Lasers
• Spontaneous emission and stimulated emission
• Application of Lasers• Classification of lasers according to the way of
pumping• Laser diodes
– What is semiconductor? – p/n junction diode– Light emitting diode and laser diode
What is Laser?
• Spontaneous and stimulated emission
• Different pumping methods
• Characteristics of laser light
Spontaneous and stimulated emission
• Spontaneous emission : Light emission by relaxation from the excited state to the ground state
• stimulated emission : Light emission due to optical transition forced by optical stimulation;
• This phenomenon is the laser=light amplification by stimulated emission of radiation
Optical transition• Transition occurs from
the ground state 1 to the excited state 2 with the probability of P12 by the perturbation of the electric field of light: This is an optical absorption.
• The excited state 2 relaxes to the ground state 1 spontaneously with a light emission to achieve thermal equilibrium
1
2
p12 Optical absorption
Energy
1
2
Spontaneous emission
Stimulated emission• Transition from the
excited state 2 to the ground state 1 occurs by the stimulation of the electric field of incident light with the transition probability of P21(=P12), leading to emission of a photon. This process is called stimulated emission.
• The number of photons is doubled since first photon is not absorbed.
1
2
p21Stimulated emission
1
2
p12 Stimulated emission
Energy
E
Emission is masked by absorption under normal condition
• Under normal condition stimulated emission cannot be observed since absorption occurs at the same probability as emission (P12=P21), and the population N1 at 1 dominates N2 at 2 due to Maxwell-Boltzmann distribution. Therefore, N2P21<N1P12
1
2
p21Stimulated emission
1
2
p12 Optical absorption
N2
N2
N1
N1
Maxwell-Boltzmann distribution
• The population at the excited state 2 located at E above the ground state 1 is expressed by a formula exp(-E/kT)
1
2
Distribution function
Energ
y1
E
exp(-E/kT)
0
population inversion for lasing• In order to obtain net emission (N2P21>N1P12),
N2, the population of the state 2 should exceed N1, the population of the state 1.
• This is called population inversion, or negative temperature, since the distribution feature behaves as if the temperature were negative.
1
2
Distribution function
Energ
y 1
E
exp(E/kT)
0
Characteristics of laser
• Oscillator and amplifier of light wave• Wave-packets share the same phase leading to
Coherence: two different lasers can make interference fringes
Directivity: laser beam can go straight for a long distance
Monochromaticity: laser wavelength is “pure” with narrow width
High energy density: laser can heat a substance by focusing
Ultra short pulse: laser pulse duration can be reduced as short as femtosecond (10-15 s)
• Bose condensation quantum state appearing macroscopically
Application of lasers
• Optical Communications
• Optical Storages
• Laser Printers
• Diplays
• Laser Processing
• Medical Treatments
Optical fiber communication Optical fiber communication system
Multi-plexer
Electro-optical conversion
Laser diode
Amplifier
Photodiode
Opto-electronic Conversion
Demulti-plexer
Optical fiber
Laser Printers
http://web.canon.jp/technology/detail/lbp/laser_unit/index.html
scanner motor/ motor driver
laser diode/ laser driver
cylindrical lensopt. box
horizontal syncmirror
polygon mirror
spherical lens
toric lens
BD lensphotosensitive drumComputer
optical fiber
DC controllerBD signal BD signal video signal
controller
Classification of lasersaccording to the way of pumping
• Gas lasers :eg., He-Ne, He-Cd, Ar+, CO2,
pump an excited state in the electronic structure of gas ions or molecules by discharge
• Solid state laserseg., YAG:Nd, Al2O3:Ti, Al2O3:Cr(ruby) :pump an excited state of luminescent center (impurity atom)
by optical excitation• Laser diodes (Semiconductor lasers)
eg., GaAlAs, InGaNhigh density injection of electrons and holes to active layer of
semiconductor through pn-junction
HeNe laser, how it works
http://www.mgkk.com/products/pdf/02_4_HeNe/024_213.pdf
•He atoms become excited by an impact excitation through collision•The ground state is 1S (1s2; L=0, S=0) and the excited states are 1S (1s12s1 ; L=0, S=0) and 3S (1s12s1 ; L=0, S=1)•The energy is transferred to Ne atoms through collision.•Ne has ten electrons in the ground state 1S0 with 1s2 2s2 2p4 configuration, and possesses a lot of complex excited states
He Ne
1S
21S
23S
HeNe laser: different wavelengths
• 3.391 m mid IR
• 1.523 m near IR
• 632.8 nm red 赤• 612 nm orange 色• 594 nm yellow 黄色• 543.5 nm green グ
リーン
He Ne
1S
21S
23S
Application of gas laser
Ar ion laser• Illumination (Laser show)• Photoluminescence
Excitation Source
Solid state laser
YAG laser YVO4laser• YAG:Nd• 1.06m• Micro fabrication• Pumping source for SH
G
http://www.fesys.co.jp/sougou/seihin/fa/laser/fal3000.html
Solid state laser
Ruby laser• Al2O3:Cr3+
• Synthetic ruby single crystal• Pumped by strong Xe lamp • Emission wavelengths; 694.3nm• Ethalon is used to select a wavel
ength of interest
Ruby rod
Ruby laser
LD (laser diode)
• Laser diode is a semiconductor device which undergoes stimulated emission by recombination of injected carriers (electrons and holes), the concentration being far greater than that in the thermal equilibrium.
