g = Γ . G - (α i + α m ) Active region Waveguide Cladding Contact Waveguide Cladding Contact To 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
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g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated
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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
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.