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LasersStimulated EmissionLasers: Trapping PhotonsTerahertz LasersCourse OverviewP-N Junctions and LEDsHigh energy electrons (n-type) fall into low energy holes (p-type)

Emitted LightBeamsDiodeTransparentPlastic CaseTerminal Pins

Energy ConservationWstored = __________WlightWelectricalWheat

WlightEnergy ConservationWstored = __________

Wstored = electron energyWlightWelectricalWheatThrough and Across VariablesTHROUGHACROSSELECTRICALLIGHT

Atomic Transitions

2p1sphotonr

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Light Emission from MagnetsMaxwells Equations couple H and E fields..

Image in the Public DomainCourtesy of Bala Krishna Juluri and Sophocles Orfanidis. Used with permission.Radiation was missing from our quasi-static approximationhttp://juluribk.com/2010/01/14/radiation-from-dipole/ Light Emission from Magnets

external fieldhigh energylow energy

Superposition state = oscillating magnet

Images in the Public Domain

Classical: Oscillating electric field drives charge oscillation

Quantum: Electric field creates superposition of energy states which have an oscillating charge densitySolar Cells and PhotodetectorsEmission

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Reverse Absorption: Stimulated Emission

ABSORPTIONSTIMULATED EMISSIONHow do you choose the color, direction, and phase of the generated photon ?GENERATED PHOTON IS AN EXACT DUPLICATE OF THE INCOMING PHOTONQuantum Mechanics and Stimulated EmissionPauli Exclusion and electrons (fermions)Stimulated emission and photons (bosons)Two is a crowd !The More the Merrier !

boson

Boson

boson !

Boson

also boson

also boson

also boson

also boson

fermion

fermion

fermion!

FermionFERMIONS GO TO DIFFERENT STATESBOSONS PREFER TO BE IN THE SAME STATE

00.511.522.533.544.55-1-0.8-0.6-0.4-0.200.20.40.60.81z/l0Ex+ (z,t)

Quantum Mechanics and Stimulated Emission

The astounding phenomenon is Stimulated Emission a purely quantum phenomenon !Two identical photons!Stimulated Emission: If one photon is present it is more likely that an atom will emit a second identical photon! In a laser there is a cascade that causes emission of many identical photons! Photon emitted by some other atomIdentical photons with the same frequency moving in the same direction Result is a coherent light source with a highly directional beam !Lasers

13Semiconductor LasersAll images are in the public domain

Active Devices for DVD PlayersDetectorLaser strained QW at 655 nmDiffraction GratingPolarizing PrismCylindrical LensCollimator Lens2-axis Device Wave Plate

All images are in the public domain

CONDUCTIONBANDelectronholeVALENCEBANDMETALCONTACTMETALCONTACTVp-type semiconductorn-type semiconductorQuantum Well LasersNARROW GAPACTIVE REGION

waveguidesresonatorsTrapping Photons: Mirrors and WaveguidesHow do we keep photons around for long enough time so they have a chance to stimulate an emission ?

boson

Boson

boson !

Boson

also boson

also bosonINTERSUBBAND LASER: chosen by design Unipolar: electrons make intraband transitionsSame subband dispersion

Longest Wavelength Semiconductor LasersINTERBAND LASER: set by bandgap Bipolar: electron-hole recombinationOpposite band dispersion

Conduction BandValence Band

Conduction Band

Quantum-Cascade Lasers(slide courtesy of Prof. Jerome Faist at Univ. Neuchtel)

Groupe de physique msoscopiqueInstitut de physique,Universit de NeuchtelCascade: N repetitions of a period 1 electron traveling through this structure may generate N photons

Courtesy of Jerome Faist. Used with permission.19

metalQuantum wellsCourtesy of Qing Hu, Millimeter-wave and Terahertz Devices Group at MIT. Used with permission.

Metal Mirror WaveguidesMetals are excellent reflectors at THz frequencies

Courtesy of Qing Hu, Millimeter-wave and Terahertz Devices Group at MIT. Used with permission.WORK AND ENERGYELECTRODYNAMICS

6.007 Applied E&M From Motors to Lasers

The course encompassed THREE THEMES with FIVE related LABS

QUANTUM MECHANICS

6.007 Applied E&M From Motors to Lasers

The course encompassed THREE THEMES with FIVE related LABSMEASUREMENT AND UNCERTAINTY - Photon Momentum- Heisenberg Microscope

ELECTRON EIGENSTATES- Calculating WavefunctionsParticle in a Box - Atoms and Quantum Dots

QUANTUM ELECTRONICS - Tunneling (STM, Flash)Energy Bands/ Conduction - Energy Band TransitionsPhotodetectors, Solar CellLED and Lasers LAB TUNNELING TOUCHPADENERGY CONVERSION and STORAGE- Energy Conservation- Across and Through Vars.- Energy Storage LAB: MOTORS

ENERGY/POWER/WORK in BASIC CIRCUIT ELEMENTS - Electric/Magnt Materials- Energy Method for Motors- Magnetostatic / Electrostatic Machines- Micro-Electro Machines LAB: COIL GUN

- Limits of StaticsEM WAVES Wave Equation - Energy in the EM Waves Polarized Light

MATERIALS RESPONSE- Lorentz Oscillator- Reflection, Absorption- Complex Refractive Index - Evanescent Waves LAB LIQUID CRYSTAL DISPLAY

DEVICES AND PHYSICS Polarizers/Birefringence LAB: FIBEROPTICS

- Photon as a Quantum of EnergyWORK AND ENERGYELECTRODYNAMICSQUANTUM MECHANICSMIT OpenCourseWarehttp://ocw.mit.edu6.007 Electromagnetic Energy: From Motors to LasersSpring 2011For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.