GaAs/AlxGa1-xAs; GaxIn1-xAsyP1-y/AlxIn1-xAs on InP; InAs1-xSb/AlGa1-xSb on GaSb
Electron states in heterostructures
0 2 4 6 8 10
2
1.595
1.19
0.785
0.38
-0.025
k || (10 6cm -1)
E (e
V)
K||, cm-10 107
1.6
E, e
V
1.2
0.4
-0.025
0 40 80 120 160 200
2
1.539
1.078
0.617
0.156
-0.305
z (Å)
E (e
V)
z, A
E, e
V
1.54
0.16
1
0 200100-0.3
AlInAs GaInAs 80 A AlInAs
,...2,1;)(2
~)( 2||
22
nkkkm
kE neff
n
0 4 8 12 16 20
2
1.1
0.2
-0.7
-1.6
-2.5
k z (10 6cm -1)
E (
eV
)
8-band kp method(4 bands x 2 spins)
Ga0.47In0.53As
Bulk semiconductorsQuantum wells
0 40 80 120 160 200
2
1.539
1.078
0.617
0.156
-0.305
z (Å)
E (e
V)
z, A
E, e
V
1.54
0.16
1
0 200100
Optical transitions in quantum wells
0 2 4 6 8 10
2
1.595
1.19
0.785
0.38
-0.025
k || (10 6cm -1)
E (e
V)
K||, cm-10 107
1.6
E, e
V
1.2
0.4
-0.025
-0.3
AlInAs GaInAs 80 A AlInAs
0.7 0.86 1.02 1.18 1.34 1.5
15000
12000
9000
6000
3000
0
E (eV)
Ab
so
rpti
on
(1
/cm
)ab
sorp
tio
n
frequency
interband
intersubband
1
2
3
Intersubband transitions: dipole moment
dzzfz
zfz nmmn )()(*
Dipole matrix element:
zz
z
z
z
LzzkL
fzkL
f ~;sin1
~,cos1
~ 1221
Typical values ~ 10-100 ACompare with atomic transitions ~ 0.2-0.5 A
1
2
3
Intersubband transitions: selection rules
dzzfz
zfz nmmn )()(*
- Dipole matrix element:
f1 and f3 are even -> z13 = 0
- Only TM-polarization (E QW plane)
-500 -400 -300 -200 -100 0
0
0.2
0.4
0.6
0.8Intersubband transitions in asymmetric coupled QWs
Control of the optical response by engineering the shape (symmetry) of envelope functions and energies of ISBT
High optical nonlinearities: e.g. (2) ~ 106 pm/V
Short relaxation time ~ 1 ps: possibility of an ultrafast modulation
Add advantages of a semiconductor medium: electron transport and Stark effect under applied voltage, integration with other components
Saturation easily reached: /ezERabi
Large coherence can be excited 2/1~21
Rich nonlinear dynamics
measured since 1980s
Rui Yang’s talk
High voltage to align levels, high current => high heat dissipation
60 nm
520
me
V
3
2
activeregion
injector (n-doped)
injector (n-doped)
e
activeregion
QC lasersJ. Faist, F. Capasso, et al. Science 264, 553 (1994)
•Control of lifetimes: phonons, tunneling; need t32 > t2
•Cascading: high power when t_stim approaches T1
From sawtooth to staircase potential
jbjbjwjw lklk ,,,,E21 = Ephonon
From sawtooth to staircase potential
0 200 400 600 800
0
100
200
300
400
500
600
700
800
900
1000
1100
Energ
y (m
eV
)
Z (Å)
V = 0
0 200 400 600 800
0
100
200
300
400
500
600
700
800
900
1000
1100
En
erg
y (m
eV
)
Z (Å)
V = Vth
Fabrication: MBE or MOCVD
TEM / SEM image
55 nm
MINIGAP
MINIBAND3
2
1
g
ACTIVEREGION
INJECTOR
I
ACTIVEREGION
I
3
2
1
55 .1 nm
725 m
eV
0.9 nm thickwell and barrier
Rui Yang’s talk
Rui Yang’s talk
Mid-Far Infrared lasers
• IV-VI lead-salt diode lasers: 3-30 m, low-T
• Type II lasers
• Interband cascade lasers
• Intersubband (quantum) cascade lasers
What makes the QC-laser special?
• Wavelength agility– layer thicknesses determine emission wavelength
• Demonstrated applications in mid/far-IR gas sensing
• High optical power ~ 1W, room-T operation– cascading re-uses electrons
• Ultra-fast carrier dynamics– no relaxation oscillations
• Pure TM-polarization – efficient in-plane light coupling– Micro-lasers
• Small linewidth enhancement factor• Intrinsic “design potential”
HITRAN Simulation of Absorption Spectra (3.1-5.5 & 7.6-12.5 m)
NO: 5.26 m
CO: 4.66 m CH2O: 3.6 m
NH3: 10.6 m O3: 10 m
N20, CH4: 7.66 m
CO2: 4.3 m
CH4: 3.3 m
COS: 4.86 m
Frank Tittel et al.
