Brought to you by: Jonathan E. Mace

What is a…

What make a laser so special?

History of Lasers -S. N. Bose postulates that light does not behave classically in the fact that statistically they will tend to travel together (1924). - From this Albert Einstein uses photon statistics to predict stimulated emission, the physical basis of lasers. - Charles Townes and Arthur Shawlow produce first Maser from a beam of ammonia and theorize existence of optical and IR lasers (1958). - T. H. Maiman produces the first optical laser from a ruby (1960). - Gordon Gould eventually granted patent for components of lasers.

Wave / Particle Duality of Light Wave-like Properties -Wavelength/Frequency -Interference -Phase Lasers Use Both!!! Particle-like Properties -Photons -Momentum -Quantum mechanics

Absorption and Emission of Light Ground State Electrons Excited State Electron s Electrons can only go to certain energy levels. They can only absorb certain frequencies of light. Electrons emit light at the same discrete frequencies at which they absorb when traveling between energy levels.

The two types of emission Excited atoms decay to a lower energy state and emit light in random directions. The different atoms can emit light at different times and may not undergo the same transitions. Light from the decay of one excited atom interacts with another similarly excited atom causing that element to emit light This light produced is: in phase, directional, and monochromatic.

Metastable States Electron in an unstable excited state. (lifetime in nanosecond timescale) Electron drops to a lower excited state through a “radiationless” transition. (heat given off) Electron is now in a metastable state which is more stable than pervious state (lifetime can be in millisecond timescale) Metastable states are required for stimulated emission to have enough time to be effective in producing laser light.

To “Lase”? - To lase is the verb form of what a laser does. Lasing, or laser action, is when stimulated emission from atoms or molecules in excited states overcomes spontaneous emission from excited atoms or molecules in the medium, thus producing laser light. - This sounds relatively simple, however a certain criteria has to be met…. - If only a few atoms or molecules are excited then it will be very hard to have stimulated emission occur because the likelihood of a photon encountering another excited atom or molecule would be small. - Therefore, we just need to get a heck of a lot of atoms or molecules into an excited state and we should get laser light…. right? Well, not exactly. Since most atoms and molecules are in the grounds state we need a population inversion.

Population Inversion - The second law of thermodynamics makes a two level laser impossible since only half of the atoms can be put in the excited state. - Three and four level lasers are possible when we take advantage of metastable states. A three level laser was the first created, but now four level lasers are the most common. - A population inversion is achieved when more atoms in a medium are in a higher energy state than are in the lower state.

Three and Four Level Lasers Ground state Electron pumped to third energy level. Electron quickly drops to metastable state. Elec- trons build up in second level. Population in- version between second and first levels Stimulated emission occurs dropping electrons back to ground state. Laser trans- ition is between second and first levels. Electron pumped to fourth energy level. Electron quickly drops to metastable state at third level. Electrons build up in third level. Population inversion between third and second levels Laser transition and thus stimulated emission occurs between third and second levels. Electron quickly drops to ground state.

Mirror Total reflection Vaporized silver or dielectric coating up to % reflective. Mirror Partial reflection From 10-90% reflective. Specialized for different types of lasers. Pumping Source Atoms An amplifier with a provision for positive feedback is known as an oscillator, which is what a laser is. The cavity allows the light to oscillate down its axis and creates a more intense laser by effectively lengthening the cavity. The cavity also spectral purity of the beam by only allowing certain wavelengths of light to oscillate. So in a way, this is actually a LOSER, but who wants that? Optical Resonant Cavity

Types of Lasers - Solid state lasers - Insulator doped with a metal - Ruby (Cr 3+ in Al 2 O 3 ), Nd:YAG, Erbium:Glass - uses: Spectroscopy, industry, laser ranging, dentistry - Gas lasers - Usually more than one type of atom involved. - HeNe, CO 2, ArF, Kr, N 2 -uses: Eye surgery, holography, cutting/welding - Semiconductor lasers - Novel mechanism for laser light. - AlGaInP, GaAlAs, InGaAsP, GaAs, GaP - uses: Laser pointers, checkouts, CD/DVD, communication - Dye lasers - Need to be connected to another laser. - Rhodamine, Coumarin, Stilbene -uses: spectroscopy, scar reduction, tattoo removal

Solid State Lasers The metal ions in the glass lattice are what produces the laser light. These can either be pulsed or continuous wave. May use Xe lamp or diode array. Efficiency of 1% to 25%. Price $300 to $15,000.

