g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no...
Transcript of g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no...
g = Γ.G - (αi + αm)
Active region
Waveguide
Cladding
Contact
Waveguide
Cladding
ContactTo achieve maximum
material gain G at given l
To provide excellent injection properties with minimum optical loss α
and heating
To optimize optical
confinement Γ
Semiconductor laser design goals
Semiconductor Lasers: 2D vs. 3D confinement
electrons E
)(3 EDρ
holes
double-heterostructure laser
Eg
quantum well (QW) laserelectrons
holes
Ec1-h1
E
)(2 EDρ
c
v
3D joint DOS
νh
Eg
gr Ehmh −⎟⎠⎞
⎜⎝⎛= ν
πνρ
23
222
21)(
h
c1
h1
c2
h2
2D joint DOS
νh
2)(hπ
νρ rmh =
11111 −−−−− +=−= hcvcr mmmmm
Ec1h1 Ec2h2
hν
hν
c1
h1
c2
h2
hνhν
hν
hν
hνhν
Advantages of intersubband scheme:
Lasing wavelength is no longer defined by Eg and can be tuned by the QW width
Hole transport is eliminated
δ-like joint DOS provides for higher gain and better temperature stability
Monopolar transport offers electron recycling
electrons
electrons
holes
Eg >> hν
hν
electrons
holes
Eg,eff ≈ hνhν
Wavelength is restricted by
material bandgap
Wavelength is uncoupledto the material bandgap
Interband and Intersubband lasing
Interband and Intersubband Lasing
interband lasing scheme intersubband lasing scheme
Bipolar laser: electron AND hole
injection
monopolar transportoffers electron recycling
electrons
holes
electrons
electrons
holeshole transport iseliminated
lasing wavelength is nolonger defined by Eg
lasing wavelength can betuned by the QW width
Eg,eff ≈ hν
Eg >> hν
hν
δ-like joint DOS providesfor higher gain and bettertemperature stability
fast electron relaxationallows HF modulation
Advantages:
electrical injection
optical pumping
Ωh hνhν
hν
• In intersubband lasers δ-like joint density of states provides for higher optical gain
• Transparency current is negligible in intersubband lasers due to small electron population in the lower lasing states
(2)
(1)
K
Monopolar laser)1()2(
ee ff >
hν
Bipolar laser
(c)
(v)
)()( ve
ce ff >
hνhν
[ ])()()()( 12 KfKfhhG −∝ νρν
δ-like joint DOS provides for higher gain
Optical Gain
interband lasing intersubband lasing
Bipolar laser
(c)
(v)
(2)
(1)
K
Monopolar laser)1()2(
ee ff >)()( ve
ce ff >
hνhνhν
[ ])()()()( 12 KfKfhhG −∝ νρν
)()()()(12 KVKV
KE−
=ρρ 2
1
122
112
2111)()()(
hh πππρ rD
mmmdK
KdEdKKdEKE =−=−=
−−
Intersubband Kinetics
(2)
(1)nrR
21τ
(0)
K
(2)
(1)
rR 11τ
2110 ττ <<
Ec
(2)
(1)(0)
3-level scheme
LOhE ν≈10
LOhE ν>21
K2 K1
K0
LOhvE ≈10
hνLO
hνLO
hνLO
hνLO
21τ
Optical phononresonance !
Tunneling
Optical phononemission
101
201
112
212 −
−−
− −∝<<−∝ KKMKKM
LOhvE >>21
( )Jnn==
1
1
2
2ττ
1221 nn >⇒<ττ
hν
Tunneling ?
Tunneling
Quantum Cascade Laser
quantum cascade laser
E ~ 50 kV/cm(2)
(1) (2)
(1) (2)
(1)
Ener
gy
Distance
(2)
(1)
E = 0
quantum well laser
νhνh
νh
νh
νh
νh
Advantages of the cascaded scheme:
• quantum efficiency in excess of 1 allows high-power RT operation;
• electric field tunability due to Stark effect;• multy-wavelength operation;• electrically uniform active region;• large confinement factor.
