Stimulated Emission Lasers Trapping Photons Terahertz Lasers Course Overview PN Junctions and LEDs High energy electrons ntype fall into low energy holes ptype Emitted Light Beams Diode ID: 328623
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Slide1
Lasers
Stimulated Emission
Lasers: Trapping PhotonsTerahertz LasersCourse OverviewSlide2
P-N Junctions and LEDs
High energy electrons (n-type) fall into low energy holes (p-type)
Emitted Light
Beams
Diode
Transparent
Plastic Case
Terminal PinsSlide3
Energy Conservation
W
stored = __________
W
light
W
electrical
W
heatSlide4
W
light
Energy Conservation
W
stored
= __________Slide5
W
stored
= electron energy
W
light
W
electrical
W
heat
Through and Across Variables
THROUGH
ACROSS
ELECTRICAL
LIGHTSlide6
Atomic Transitions
2p
1s
photon
rSlide7
Light Emission from Magnets
Maxwell
’s Equations couple H and E fields..
Image in the Public Domain
Courtesy of
Bala
Krishna
Juluri
and Sophocles
Orfanidis
. Used with permission.
Radiation was missing
from
our quasi-static approximation
http://juluribk.com/2010/01/14/radiation-from-dipole/ Slide8
Light Emission from Magnets
external field
high energy
low energy
Superposition state = oscillating magnet
Images in the Public DomainSlide9
Classical
: Oscillating electric field drives charge oscillation
Quantum: Electric field creates superposition of energy states – which have an oscillating charge densitySolar Cells and Photodetectors
Emission
2p
1s
photon
rSlide10
Reverse Absorption: Stimulated Emission
ABSORPTION
STIMULATED EMISSION
How do you choose the color, direction, and phase
of the generated photon ?
GENERATED PHOTON IS
AN
EXACT DUPLICATE
OF THE INCOMING PHOTONSlide11
Quantum Mechanics and Stimulated Emission
Pauli 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!
Fermion
FERMIONS GO TO DIFFERENT STATES
BOSONS PREFER TO BE IN THE SAME STATESlide12
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
z/
l
0
E
x
+
(z,t)
Quantum Mechanics and Stimulated EmissionSlide13
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 atom
Identical photons with the same frequency moving in the same direction –
Result is a
coherent light source with a highly directional beam !
LasersSlide14
Semiconductor Lasers
All images are in the public domainSlide15
Active Devices for DVD Players
Detector
Laser strained QW at 655 nm
Diffraction Grating
Polarizing Prism
Cylindrical Lens
Collimator Lens
2-axis Device
¼ Wave Plate
All images are in the public domainSlide16
CONDUCTION
BAND
electron
hole
VALENCE
BAND
METAL
CONTACT
METAL
CONTACT
V
p-type semiconductor
n-type semiconductor
Quantum Well Lasers
NARROW GAP
ACTIVE REGIONSlide17
waveguides
resonators
Trapping Photons: Mirrors and Waveguides
How 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
bosonSlide18
INTERSUBBAND LASER:
chosen by design Unipolar: electrons make intraband transitionsSame subband dispersion
Longest Wavelength Semiconductor LasersINTERBAND LASER:
set by
bandgap
Bipolar: electron-hole recombination
Opposite band dispersion
Conduction Band
Valence Band
Conduction BandSlide19
Quantum-Cascade Lasers(slide courtesy of Prof. Jerome Faist at Univ.
Neuchâtel)
Groupe de physique mésoscopiqueInstitut de physique,Université de Neuchâtel
Cascade
: N
repetitions
of a
period
1
electron traveling through this structure may generate N photons
Courtesy of Jerome
Faist
. Used with permission.Slide20
metal
Quantum wells
Courtesy of Qing
Hu
,
Millimeter-wave and Terahertz Devices Group
at MIT. Used with permission.
Metal Mirror Waveguides
Metals are excellent reflectors at THz frequencies
Courtesy of Qing
Hu
,
Millimeter-wave and Terahertz Devices Group
at MIT. Used with permission.Slide21
WORK AND ENERGY
ELECTRODYNAMICS
6.007 – Applied E&M – From Motors to Lasers
The course encompassed THREE THEMES with FIVE
related
LABS
QUANTUM MECHANICSSlide22
6.007 – Applied E&M – From Motors to Lasers
The course encompassed THREE THEMES with FIVE related LABSMEASUREMENT AND UNCERTAINTY
- Photon Momentum- Heisenberg MicroscopeELECTRON EIGENSTATES- Calculating WavefunctionsParticle in a Box - Atoms and Quantum DotsQUANTUM ELECTRONICS - Tunneling (STM, Flash)Energy Bands/ Conduction - Energy Band TransitionsPhotodetectors, Solar CellLED and Lasers•• LAB •• TUNNELING TOUCHPAD
ENERGY 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 LightMATERIALS 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 ENERGYELECTRODYNAMICS
QUANTUM MECHANICSSlide23
MIT
OpenCourseWarehttp://ocw.mit.edu6.007 Electromagnetic Energy: From Motors to Lasers
Spring 2011For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.