What is semiconductor?• Semiconductors possess electrical conductivity
between metals and insulators
Resistivity (cm)
Conductivity (S/cm)
Ene
rgy
band
gap
(eV
)
Ene
rgy
band
gap
(eV
)
semiconductor
metal
insulator
diamond
Electric resisitivity of K
Temperature (K) Temperature (K)
Ele
ctric
res
itivi
ty (
cm)
Ele
ctric
res
itivi
ty (
cm)
log
scal
e
Temperature dependence of electrical conductivity in metals and semiconductors
• Resistivity of metals increases with temperature due to electron scattering by phonon
• Resistivity of semiconductors decreases drastically with temperature due to increase in carrier concentration
Conductivity, carrier concentration, mobility
• Relation between conductivity and carrier concentration n and mobility
= ne• Resistivity and conductivity is related by
=1/• Mobility is average velocity v[cm/s] introduced
by electric field E[V/cm] , expressed by equation v= E
Periodic table and semiconductors
IIB IIIB IV V VI
B C N O
Al Si P S
Zn Ga Ge As Se
Cd In Sn Sb Te
Hg Tl Pb Bi Po
IV (Si, Ge)III-V (GaAs, GaN, InP, InSb)II-VI (CdS, CdTe, ZnS, ZnSe)
I-VII (CuCl, CuI)I-III-VI2 (CuAlS2 , CuInSe2)II-IV-V2 (CdGeAs2, ZnSiP2)
Crystal structures of semiconductors
• Si. Ge: diamond structure• III-V, II-VI: zincblende structure
• I-III-VI2, II-IV-V2: chalcopyrite structure
Diamond structure
Energy band structure for explanation of metals, semiconductors and insulators
Fermi level
3s,3pConduc
tionband
3s,3pValence
band
3sband
2pshell
2sshell
1sshell
2pshell
2sshell
1sshell
3s,3pConduc
tionband
3s,3pValence
band
intrinsic extrinsic
Metals SemiconductorsInsulatorsand semiconductors at 0K
Difference of metals, semiconductors and insulators
Concept of Energy BandTwo approaches
• Approximation from free electron– Hartree-Fock approximation– Electron is treated as plane waves with wavenumb
er k– Energy E=(k)2/2m (parabolic band)
• Approximation from isolated atoms– Heitler-London approximation– Linear combination of s, p, d wavefunctions
Band gap of silicon
Si-Si distanceSchematic illustration of variation of electronic states in silicon with Si-Si distance
valence band
conduction band
lattice constant of Si
covalent bondingisolated
atom
Energy gap
3p
3s
Ene
rgy
Bonding orbitals
Antionding orbitals
Band gap and optical absorption edge
・When photon energy E=h is less than Eg, valence electrons cannot reach conduction band and light is transmited.・When photon energy E=h reaches Eg, optical absorption starts.
h/1240
valence band
h>Egh Eg
conduction band
Color of transmitted light and band gap
1.5eV
CdS
GaP
HgS
GaAs
3eV 2.5eV 2eV
800nm300nm
ZnS
Eg=2eV
Eg=2.2eV
Eg=2.6eV
Eg=3.5eV
Eg=1.5eV
白
黄
橙
赤
黒
3.5eV4eV
transparent region
Semiconductor pn junction
N typeP type
++++
----
Carrier diffusion takes place when p and n semiconductors are contacted
space charge potential
Energy
space charge potential
+
-
LED, how it works?
• Forward bias to pn junction diode• electron is injected to p-type region• hole is injected to n-type region• Electrons and holes recombine at t
he boundary region• Energy difference is converted to p
hoton energy
p n
recombination
Space charge layer
++++
----
electron
+ -
electron drift
hole drift
recombination
light emission
electronhole
energy gapor
band gap
hc
hE (nm)
8.1239(eV)
E
Semiconductors for LD
• Optical communication : 1.5m; GaInAsSb, InGaAsP
• CD : 780nm GaAs
• DVD : 650nm GaAlAs MQW
• DVR : 405nm InGaN MQW
Double hetero structure
• Electrons, holes and photons are confined in thin active layer by using the hetro-junction structure
http://www.ece.concordia.ca/~i_statei/vlsi-opt/
Invention of DH structure (1)• Herbert Kroemer and Zhores Alferov suggested in 19
63 that the concentration of electrons, holes and photons would become much higher if they were confined to a thin semiconductor layer between two others - a double heterojunction.
• Despite a lack of the most advanced equipment, Alferov and his co-workers in Leningrad (now St. Petersburg) managed to produce a laser that effectively operated continuously and that did not require troublesome cooling.
• This was in May 1970, a few weeks earlier than their American competitors.
• from Nobel Prize Presentation Speech in Physics 2000
Invention of DH structure (2)
• In 1970, Hayashi and Panish at Bell Labs and Alferov in Russia obtained continuous operation at room temperature using double heterojunction lasers consisting of a thin layer of GaAs sandwiched between two layers of AlxGa1-xAs. This design achieved better performance by confining both the injected carriers (by the band-gap discontinuity) and emitted photons (by the refractive-index discontinuity).
• The double-heterojunction concept has been modified and improved over the years, but the central idea of confining both the carriers and photons by heterojunctions is the fundamental philosophy used in all semiconductor lasers.
from Physics and the communications industry W. F. Brinkman and D. V. Lang Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974
http://www.bellsystemmemorial.com/pdf/physics_com.pdf