAG and CO2 lasers. One reason is that Nd:YAG light is emitted at a wavelength of 1.06 microns in the near infrared, while CO2 light is emitted at 10.6 microns. The material interactions at these wavelengths differ. Most organics don't absorb 1 micron light very well, while they absorb 10 micron light. So, non-metal processing is generally a CO2 application. Metals are more reflective at 10 microns than at 1 micron, so CO2 lasers only weld effectively in the "keyhole" mode, where the irradiance is high enough to generate a vapor channel in the workpiece. Once you get into keyhole mode, the high average power of CO2 lasers makes high speed welding possible. For small spot welds, Nd:YAG lasers are far more controllable. Also, since there are a lot more Nd atoms in a YAG rod than there are CO2 atoms in laser gas, Nd:YAG lasers can deliver much higher peak powers than CO2 lasers. This makes them better for drilling. Conversely, since it's hard to cool a solid rod, Nd:YAG lasers have problems with high average powers. You can build a CO2 laser with very high power; Convergent has commercial 45 kW units, and much bigger ones have been built. w.wang 298 w.wang "Laser Applications and Processing in Precision Manufacturing" was help at the EOC299 on December 7, 2000 w.wang 300 w.wang "Laser Applications and Processing in Precision Manufacturing" was help at the EOC301 on December 7, 2000 w.wang "Laser Applications and Processing in Precision Manufacturing" was help at the EOC302 on December 7, 2000 Lasers in General Surgery

Almost every medical surgery in which a removal of tissue is required, or a cut needs to be made, can be done with a laser. In general, the results using lasers are better than the results using a surgical knife.

•When a bleeding need to be stopped, a Nd-YAG laser can be used. Its radiation enters deep into the tissue, and heats and coagulates a large area. •When a clean cut need to be done, an Excimer laser is used.

•A more general cutting tool is the CO2 laser.

w.wang 303 Lasers in Dentistry

Dental soft tissue treatment applications are similar to those of other soft tissue in the body, and are common for many years. •In case of gum surgery, with the laser most of the patients suffer less postoperative pain. •Almost no bleeding. •No stitches required. Dental hard tissue treatment is new, and only in May 1997 the FDA (Federal Drug Administration) approved the Er- YAG laser for use on the hard tissue (teeth) in humans.

Rami Arieli: "The Laser Adventure" Chapter 9.2.1 page 11 w.wang 304 Examples of Laser used in Dentistry

Argon: laser teeth whitening Dual Wavelength Surgical Argon: teeth whitening; soft tissue surgeries, such as gingivoplasty, frenectomy and biopsy; composite curing, which reinforces and strengthens the tooth Nd:YAG: soft tissue surgery, such as gingivoplasty, frenectomy, gingivectomy, bacteria reduction when treating gum disease Er:YAG: removal of tooth decay/cavities while also decontaminating the area, frenectomy, crown lengthening Perio Diode: gum treatment and soft tissue surgery, such as gingivoplasty Low Level Laser Therapy (LLLT): biophotomodulation, treats canker sores, herpes, sore jaws, helps relieve post-op discomfort and promotes post-op healing Diagnodent Laser: early detection of cavities

w.wang 305 Lasers for eye treatment

Soldering Detached Retina

As a result of mechanical shock, the retina inside the eye can be torn, and detached from the tissue it is connected to

The electromagnetic radiation from the laser heats the detached retina, and as a result the damaged blood vessels around the retina are closed and solder to place.

Because of the focusing effect of the eye, small amount of laser power is needed to solder the detached retina. Rami Arieli: "The Laser Adventure" Chapter 9.2.1 Eye page 1 Excimer laser

w.wang 306 Lasers in Diagnostic Medicine, and in combination with Drugs

Diagnostics of cancer cells using Fluorescence, and Photo Dynamic Therapy (PDT)

One of the biggest problems in medicine today is to find a cure for cancer.

There are many treatments for cancer to destroy the cancer cells, such as: • Disectomy of the infected organ. • Radioactive irradiation. • Heat treatment. Rami Arieli: "The Laser Adventure" Chapter 9.2.2 page 1 w.wang 307 Types of Lasers Although there are several different kinds of lasers, only three kinds have gained wide use in medicine: Carbon dioxide (CO2) laser—This type of laser can remove thin layers from the skin's surface without penetrating the deeper layers. This technique is particularly useful in treating tumors that have not spread deep into the skin and certain precancerous conditions. As an alternative to traditional scalpel surgery, the CO2 laser is also able to cut the skin. The laser is used in this way to remove skin cancers. Neodymium:yttrium-aluminum-garnet (Nd:YAG) laser—Light from this laser can penetrate deeper into tissue than light from the other types of lasers, and it can cause blood to clot quickly. It can be carried through optical fibers to less accessible parts of the body. This type of laser is sometimes used to treat throat cancers. Argon laser—This laser can pass through only superficial layers of tissue and is therefore useful in dermatology and in eye surgery. It also is used with light- sensitive dyes to treat tumors in a procedure known as photodynamic therapy (PDT).

w.wang 308 Soft lasers Most of the medical laser applications were until recently based on the thermal effects caused by the electromagnetic radiation which was absorbed in the biological tissue.

