Atomic model
MIT 2.71/2.710 Optics 10/20/04 wk7-b-21
Semi-classical view of atom excitations
Energy
Energy
Atom in ground state
Atom in excited state
Absorption
Spontaneous emission
Stimulated emission
E1
E2
• n1 - the number of electrons of energy E1
• n2 - the number of electrons of energy E2
2 2 1
1
( )exp
n E E
n kT
Boltzmann’s equation
example: T=3000 K E2-E1=2.0 eV
42
1
4.4 10n
n
Einstein’s theory of spontaneous and stimulated emission
Einstein’s coefficients
Probability of stimulated absorption R1-2
R1-2 = r (n) B1-2
Probability of stimulated and spontaneous emission :
R2-1 = r (n) B2-1 + A2-1
assumption: n1 atoms of energy e 1 and n2 atoms of energy e 2 are in
thermal equilibrium at temperature T with the radiation of spectral density r (n):
n1 R1-2 = n2 R2-1 n1r (n) B1-2 = n2 (r (n) B2-1 + A2-1)
2 1 2 1
1 1 2
2 2 1
/ =
1
A Bn Bn B
E1
E2
B1-2/B2-1 = 1
According to Boltzman statistics:
r (n) = =
12 1
2
exp( ) / exp( / )n
E E kT h kTn
1)exp(
/
12
21
1212
kT
h
B
BBA 1)/exp(
/8 33
kTh
ch
3
3
12
12 8
c
h
B
A
Planck’s law
The probability of spontaneous emission A2-1 /the probability of stimulated
emission B2-1r( )n :
1. Visible photons, energy: 1.6eV – 3.1eV.
2. kT at 300K ~ 0.025eV.
3. stimulated emission dominates solely when h n /kT <<1!(for microwaves: hn <0.0015eV) The frequency of emission acts to the absorption:
if h n /kT <<1.
1)/exp()(12
12
kThB
A
1
2
1
2
12
12
211
122122 ])(
1[)(
)(
n
n
n
n
B
A
Bn
BnAnx
x~ n2/n1
Population inversion
For lasing action
• Active medium• Pumping mechanism
– Optical– Electrical discharge– Chemical pumping
• Optical resonator Resonator
Laser characteristics
Carbon Di Oxide LASER Principle
The transition between the rotational and vibrational energy levels lends to the construction of a molecular gas laser. Nitrogen atoms are raised to the excited state which in turn deliver energy to the CO2 atoms whose
energy levels are close to it. Transition takes place between the energy levels of CO2 atoms and the laser beam is emitted.
Type : Molecular gas laser
Active Medium : Mixture of CO2, N2, He or H2O vapour
Active Centre : CO2
Pumping Method : Electric Discharge Method
Optical Resonator : Gold mirror or Si mirror coated with Al
Power Output : 10 kW
Nature of Output : Continuous or pulsed
Wavelength Emitted : 9.6 μm or 10.6 μm
Symmetric 100 C - stationaryO - vibrates simultaneously along molecular axis
Bending 010,020
C & O vibrate perpendicular to molecular axis
Asymmetric Stretching
001, 002
C & O atoms vibrate in opposite directions along molecular axis
Applications• Bloodless surgery• Open air
communication
• Military field
Nd (Neodymium) – YAG (Yttrium Aluminium Garnet) LASER Principle Characteristics
Doped Insulator laser refers to yttrium aluminium garnet doped with neodymium.
The Nd ion has many energy levels and due to
optical pumping these ions are raised to excited levels. During the transition from the metastable state to E1,
the laser beam of wavelength 1.064μm is
emitted
Type : Doped Insulator Laser
Active Medium : Yttrium Aluminium Garnet
Active Centre : Neodymium
Pumping Method
: Optical Pumping
Pumping Source
: Xenon Flash Pump
Optical Resonator
: Ends of rods silver coatedTwo mirrors partially and totally reflecting
Power Output : 20 kWatts
Nature of Output
: Pulsed
Wavelength Emitted
: 1.064 μm
Nd (Neodymium) – YAG (Yttrium Aluminium Garnet) LASER
Power Supply
Capacitor
Resistor
Laser Rod
Flash Tube
M1– 100% reflector mirror
M2 – partial reflector mirror
E1, E2, E3 – Energy levels of NdE4 – Meta Stable StateE0 – ground State Energy Level
ApplicationsTransmission of signals over large distancesLong haul communication systemEndoscopic applicationsRemaote sensing
Energy Level Diagram of Nd– YAG LASER
Non radiative decay
Laser1.064μm
Non radiative decay
E3
E2
E0
E1
E4
Nd
Principle• The electron in the
conduction band combines with a hole in the valence band and the recombination produces radiant energy. This photon induces another electron in the CB to combine with a hole in the VB and thereby stimulate the emission of another photon.
