LASERs Joshua Bedsole December 2 nd, 2014 Fondren Science Building Room 158 Southern Methodist University Physics Department Bedsole1 12/2/14, Southern Methodist University Physics Department outlet-for-red-green-and-blue-laser-beams-1205-p.asp

A Look Ahead Interaction of photons and atoms LASER production and components Types of LASERs LASER – what does it mean? Bedsole2 12/2/14, Southern Methodist University Physics Department

Practical Applications of LASERs Bedsole3 Medical – LASER eye surgery to remove cataracts. Assembly Lines – automobile industry uses LASERs for precision cutting Welding – welding stainless steel to copper in cooking pots Telecommunications 12/2/14, Southern Methodist University Physics Department

LASERs Light Amplification by Stimulated Emission of Radiation Stimulated Emission? – More on this later! Coherent vs. Incoherent Light Bedsole4 12/2/14, Southern Methodist University Physics Department

Photon Interaction with Atoms Spontaneous Emission – incoherent Absorption Stimulated Emission – coherent Bedsole5 12/2/14, Southern Methodist University Physics Department

Equations of the Above Phenomena R spon =A spon N 2 R abs =B abs N 1 Y(ΔE) R stim =B stim N 2 Y(ΔE) Bedsole6 Einstein A and B Coefficients What is Y(ΔE)??? 12/2/14, Southern Methodist University Physics Department Principle of Detailed Balance

Y(ΔE) Bedsole7 A spon N 2 + B stim N 2 Y(ΔE) = B abs N 1 Y(ΔE) Y(ΔE) = [A spon /B abs ]/[exp(ΔE/k B T)-B stim /B abs ] Y(ΔE)=[A spon /B abs ]/[(N 1 /N 2 )-(B stim /B abs )] Since the gas atoms are diffuse, they follow the Boltzmann distribution. The Boltzmann distribution is just a way to describe the probability that a particle is at a certain energy level. P(E n ) = A*exp(-E n /k B T) So, N 1 /N 2 becomes exp(-E 1 /k B T)/exp(-E 2 /k B T) = exp(ΔE/k B T) 12/2/14, Southern Methodist University Physics Department

Y(ΔE) Bedsole8 Since Y(ΔE) is the number of photons of energy ΔE, we know that it must be the spectral energy density dU=[hf 3 ]/[exp(hf/k B T)-1] So, B stim = B abs So how do we ensure that stimulated emission is more likely than absorption? Make N 2 >>N 1 – Population Inversion This is unnatural, so it must be set up by external means – more on this soon! 12/2/14, Southern Methodist University Physics Department

Components of a LASER Bedsole9 Mirrors/Resonant Cavity Optical Pumping Electric Discharge Pumping 12/2/14, Southern Methodist University Physics Department

Types of LASERs Bedsole10 Three Level LASER Metastable State Four Level LASER 12/2/14, Southern Methodist University Physics Department

In Conclusion Bedsole11 Light Amplification by Stimulated Emission of Radiation Photon/Atom Interaction Y(ΔE) Population Inversion LASER Components Types of LASERs 12/2/14, Southern Methodist University Physics Department green-lasers-beyond-100-mW.html

Sources Bedsole12 Modern Physics, Second Edition. Randy Harris The Physics of Lasers in general and Lasers for Telecommunication Femtosecond laser-assisted cataract surgery – American Society of Cataract and Refractive Surgery Cataract%20Surgery%20Review_0.pdf 12/2/14, Southern Methodist University Physics Department