(1)
(1)
(1)
(1)
Laser design elements: Active Region
2QW, direct(intrawell)
Optical phononresonance
3QW, indirect(interwell)
Optical phononresonance
4QW, directtransition
Double optical phononresonance
Bound-to-continuuminjectorless
QW
SL
Continuum-to-continuum(superlattice QCL)
SL1
SL2
Tunneling
Tunneling
Tunneling
Tunneling
(2)
(1)
minibandsminigap
(2)
(1)
energylevels
tunneling
miniband transport
(2)
(1)
Laser design elements: Superlattice Injector
Laser design elements: Superlattice Injector
GaInAs/AlInAs superlattice
Lw/Lb = 6.0/1.8 nm
infinite superlattice
finite superlattice with 9 periods
(2)
(1)
E = 0
(2)
(1)
i = 1
i = 9
i = 1
i = 9
i = 1
i = 9
i = 1
i = 9
zkz k||π/d
Ene
rgy
d=Lw+Lb
minigap minibands
(2)
(1)
i = 1
(2)
(1)
i = 2
. . .+
isolated quantum wells
Advantages of the cascaded scheme:
• quantum efficiency in excess of 100%•electrically uniform active region• large confinement factor• multy-wavelength operation
Ec
(2)
(1)(0)
3-level scheme
LOhE ν≈10
LOhE ν>21
E ~ 50 kV/cm(2)
(1) (2)
(1) (2)
(1)
Ener
gy
Distance
(2)
(1)
E = 0
νh
νh
νh
Monopolar transport offers electron recycling
60 nm52
0 m
eV
3
2
activeregion
injector
injector
e
activeregion
Doped injectors separate
active regions electrically
Courtesy of Claire Gmachl - Princeton University
Intersubband-based QC-laser λ ~ 7.5 µm
F. Capasso et al. Physics Today 55, 34 (May 2002)
Real laser design: three level QCL
(x 25 stages)
8 µm
ban
d o
ffse
t 725 m
eV
QW1 QW2 QW3ACTIVE QWs - layer sequence (nm): 4.4/0.9/0.9/4.8/1.7/4.4/2.8
AQWs
INJECTOR
INJECTOR
minigap
miniband
miniband
minigap
Ga0.38In0.62As/Al0.48In0.52As
E32 = 280 meVE21 = hνLO = 30 meV
τ21 = 0.3 psτ32 = 2.6 psτ31 = 3.0 psNdop = 2.4×1011 cm-2
R. Kohler et al. APL 76, 1092 (Feb. 2000)
n-InP substrate
λ = 4.5 µm, pulsed
Voltag
e (V
)
Peak
outp
ut
pow
er (
mW
)
0 1 2 3 4
10
8
6
4
2
0
400
300
200
100
0
Current (A)
cladding regionactive regioninsulating layertop metal contact
Related problem: Active Region Heating
Double-phonon depopulationscheme
M. Beck et al. Science 295, 301 (2002)
E3 - E2 = E2 - E1 = hνLO~30 meVE4 - E3 = 135 meV (λ = 9.1 µm)
CW-RT Operation:• buried stripe geometry• epilayer-down mounting
(x 35 stages)
TAR = THS + U×Jth/G ; ∆T = TAR-THS
Jth = J0exp(TAR/T0)
J0= 560 A/cm2 ; T0= 170KJth~ 4.3 kA/cm2 (RT, CW)Jth~ 3.1 kA/cm2 (RT, pulsed)
hνLO
AR
HS
Pea
k w
avel
ength
(µm
)
Pea
k en
ergy
(meV
)
250
225
200
175
150
8
7
6
5
8 10 20 30
Ultra-broadband QCL
C. Gmachl et al. Nature 415, 883 (Feb. 2002)
Active region index
Many-Wavelength Operation:• all stages are designed for different wavelengths(heterogeneous cascade);
• optical gain compensates opticalloss (Ith= const) in the wholewavelength spectrum.
Wavelength (µm)
Wav
eguid
e lo
ss
Modal
gai
n
5 6 7 8
Wavelength (µm)
Optica
l pow
er (
arb.u
n.)
5 6 7 8 9
10
1
0.1
2 A3 A4 A
5 A9 A
Wav
elen
gth
(µm
)
Grating period (µm)
neff= 3.18
C. Gmachl et al. IEEE Journal QE 38, 569 ( 2002)
0.5 1.0 1.5 2.0
20
15
10
5
Wavelength (µm)
Pow
er (
log)
Pow
er (
linea
r)
Wavelength (µm)8.6 8.7 8.8 8.5 8.6 8.7
DFB and Tunable QCL
M.Kisin and S Luryi. Appl. Phys. Lett. 82, 847 ( 2002)
[110]
λac
[001]z
λ0 = 2 λac
Piezo-Acoustic Wave
( )αλπβ −Γ−= Gineff 2
2
DFB mechanisms:
• refractive index modulation;• optical gain modulation.
Tem
per
ature
(K)
Wavelength (µm)
T (
%)CO2 CO2H2O H2O
4 5 6 7 8 10 12 14 18
4.59 4.65 5.35 5.4 8.5 8.6 9.5 9.6 9.9510.05
16.2 16.22
100
200
300
0100
0
Applications Example: Environmental Monitoring
Mid IR spectrum is called molecular fingerprint region.
Two atmospheric transparency windows3-5 µm and 8-13 µm lack water-vaporabsorption and are particularly importantfor chemical-sensing applications.
Advantages of laser-based optical methodsin trace-gas analysis include:
• noninvasive character,• high sensitivity and selectivity,• real-time detection.
Other exemplary applications:
• combustion diagnostics in the power and automobile industries, medical diagnostics, • detection of explosives and drugs, chemical and biological weapons of mass destruction,• military countermeasures as blinding the IR sensor of a heat-seeking missile,• optical wireless communications in the eye-safe atmospheric transmission windows.
TIME
Carbon monoxide concentrations in ambient air(monitored at Rice University on March 2001, A. Kosterov et al.)
CARBO
N M
ON
OXID
E C
ON
CEN
TRATIO
N(p
arts
per
bill
ion in v
olu
me)
500
400
300
200
Mid
nig
ht
8 A
M
F. Capasso et al. Physics Today 55, 34 (May 2002)
Recommended Literature
• J. Faist et al. Science (Apr. 1994), v.264, p.553.• J. Faist et al. Nature (June 1997), v.387, p.777.• C. Gmachl et al. Nature (Feb. 2002), v.415, p.883.• M. Beck et al. Science (Jan. 2002), v.295, p.301.• F. Capasso et al. IEEE Journal on Selected Topics in Quantum Electronics (Nov. 2000),
v.6, p.931.• J. Faist et al. IEEE Journal on Quantum Electronics (June 2002), v.38, p.533. • F. Capasso et al. Physics Today (May 2002), v.55, p.34.