In the last few years, some new applications are using low power lasers with output power less than 1 Watt.

Some of the effects of these low power levels on the biological tissue is not thermal, and in effect the mechanism of interaction is not yet clear.

It is sometimes referred to as Biostimulation, which does not explain a lot.

Rami Arieli: "The Laser Adventure" Chapter 9.2.3 page 1 w.wang 309 Lasers in Dermatology

Among these defects are: •Pigmented skin, Abnormal skin growth, Blemishes. •Tattoos. Today, with the wide variety of lasers in use, tattoos can almost completely be erased from the skin. Different wavelengths are used to remove different ink colors from the skin. The specific laser wavelength is selectively absorbed by the specific color, without damage to surrounding cells. Usually the treatment is made in a number of treatments. After each treatment checking what was left in the damaged skin. (Ruby, Nd: YAG and CO 2 laser) A nice Web site about the laser tattoo removal is at Beckman Laser Institute and Medical Clinic: http://www.bli.uci.edu/clinic/tattoos.html

• Carcinomas and malignancies. Rami Arieli: "The Laser Adventure" Chapter 9.2.1 Dermatology page 1

w.wang 310 This is an example of a laser system for hair removal procedures.

(810 ± 10) nm Classification: IIIb Emitting wavelength Output power User-adjustable 0 to 15 Joules Designation: OEM Generation modes CW Manufacturer: LaserTec, UK Beam characteristic Semiconductor Diode Warranty: 1 Year Pulse duration Manual Adjust Emission Indicator: Yes Weight 1.2 kg max Key Lock: Yes Optics Pinpoint 2mm Beam Shutter: No 21CFR 1040, IEC 825-1:1993: Dimensions 10 x 19 x 7.5 inches No w.wang 311 Diode laser arrays

Specifications •Emitting Dimensions: 10 x 17.2mm •CW Output Power: 240W •Threshold Current: 7A •Operating Current: 25A typ, 30A max •Operating Voltage: 22.8V typ, 25.8V max •Series Resistance: 0.19 ohm •Beam Divergence: 35 x 10 degrees •Peak Wavelength: 803nm •Spectral Width: 3nm typ, 4nm max

Cooling Requirements •Flow: 1.5 LPM •Pressure: 15 PSI •Water Temperature: 25C •DI Water not required •<20um particle filter required

Courtesy E-Bay laser diode bar array made by Spectra Diode Labs (SDL). This is model 3474-MB. It consists of 12 20W diode bars in a 10 x 20mm G stack package producing an amazing 240W with an energy density of 120W/cm^2. w.wang 312 A diode laser bar

Electric current out

Light out spreads at ~60 deg. up and down and ~20 deg. left and right.

Electric current in Heat out w.wangLouis Chow, University of Central Florida 313 A diode laser array pumped solid state laser

Waste heat out Electric current in Electric Slab shaped current out gain medium

Resonator Resonator mirror mirror

Louis Chow, University of Central Florida

Diode laser arrays (2) with beam control prisms pumping a slab laser through both mid sized edges w.wang 314 Other Light Sources

w.wang 315 Electroluminescent Light A thin copper wire coated in a phosphor which glows when an alternating current is applied to it. It can be used in a wide variety of applications—vehicle and/or structure decoration, safety and emergency lighting, toys, clothing etc.—much as rope light or Christmas lights are often used. Unlike these types of strand lights, EL wire is not a series of points, but produces a 360 degree unbroken line of visible light. Its thin diameter makes it flexible and ideal for use in a variety of applications such as clothing or costumes. w.wang Wikipedia 316 w.wang 317 FireFlies

Bioluminescence is an enchanting process in which living organisms convert chemical energy into light. The light results from the oxidation of an organic substrate, a luciferin, catalyzed by an enzyme called a luciferase.

w.wang 318 Luminescence of FireFlies

Fireflies have organs underneath their abdomens that are dedicated to producing light. As they take in oxygen, specialized cells called photocytes produce light as the oxygen combines with luciferin. In a firefly's tail, there are two different chemicals that aid light production: luciferase and luciferin. Luciferase plays a role in heat resistance, and luciferin is an enzyme that triggers light release. Both of these chemicals light up in the presence of ATP (principal molecule for storing and transferring energy in cells), which is the energy source for these reactions. Even as underground larvae or eggs, fireflies produce light.

w.wang 319 w.wang Takaharu Sakiyama 320