Wide Range of Gas Sensing Applications• Urban and Industrial Emission Measurements
Industrial Plants Combustion Sources and Processes (eg. early fire detection) Automobile and Aircraft Emissions
• Rural Emission Measurements Agriculture and Animal Facilities
• Environmental Gas Monitoring Atmospheric Chemistry of Cy gases (eg global and ecosystems) Volcano Gas Emission Studies and Eruption Forecasting
• Chemical Analysis and Industrial Process Control Chemical, Pharmaceutical, Food & Semiconductor Industry Toxic Industrial Chemical Detection
• Spacecraft and Planetary Surface Monitoring Crew Health Maintenance & Advanced Human Life Support
Technology• Biomedical and Clinical Diagnostics (eg. non-invasive breath analysis)• Forensic Science and Security• Fundamental Science and Photochemistry
Life SciencesFrank Tittel et al.
Air Pollution: Houston, TX
Non-invasive Medical Diagnostics:Non-invasive Medical Diagnostics:Breath analysisBreath analysis
NO: marker of lung diseases
• Concentration in exhaled breath for a healthy adult: 7-15 ppb• For an asthma patient: 20-100 ppb
Appl. Opt. 41, 6018 (2002)
NH3: marker of kidney and liver diseases
Need fast and compact sensors
NASA Atmospheric & Mars Gas Sensor Platforms
Tunable laser sensors for earth’s stratosphere
Aircraft laser absorption spectrometers
Tunable laser planetary spectrometer
Frank Tittel et al.
Generation in the THz range
Why THz range is important
~ 100-1000 m, f ~ 0.3-3 THz
T-rays allow you to see through any dry optically opaque cover: envelope, clothing, suitcase etc, and locate non-metallic things, even read letters.
T-rays have enough specificity to distinguish “big” molecules; they can be used to detect explosives, drugs, etc.
THz spectroscopy and imaging
Three different drugs: MDMA (left), aspirin (center), and methamphetamine (right), have different images in T-rays
K. Kawase, OPN, October 2004
Q. Hu, QCL Workshop
Q. Hu, QCL Workshop
Terahertz QCLs
Highest operating temperature ~ 175 K in pulsed regime
Narrow tunability
Q. Hu (MIT), F. Capasso (Harvard), J. Faist (ETH), A. Tredicucci (Pisa)
12
Terahertz QCLs: 3 QW design
GaAs/AlGaAs
Belkin et al.
Free carriers help to reduce losses!
Metal-metal waveguide
z
xActive region
GaAs substrate
Gold
01075150m
(a)
(b)
GoldActive region
01075150m
GaAs substrate
Fig. 2. Schematic representation of (a) the semi-insulating surface-plasmon waveguide (b) the metal-metal plasmon waveguide, used in THz QCLs. The component of the magnetic field of the mode parallel to the layers of the active region (Hy) is plotted.
Heavily doped GaAs
Heterogeneous Cascades (multi- generation)
Homogeneous cascade: single stack of
~ 30 identical active regions & injectors
Stacked cascades: Interdigitated cascades:
Cooperative cascades:
Different electric field across sub-stacks
Charge transport between stages
How to design cooperation
So far:
Now:
Heterogeneous Cascades (multi- generation)
9.45 9.50 9.55Wavelength (m)
8.0 8.2Wavelength (m)
9.5 mactive region
9.5 mactive region
8.0 mactive region
Distance
Ene
rgy
Current flows in series
Design of the ultrabroadband quantum cascade laser
Act
ive
wav
egui
de c
ore
Sho
rter
wav
elen
gths
ge
nera
tion
Long
er w
avel
engt
hs
gene
ratio
n 5
6
7
8
Pea
k w
avel
engt
h ( m
)
150
175
200
225
Pea
k en
ergy
(m
eV)
250
Active region index 'i'
0 10 20 30
3
3
212
1
Ultrabroadband (6 - 8 m) spectrum
a
0.1
1
10
Pow
er (
arb.
uni
ts, l
og. s
cale
)
Wavelength (m)
5 6 7 8 9
2, 3, 4 A5 ... 13 A
activeregion
1
g3
2
4
I5
ener
gy
z 2
1
3
4
I.
5
II.
(2) ~ 105 pm/V
• Maximizing the product of dipoles d23d34d24 • Quantum interference between cascades I and II
Monolithic integration of quantum-cascade lasers Monolithic integration of quantum-cascade lasers with resonant optical nonlinearitieswith resonant optical nonlinearities
Frequency down-conversion to the THz range
• Difference frequency generation
• Stokes Raman and cascade lasing
• Parametric down-conversion
Three ways to achieve using nonlinear optics:
~ 100-1000 m, f ~ 0.3-3 THz
Current THz semiconductor lasers require cryogenic temperaturesThey are not tunable
Difference frequency generation in two-wavelength QCLs
M. Belkin, F. Capasso, A. Belyanin et al. Nature photonics 1, 288 (2007).M. Belkin, F. Xie et al., APL 96, 201101 (2008)
Difference frequency generation in two-wavelength QCLs
*)2(( qpqp EEP
1
2
3ωq
ωp
cladding
Laser1 section
Side contact layer
Laser 2 section
substrate
M. Belkin, F. Capasso, A. Belyanin et al. Nature photonics 1, 288 (2007).
M. Belkin, F. Xie et al., 2008
Results obtained by Feng Xie in Harvard in summer 2007