Type : Homojunction Semiconductor laser
Active Medium : P – N junction
Active Centre : Recombination of electrons and holes
Pumping Method
: Direct Pumping
Optical Resonator
: Polished junction of diode
Power Output : 1 mW
Nature of Output
: Continuous or pulsed
Wavelength Emitted
: 8400 – 8600 Angstrom Units
HOMOJUNCTION SEMICONDUCTOR LASER (Ga-As Laser)
P- and N-type Semiconductors
• In the compound GaAs, each gallium atom has three electrons in its outermost shell of electrons and each arsenic atom has five. When a trace of an impurity element with two outer electrons, such as zinc, is added to the crystal. The result is the shortage of one electron from one of the pairs, causing an imbalance in which there is a “hole” for an electron but there is no electron available. This forms a p-type semiconductor.
• When a trace of an impurity element with six outer electrons, such as selenium, is added to a crystal of GaAs, it provides on additional electron which is not needed for the bonding. This electron can be free to move through the crystal. Thus, it provides a mechanism for electrical conductivity. This type is called an n-type semiconductor.
Reverse-biased pn Junction
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
A reverse bias widens the depletion region, but allows minority carriers to move freely with the applied field.
Forward-biased pn Junction
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Lowering the barrier potential with a forward bias allows majority carriers to diffuse across the junction.
Applications• Compact & used in fibre optic communications• CD writer• Relieves pain• Laser printers
Excimer LASER
• Excited dimer– Short lived molecule formed from one or two
species, at least one of which is in an electronically excited state
– May not be stable in ground state• Excimer LASER:
– Electron pumped LASER– Dimer (excimer)/complex (exciplex) formation– LASER radiation: relaxation from excited state
dimer to ground state
ExcimerFunctionChemicalsCharacteristicapplications
Organic DyeChemicalsFunction Characteristicapplications
Excimer
e- + A → A*A* + B → AB* → AB + hν
ImmediatelyAB → A + B
Two important facts:1. The lower state does not exist!2. No rotational/vibrational bands
ExcimerFunctionChemicalsCharacteristicapplications
Organic DyeChemicalsFunction Characteristicapplications
Excimer LASER
ExcimerFunctionChemicalsCharacteristicapplications
Organic DyeChemicalsFunction Characteristicapplications
Energy states of an excimer
Excimer
• Excited Dimers– F2, Xe2 ect.
• Excited Complexes (Exciplex)– Combination of rare gas atoms and halogen
atoms– Ar, Kr, Xe– F, Cl, Br
ExcimerFunctionChemicalsCharacteristicapplications
Organic DyeChemicalsFunction Characteristicapplications
Excimer LASER
ExcimerFunctionChemicalsCharacteristicapplications
Organic DyeChemicalsFunction Characteristicapplications
Excimer Wavelength
Ar2 126 nm
Kr2 146 nm
F2 157 nm
Xe2 172 and 175
ArF 193 nm
CaF2 193 nm
KrCl 222 nm
KrF 248 nm
Cl2 259 nm
XeBr 282 nm
XeCl 309 nm
N2 337 nm
XeF 351 nm
•Many wavelength possibilities
• Depends upon the excited dimer
•Repetition rate from 0.05 Hz to 20 kHz
•High power:
• several 10-200 W
Excimer LASER
• Micromaching– Ink jet cartidges (drilling the nozzles)
• Radiation for changing the structure and properties of materials– Active matrix LCD monitors– Fiber bragg gratings– High temperature superconducting films
• “Short wavelength light bulb” in optical litography– Computer chips
ExcimerFunctionChemicalsCharacteristicapplications
Organic DyeChemicalsFunction Characteristicapplications
1. Introduction
Magnetostatic “wiggler” field
Relativistic electron beam
EM radiationN
S N
S N
S N
S N
S
The Free Electron Laser (FEL) consists of a relativistic beam of electrons (v≈c)
moving through a spatially periodic magnetic field (wiggler).
Principal attraction of the FEL is tunability :- FELs currently produce coherent light from microwaves through visible to UV- X-ray production via Self- Amplified Spontaneous Emission (SASE) (LCLS – 1.5Å)
(wavelength lw)
l lw /g2 << lw
PrincipleTwo beams (object beam and reference beam) are superimposed on a
holographic plate to form an image called a hologram.
PrincipleA beam of light
(reading beam) having the same wavelength as that of the reference beam used for constructing the hologram, is made to fall over the hologram, which in turn gives rise to a 3-D image in the field of view.
Extra slides
Review of Semiconductor Physics
a) Energy level diagrams showing the excitation of an electron from the valence band to the conduction band.The resultant free electron can freely move under the application of electric field.b) Equal electron & hole concentrations in an intrinsic semiconductor created by the thermal excitation of electrons across the band gap
-123 JK 1038.1 Bk
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
n-Type Semiconductor
a) Donor level in an n-type semiconductor. b) The ionization of donor impurities creates an increased electron concentration distribution.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
p-Type Semiconductor
a) Acceptor level in an p-type semiconductor.
b) The ionization of acceptor impurities creates an increased hole concentration distribution
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
The pn Junction
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Electron diffusion across a pn junction creates a barrier potential (electric field) in the depletion region.