what do we study and do? - jacobs school of...

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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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Lecture 4

Semiconductor Fabrication,

Laser

3

I am still confused about energy levels

•  A. Yes

•  B. No

4

I am still confused about energy band gap

•  A. Yes

•  B. No

5

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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The Fabrication of Integrated Circuits (or LEDs, lasers)

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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

12

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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Population Inversion

Optical resonant cavity

Mirror Mirror

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Purpose of the Cavity

Natural Emission

Cavity Modes

λ

Standing wave

Monochromatic Directional

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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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Output Power vs. Pump Rate

http://www.ni.com/white-paper/14878/en/

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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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Ruby Laser

http://universe-review.ca/F13-atom07.htm

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Ruby Laser

~4 millisec

Heat

http://universe-review.ca/F13-atom07.htm

Xenon-filled flash tube

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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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He-Ne Laser

http://www.thorlabs.com/images/TabImages/HeNe_EnergyLevels.jpg

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He-Ne Laser

http://en.wikipedia.org/wiki/Helium%E2%80%93neon_laser

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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


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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


(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)


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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How a Laser Works

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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)

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In a scale of 1 (easy) to 5 (difficult), rate today’s lecture on laser.

•  A. 1 (easy)

•  B. 2

•  C. 3

•  D. 4

•  E. 5 (difficult)

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