Gas Lasers Gas lasers are pumped by passing electrical discharge across the gas tube. In HeNe lasers the He are excited and collide with Ne which then lase, allowing continuous pop. inv. Argon lasers us a plasma of Ar ions moving in a helical path and emit at a range of wavelengths. CO 2 lasers use vibrational modes of the molecule to produce light and require very long cavities. Efficiency 1-20%, cost $500-$20,000.

Semiconductor Lasers Basics Conducting Band Valence Band p-type doped When a potential difference is applied, electrons move from the full valence band to holes created by empty energy levels in the atoms of the doping substance, and the holes in the valence band permit conductivity n-type doped Upon application of a potential difference, electrons move from the atoms on the doping substance to the empty conducting band, and are free to move and thus conduct electricity } ΔE

Semiconductor Lasers As free electrons move from negatively doped region to holes in positively doped region they emit light. The heavily doped active region ensures an abundance of holes for electrons to move into. Flat polished ends and roughened sides trap light inside the crystal to cause stimulated emission. These types of lasers are the most common. They are cheap and easy to build and can be made very small and are only continuous wave % efficiency, cost negligible - $10,000

Dye Lasers A laser or flash lamp is used to excite a dye consisting of a fluorescent organic molecule in solution. This molecule then relaxes to a metastable and re-emits the light at a different wavelength via stimulated emission. Special optics can then be used to tune the frequency of the laser over a distribution of wavelengths. Efficiency 20-50% cost $

Laser Classification Lasers are divided into classes depending on the power or energy of the beam and the wavelength of emitted radiation. Laser classification is based on the potential for causing immediate injury to the eye or skin and/or potential for causing fires from direct exposure to the beam or from reflections from diffuse reflective surfaces. Class 1 lasers - Considered to be incapable of producing damaging radiation levels, and are exempt from most control measures or other forms of surveillance. Ex. laser printers. Class 2 lasers - Emit radiation in the visible portion of the spectrum, and protection is normally afforded by the normal human aversion response (blink reflex) to bright radiant sources. May be hazardous if viewed directly for long periods of time. Ex. laser pointers. Class 3a lasers - Normally would not produce injury if viewed only momentarily with the unaided eye. May present a hazard if viewed using collecting optics, e.g., telescopes, microscopes. Example: HeNe lasers between 1 and 5 milliwatts radiant power. Class 3b Lasers - Can cause severe eye injuries if beams are viewed either directly or as a result of specular reflection. Ex. visible or invisible lasers operating at power less than 500 milliwatts for continuous wave lasers or less than 10 J/cm 2 for a 0.25 s pulsed laser. Class 4 lasers - Hazard to the eye from direct beam and specular reflections and sometimes even from diffuse reflections. Can also start fires and damage skin. Ex. Lasers operating at power levels greater than 500 mW for continuous wave lasers or greater than 0.03 J for a pulsed system.

Optical Resonant Cavity Mirror Total reflection Mirror Partial reflection Pumping Source Atoms

Pumping Source Pump Cycle Excited Atoms

Pumping Source Emission and Lasing Pumping Source

Frontiers of laser science -Using blue light to read CD/DVD instead of red light… Shorter wavelength means that way more information can be stored on a disc. - Multiline lasers capable of producing different colors of light consecutively hold much potential for spectroscopy. - X-ray lasers will allow better surface characterization and biomolecule imaging should be allow us to observe molecules undertaking key biological processes while still in a cell. - Tunable lasers for fiber optic networks will increase speed by moving data to different wavelengths and making faster color laser printers. - More precise and smaller band spreading lasers for dermatological surgery, especially for ethnic skin. - Quantum cascade lasers for precise measuring of fluids and gravity waves.

Some Laser Wavelengths MediumTypeWavelength (nm) RubySolid State694 HeNeGas594, 612, 633, 1152 Nd:YAGSolid State1064, 532 (doubled) ArFGas193 XeClGas308 Er:GlassSolid State1540 CO 2 Gas10600 InGaAlPSemiconductor GaAs/GaAlAsSemiconductor RhodamineDye ArGas364, 514