Post on 01-Jan-2020
Freedom from bandgap slavery: from diode
lasers to quantum cascade lasers
FEDERICO CAPASSO
School of Engineering and Applied Sciences
Harvard University
capasso@seas.harvard.edu
http://www.seas.harvard.edu/capasso
American Physical Society April meeting, Feb. 16, 2010, Washington DC
Support : NSF, DARPA, AFSOR, ARO, APS
Collaborators: A. Y. Cho; J. Faist, C. Sirtori, D. L. Sivco, A. L.
Hutchinson and Many others around the world ( > 50)!
Convergence of different fields in highly
interdisciplinary environments, primarily at industrial and Government
labs (Bell Labs, GE, IBM, Lincoln Lab, Ioffe Inst. ) led to revolutionary
device advances
• Materials research and in particular the emergence of modern thin films growth
techniques : MBE (Cho and Arthur, 1968-1969) and MOVPE
(Metallorganic Vapour Phase Epitaxy (Manasevit, 1968, and Dupuis, 1980s) with
their unprecedented control of composition, interface abruptness, layer
thickness, doping. They are the growth platform of semiconductor photonics
• Solid state physics and Solid-state Electronics: laser action in pn junctions :
injection lasers (1962); semiconductor heterostructures, optical and transport
properties, heterojunction laser concept (Kroemer, Alferovn & Kazarinov, 1963),
CW Room temperature operation Alferov et al.; Hayashi and Panish, 1970)
• Bandstructure engineering : designer materials with man made properties
which can be designed bottom up using the laws of quantum mechanics:
quantum size effect, tunneling phenomena, superlattices and applications
such as quantum well lasers and quantum cascade lasers (Esaki, Tsu & Chang
1969-1974; Dingle, Gossard and Henry, 1974, Capasso & Faist 1994)
PN Junction lasers (With Homojunction!) operated only in pulsed mode
and at high threshold because of poor carrier and photon confinement
HETEROSTRUCTURES
Independent control of electron and hole motion
Refractiveindex
Photondensity
Active
region
n ~ 5%
2 eV
Holes in VB
Electrons in CB
AlGaAsAlGaAs
1.4 eV
Ec
Ev
Ec
Ev
(a)
(b)
pn p
Ec
(a) A doubleheterostructure diode hastwo junctions which arebetween two differentbandgap semiconductors(GaAs and AlGaAs).
2 eV
(b) Simplified energyband diagram under alarge forward bias.Lasing recombinationtakes place in the p-GaAs layer, theactive layer
(~0.1 m)
(c) Higher bandgapmaterials have alower refractiveindex
(d) AlGaAs layersprovide lateral opticalconfinement.
(c)
(d)
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
GaAs
Double Heterostructure Laser Diode
Important spectral regions
Infrared countermeasures; Free-space communication; LIDAR; remote
sensing
● Atmospheric transparency windows ( =3-5 m and =8-12 m)
● Rayleigh scattering (~1/ 4)
“Mid-infrared” region, =3-30 m
Chemical sensing (molecular “fingerprint” region)
● Bio, medical, environmental, chemistry, security…
“THz” region, 1-5 THz ( =60-300 m)
Security screening, materials inspection, remote sensing,
spectroscopy, local oscillators
ELIMINATION OF BAND-GAP SLAVERY: USE STATE OF THE ART
InP BASED and GaAs BASED EPITAXIAL MATERIALS USED FOR NEAR IR!
Spectral coverage of lasers
100 200 300 400 500 600 700 800 900 1,000 3,000 30,000
Ultraviolet Visible Near-infrared Mid-infrared terahertz
HeNe 633 nm
Ruby 694 nm
Nd:YAG 1064 nm
XeF 351 nm
XeCl 308 nm
KrF 248 nm
ArF 193 nm
Ti:sapphire 700-1000 nm
Ar-ion 364-514 nm
Diode
Dye
CO2
10.6 µm
QCLs
Er:YAG
2.94 µm
Birth of the Quantum Cascade Laser
Jan. 1994 at Bell Labs
J. Faist, F. Capasso, D. L. Sivco,
C. Sirtori, A. L. Hutchinson, A. Y. Cho,
Science 264, 553 (1994).
