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