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What do we study and do? • Light comes from electrons transitioning from higher
energy to lower energy levels. • Wave-particle nature of light
– Wave nature: refraction, diffraction, interference (labs) – Particle nature: photons
• Generating light – LED, laser
• Detecting light – Solar cell, photodetector
• Transmitting light – Fiber
• Manipulating light – Lens, structural coloration
• Using light (projects) – Spectrometer, holography, plastic lens,
(berry-juice) dye-sensitized solar cell
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After getting a wafer, we do microfabricationà
Photolithography Steps (1)
http://conocimientosintegratedcircuit.blogspot.com/2010/05/integrated-circuit-fabrication-process.html
Photo Resist
7 http://conocimientosintegratedcircuit.blogspot.com/2010/05/integrated-circuit-fabrication-process.html
Remove photoresist
Photolithography Steps (2)
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Properties of Laser Light
• It’s coherent – no natural source is. • Two types of coherence:
– Temporal – highly monochromatic – Spatial – highly directional
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Occupation of Energy Levels in Thermal Equilibrium
⎟⎟⎠
⎞⎜⎜⎝
⎛ −−=
TkEE
nn
B
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1
2 exp
• Boltzmann Distribution
At 300 K, for E2 – E1 = 1 eV, n2/n1 = 1.7 x 10-17 0.1 eV 2%
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How is the Boltzmann Distribution different from the probability distribution for electrons?
Electrons are fermions, and atoms are bosons.
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Optical Processes spontaneous
emission absorption stimulated emission
Light Amplification by Stimulated Emission of Radiation
Before
After
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Can population inversion achieved with a 2-level system when you “pump” energy into the system continuously?
• A – Yes
• B – No
• C – Don’t know
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Principle of LASER
E1
hυ13E2
Metastablestate
E1
E3
E2
hυ32
E1
E3
E2
E1
E3
E2
hυ21hυ21
Coherent photons
OUT
(a) (b) (c) (d)
E3
The principle of the LASER. (a) Atoms in the ground state are pumped up to the energy level E3 byincoming photons of energy hυ13 = E3–E1. (b) Atoms at E3 rapidly decay to the metastable state atenergy level E2 by emitting photons or emitting lattice vibrations; hυ32 = E3–E2. (c) As the states at E2are long-lived, they quickly become populated and there is a population inversion between E2 and E1.(d) A random photon (from a spontaneous decay) of energy hυ21 = E2–E1 can initiate stimulatedemission. Photons from this stimulated emission can themselves further stimulate emissions leading to anavalanche of stimulated emissions and coherent photons being emitted.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
IN
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Directional Beam divergence
D
φ
φ = (4/π)(λ/D)
Nd:YAG, λ = 1.06 µm, D = 3 mm, φ = 0.020o
He-Ne, λ = 0.6328 µm, D = 1 mm, φ = 0.036o
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• A – increases • B – becomes constant • C – decreases • D – don’t know
As we increase the pumping power, so does the population inversion (N2 – N1). What happens to population inversion after threshold?
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What does it take to make a laser?
• Find an appropriate material
• Find an appropriate pumping method
• Find an appropriate optical cavity
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Type of Lasers
• Doped insulator lasers – Ruby laser – Nd:YAG laser
• Gas laser – HeNe laser – Argon-ion laser – Carbon dioxide laser – Excimer laser
• Semiconductor laser
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Nd:YAG Laser
http://perg.phys.ksu.edu/vqm/laserweb/ch-6/f6s2t2p2.htm
How many level (broadly speaking) system is this?
Is it easier to achieve population inversion than a ruby laser? Why?
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Nd:YAG Laser
http://directedlight.com/laser-components/catalog/nd-yag-rods/
neodymium ions in yittrium aluminum garnet
Ophthalmology Skin cancer removal Prostate surgery Hair removal Engraving Drilling Rangefinder Spectroscopy
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Carbon Dioxide Laser
http://www.intechopen.com/books/laser-pulses-theory-technology-and-applications/longitudinally-excited-co2-laser
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Applications of CO2 Lasers
• Medical, laser scalpel – 80% of soft biological tissue (skin) is water, which
absorbs light at ~10 µm wavelength. – à Penetration length ~ 50 µm. – Vaporizes tissue
• Industrial – Laser cutting (~100-200 W)
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Semiconductor Laser – Population Inversion p+ n+
EF n
(a)
Eg
Ev
Ec
Ev
Holes in V BElectrons in C B
Junction
Electro ns Ec
p+
Eg
V
n+
(b)
EF n
eV
EF p
The energy band diagram of a degenerately doped p-n with no bias. (b) Banddiagram with a sufficiently large forward bias to cause population inversion andhence stimulated emission.
Inv ers ionreg io n
EF p
EcEc
eVo
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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Semiconductor Laser Diode
L Electrode
Current
GaAs
GaAsn+
p+
Cleaved surface mirror
Electrode
Active region(stimulated emission region)
A schematic illustration of a GaAs homojunction laserdiode. The cleaved surfaces act as reflecting mirrors.
L
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
(001)
(110) Cleavage plane
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Light Output vs. Current (L-I Curve)
threshold
http://www.newport.com/Tutorial-Laser-Diode-Technology/852182/1033/content.aspx
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Heterostructure Laser
Refractiveindex
Photondensity
Activeregion
Δn ~ 5%
2 eV
Holes in VB
Electrons in CB
AlGaAsAlGaAs
1.4 eV
Ec
Ev
Ec
Ev
(a)
(b)
pn p
ΔEc
(a) A doubleheterostructure diode hastwo junctions which arebetween two differentbandgap semiconductors(GaAs and AlGaAs).
2 eV
(b) Simplified energyband diagram under alarge forward bias.Lasing recombinationtakes place in the p-GaAs layer, theactive layer
(~0.1 µm)
(c) Higher bandgapmaterials have alower refractiveindex
(d) AlGaAs layersprovide lateral opticalconfinement.
(c)
(d)
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
GaAs
Nobel Prize in Physics 2000
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Common Lasers
Type" Wavelength" Power" Efficiency"
Helium-Neon" 632.8nm" 0.1-50mW" <0.1%"
Ruby" 694.3nm" 0.03-100J" <0.5%"
Carbon-Dioxide" 10.6µm" 3-100W" 5-15%"
Nd:YAG" 1.064µm" 0.04-600W" 0.1-2%"
Argon" 488/514nm" 5mW-20W" <0.1%"
Dye" 400-900nm" 20-800mW" 10-20%"
Hydrogen-Fluoride" 2.6-3µm" 0.01-150W" 0.1-1%"
Gallium-Arsenide" 780-900nm" 1-40mW" 1-20%"
# photons/#electrons
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What We Learned
Principles of laser
Population inversion in a 3-level system
Optical cavity
Types of laser
Ruby laser
YAG laser
HeNe laser
Semiconductor laser
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In a scale of 1 (easy) to 5 (difficult), rate today’s lecture on semiconductor fabrication.
• A. 1 (easy)
• B. 2
• C. 3
• D. 4
• E. 5 (difficult)