Pulsed 90K operation
Quantum design: all laser properties (wavelength, gain spectrum etc) can designed bottom
Up starting from wavefunctions, energy levels, matrix elements, population inversion
In0.53Ga0.47As/Al0.48In0.52As/InP GaAs/AlxGa1-xAs
What makes the QC Laser special?
Wavelength agility: layer thicknesses determines wavelength; huge
λ design range; ultrabroadband band lasing and tuning
Unipolar nature & cascading which reuses electrons: high optical power
Intersubband transition; broadening insensitive to temperature:
temperature insensitivity of laser threshold (high T0: hundreds of K)
Ultra-fast carrier lifetime: no relaxation oscillations
Negligible spontaneous emission rate compared to (lifetime)-1
(dominated by non-radiative processes) and very small alpha
parameter: linewidth limit smaller than Schawlow Townes value
Large Rabi-frequency and can be designed with giant nonlinear
susceptibilities in active region: coherent phenomena at room
temperature and new nonlinear optical sources
AlInAs/InGaAs lattice matched to InP is best material for
15 m > > 4.5 m
Beck et al., Science (2002)
Two phonon resonance design
States 3,2,1 equally spaced by an optical phonon
This double phonon resonance further improves population inversion
and useful high power CW room temperature operation
Bound-to-continuum QC Laser
1. selective injection by
resonant tunneling
2. Oscillator strength spreading
3. diagonal transition: large
gain bandwith & tunability
J. Faist et al., Appl. Phys. Lett., 78(2), 147 (2001).
2009: Commercialization in full swing
High performance QCL by both MBE and MOVPE
High Power CW Room Temperature Operation
= 4.6 µm
A. Lyakh, et al., APL 92, 111110 (2008)
Strain compensation for large barrier heights (0.7-0.8 eV) : low carrier
leakage; Diamond sub-mount.
www.pranalytica.com
Grating on top of active region selects single mode:
Distributed Feedback Laser
ATMOSPHERIC (Troposphere & Stratosphere) TRACE GAS
MEASUREMENTS WITH QCLs
TRACE
GAS
cm-1 std dev 1s
ppb
76 m path
LoD
ppb
100 s
NH3 967 0.2 0.06
C2H4 960 1 0.5
O3 1050 1.5 0.6
CH4 1270 1 0.4
N2O 1270 0.4 0.2
H2O2 1267 3 1
SO2 1370 1 0.5
NO2 1600 0.2 0.1
HONO 1700 0.6 0.3
HNO3 1723 0.6 0.3
HCHO 1765 0.3 0.15
HCOOH 1765 0.3 0.15
NO 1900 0.6 0.3
OCS 2071 0.06 0.03
CO 2190 0.4 0.2
N2O 2240 0.2 0.1
13CO2/ 12CO2 2311 0.5 ‰ 0.1 ‰
LIGHTWEIGHT
MULTIPASS
CELL (76m)
LASER 1
CH4
1270.785
N2O
1271.078
CO
2179.772
LASER 2
ABSORPTION SPECTRUM
DUAL-LASER INSTRUMENT DESIGN
NSF HIAPER Pole-to-Pole Observations (HIPPO)
of Carbon Cycle and Greenhouse Gases
Gulf Stream V Aircraft
QCLs for CO2, CO, CH4, N2O
LATITUDE AND ALTITUDE
PROFILES OF TRACERS FOR
GLOBAL CIRCULATION MODELS
PRECISION: (MIXING RATIO)
CO2 30 ppb (340 ppm)
CO 0.2 ppb (80 ppb)
CH4 0.8 ppb (1800 ppb)
N2O 0.1 ppb (320 ppb)
PI: STEVEN WOFSY, HAVARD U.
Fre
e
tro
po
sp
her
e
Lo
wer
str
ato
sp
here
ALTITUDE PROFILES
Stratospheric
intrusion
CO CH4 N2O CO2
• The measurements resolve the vertical and horizontal structure of the
atmosphere: first to provide a high-resolution section of the atmosphere—
the QCL spectrometers are uniquely capable of making this kind of
observation.
• The patterns provide new information about the locations and strengths of
emissions of greenhouse gases to the atmosphere.
Broadband external cavity quantum cascade laser
Broadband QCLs design by combining dissimilar active regions
Continuous wave: 2 active regions,
201cm-1 tuning (8.0 m – 9.6 m)
135 mW average Power
Pulsed operation: 5 active regions,
432 cm-1 tuning (7.5 m – 11.4 m)
1 W peak power
Grating coupled external cavity
Ozone – measured at Beijing Olympics 2008
Ozone
High of 90 – 100 ppb
Low of < 1 ppb
US EPA 8 hour average standard
= 75 ppb
Start of Olympics
Quantum Cascade Laser Open Path
System – ―QCLOPS‖
Ozone, ammonia, CO2, water vapor
External cavity
QC laser
TE-cooled
detector
Telescope
75m roundtrip
C. Gmachl, Princeton; Zifa Wang; Chinese Acad. Sci. Daylight Solutions Inc.
Real-Time Breath Sensor Architecture for NH3 Detection
• Controlled flow • Continuous control of mouth pressure • Continuous monitoring of CO2 concentration (capnograph) and its use in QEPAS data processing
19:43 19:46 19:49 19:52 19:55-100
0
100
200
300
400
500
600
0
2
4
6
8
Exh
ale
d N
H3 [p
pb
]
Time [HH:MM]
NH3
CO2
Exh
ale
d C
O2 [%
]
EC-QCL
(914-972 cm-1)
Daylight Solutions, Inc
Broadband QCL spectrometer on a CHIP
• Broadband mid-IR QCL material (8-10 m)
• Distributed feedback (DFB) laser array
spanning laser gain curve
• Temperature tuning for continuous
spectral coverage
• Computer control
Technical Approach
pulser
controller
multiplexer
DFB laser array
fluid
cell
detector
… 9.25µm 9.2µm 9.15µm 9.1µm 9.05µm 9.0µm
to detector
~3mm
~3mm
Wavelength, m
Gain
Harvard group: Lee, Belkin et al., APL 91, 231101 (2007)
Performance
20 cm
Emission spectrum of the array
Comparison with FTIR spectrometer
• Much higher S/N due to laser rather than
thermal source: remote trace gas detection
• Higher spectral resolution due laser linewidth
• Compact
B.G. Lee et al., IEEE Photon. Technol. Lett. 21 (2009) 914.
Isopropanol Absorption
0
0.5
1
1.5
2
2.5
3
1050 1080 1110 1140
wavenumber (1/cm)
ab
so
rban
ce
Acetone Absorption
0
0.1
0.2
0.3
0.4
0.5
0.6
1040 1070 1100 1130 1160
wavenumber (1/cm)
ab
so
rba
nc
e
Methanol Absorption
0
0.2
0.4
0.6
0.8
1
1.2
1040 1070 1100 1130 1160
wavenumber (1/cm)
ab
so
rban
ce
QCL spectrometer
Commercial spectrometer
QCL spectrometer
Commercial spectrometer
QCL spectrometer
Commercial spectrometer
Spectroscopy
(3 sec accumulation time) (3 sec accumulation time)
(3 sec accumulation time)
Important spectral regions
Chemical sensing (molecular “fingerprint” region)
● Bio, medical, environmental, chemistry, security…
Infrared countermeasures; Free-space communication; LIDAR; remote
sensing
● Atmospheric transparency windows ( =3-5 m and =8-12 m)
● Rayleigh scattering (~1/ 4)
“Mid-infrared” region, =3-25 m
“THz” region, 1-5 THz ( =60-300 m)
Security screening, materials inspection, remote sensing, spectroscopy, local oscillators
Terahertz quantum cascade lasers
(1-5 THz ; =60-300 m)
• Applications:
– Medical imaging
– Local oscillators
– Spectroscopy
• Needed:
– Expand wavelength coverage
– Raise operating temperature
– Mode control, tunability
Astronomy
THz imaging
far-infrared Gas Lasers: discrete frequencies,
not tunable, large, power hungry, etc
Waveguide design
Mode intensity
10 m
Doped GaAs
substrate
e
0
Metal-metal plasmon Semi-insulating surface plasmon
Mode intensity
0
Undoped GaAs
substrate
High-temperature operation (186 K) with Phonon
depopulation
Kumar, Hu, and Reno, Appl. Phys. Lett. 94, 131105 (2009)
Best temperature performance: pulsed at 185 K (Kumar, Hu, and Reno, Appl. Phys. Lett. 94, 131105 (2009))
• The maximum operating temperature is Tmax ≈ 186 K.
Room-temperature THz source?
Develop semiconductor THz sources that do
not require population inversion across the
THz transition
Pumps
1
2 THz= 1- 2
THz Difference Frequency Generation )
THz QCL source using intra-cavity DFG
• Dual-frequency mid-infrared QCLs with (2)
• THz radiation is generated via intra-cavity DFG
• Widely tunable THz source at RT 1
THz 2
Mid-IR QCLs help solve the problem of THz QCLs!
Giant (2) with population inversion
1
THz 2
Active region design
Laser action instead of absorption!
1
2
THz
', 1121'1'1''
1''1
0
2
3)2( 11
nn nnnnnnnn
nnnne
iii
zzzeN
1
2
3 . . .
Section 1, (2) and 1
Section 2, 2
Terahertz output at different T
• Peak positions agree with
mid-IR data
• Red-shift with temperature
can also be observed in mid-
IR data
• THz DFG signal observed
up to room temperature
2 3 4 5 6 7 8 9 10 11 120
20
40
60
02468
0.0
0.5
1.0
1.5
2.0
2.5
300K
80K
250K
Frequency, THz
Inte
nsi
ty, a
.u.
7 W
1 W
0.3 W
25-µm-wide, tapered to 50-µm-wide, 2-mm-long, back facet HR coating, + Silicon lens
Testing in pulsed mode (60ns pulses at 250kHz).
Large design potential still far to be exhausted
Wide range of chemical sensing applications and increasing
importance of high power applications
High power efficiency QCLs ~ 30 % and high power ~ 10 W
Recently the Princeton (Gmachl) and Northwestern (Razeghi) groups
reported 50% WPE at cryogenic temperature
Higher performance at short wavelength ( down to 3 microns) and
high performance (power and temperature in THz gap)
QCL at telecom wavelengths? High temperature (T0 = 1000), high
power QCL using Nitrides; chirp free QCLs
Mode-locked / pulsed shaped QCL and midi-ir frequency combs will
open new frontiers in molecular spectroscopy and coherent control
Increased functionality using plasmonics and metamaterials
THE FUTURE
Population Inversion in THz QCLs
Unfavorable ratio
of lifetimes
Use selective depopulation of lower
State by resonant tunneling
Optical
phonon
Temperature effects
Nonradiative decay rates grow quickly with
temperature in THz QCLs
1
2
k//
LO phonon
Beam Engineering of QCLs
• The big question:
Can we design semiconductor lasers and in particular QCLs,
with ―arbitrary‖ wavefront (beam engineering): high
collimation, multidirectional, control of polarization, super
focusing, beam steering, special beams (Bessel beams, beams
carrying orbital angular momentum)?
• Approach:
- Control the amplitude and phase of the optical near field by
patterning plasmonic structures (antennas, apertures, gratings,
etc.) and more generally metamaterials on the laser facet
• Relevance:
Highly collimated beams are important for LIDAR and standoff
detection applications in the MWIR and LWIR bands
2D Plasmonic Collimation: Original Device
Hamamatsu MOCVD-grown buried
heterostructure QCL: =8.06 m
FWHM divergence angles:
=74o
||=42o
SEM image of the laser facet Measured far-field mode profile
N. Yu et al (2008) Collaboration with Hamamatsu
2D Collimation: Design
Simulation: |E|2 at 100 nm above surface
Collaboration with Hamamatsu
Experiment: far-field mode profile (20 rings)
SEM image of a fabricated device ( =8.06 m) Measured far-field mode profile
FWHM divergence angles:
=2.7o (reduction by a factor of ~30)
||=3.7o (reduction by a factor of ~10)
Apertur
e size
w1
w2
( m2)
grating
period
( m)
groove
width w
( m)
groov
e
depth
d
( m)
radius of
the first
groove r1
( m)
2.1
1.9 7.8 0.6 1.0 6.0