Freedom from bandgap slavery: from diode lasers to quantum cascade lasers

FEDERICO CAPASSO

School of Engineering and Applied Sciences Harvard University

[email protected] 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 Double Heterostructure Laser Diode

(a) A double n p p heterostructure diode has two junctions which are (a) AlGaAs GaAs AlGaAs between two different bandgap semiconductors (~0.1 m) (GaAs and AlGaAs).

Electrons in CB Ec Ec (b) Simplified energy Ec 2 eV band diagram under a 1.4 eV 2 eV large forward bias. Lasing recombination (b) Ev takes place in the p- Ev GaAs layer, the active layer Holes in VB

Refractive (c) Higher bandgap index materials have a n ~ 5% (c) Active lower refractive region index Photon density (d) AlGaAs layers provide lateral optical (d) confinement.

© 1999 S.O. Kasap,Optoelectronics (Prentice Hall)

Important spectral regions “Mid-infrared” region, =3-30 m

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) “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

Ultraviolet Visible Near-infrared Mid-infrared terahertz

100 200 300 400 500 600 700 800 900 1,000 3,000 30,000

Ruby XeF Er:YAG ArF 694 nm 351 nm 2.94 µm Nd:YAG CO2 193 nm HeNe KrF 1064 nm 10.6 µm 248 nm 633 nm

XeCl Ti:sapphire 308 nm 700-1000 nm QCLs Ar-ion Diode 364-514 nm

Dye 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

Two phonon resonance design

Beck et al., Science (2002)

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

DUAL-LASER INSTRUMENT DESIGN TRACE cm-1 std dev 1s LoD ppb ppb GAS 76 m path 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 H O 1267 3 1 LIGHTWEIGHT 2 2 SO 1370 1 0.5 MULTIPASS 2 NO 1600 0.2 0.1 CELL (76m) 2 HONO 1700 0.6 0.3

ABSORPTION SPECTRUM 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 CH N O CO 4 2 CO 2190 0.4 0.2 1270.785 1271.078 2179.772 N2O 2240 0.2 0.1 13 12 CO2/ CO2 2311 0.5 ‰ 0.1 ‰ LASER 1 LASER 2 NSF HIAPER Pole-to-Pole Observations (HIPPO) of Carbon Cycle and Greenhouse Gases

Gulf Stream V Aircraft PRECISION: (MIXING RATIO) CO 30 ppb (340 ppm) QCLs for CO2, CO, CH4, N2O 2 CO 0.2 ppb (80 ppb) CH 0.8 ppb (1800 ppb) LATITUDE AND ALTITUDE 4 N O 0.1 ppb (320 ppb) PROFILES OF TRACERS FOR 2 GLOBAL CIRCULATION MODELS

ALTITUDE PROFILES PI: STEVEN WOFSY, HAVARD U.

CO CH4 N2O CO2

Lower

stratosphere

e

Free Free tropospher

Stratospheric intrusion • 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

Grating coupled external cavity

Broadband QCLs design by combining dissimilar active regions Continuous wave: 2 active regions, Pulsed operation: 5 active regions, 201cm-1 tuning (8.0 m – 9.6 m) 432 cm-1 tuning (7.5 m – 11.4 m) 135 mW average Power 1 W peak power

Ozone – measured at Beijing Olympics 2008

C. Gmachl, Princeton; Zifa Wang; Chinese Acad. Sci. Daylight Solutions Inc.

External cavity Ozone Start of Olympics QC laser High of 90 – 100 ppb Telescope TE-cooled 75m roundtrip detector

Quantum Cascade Laser Open Path Low of < 1 ppb System – ―QCLOPS‖

US EPA 8 hour average standard Ozone, ammonia, CO2, water vapor = 75 ppb Real-Time Breath Sensor Architecture for NH3 Detection

600 8 NH 3 CO 500 2 6

400

[%]

2 [ppb] EC-QCL 3 300 4 (914-972 cm-1) 200 2

100

Exhaled CO ExhaledNH 0 0

-100 19:43 19:46 19:49 19:52 19:55 Time [HH:MM]

• Controlled flow • Continuous control of mouth pressure

• Continuous monitoring of CO2 concentration (capnograph) and its use in QEPAS data processing Daylight Solutions, Inc Broadband QCL spectrometer on a CHIP Harvard group: Lee, Belkin et al., APL 91, 231101 (2007) Technical Approach … 9.25µm 9.2µm 9.15µm 9.1µm 9.05µm 9.0µm • Broadband mid-IR QCL material (8-10 m) • Distributed feedback (DFB) laser array spanning laser gain curve • Temperature tuning for continuous ~3mm spectral coverage to detector • Computer control ~3mm controller

Gain DFB laser array detector

Wavelength, m

pulser fluid multiplexer cell Performance B.G. Lee et al., IEEE Photon. Technol. Lett. 21 (2009) 914. Emission spectrum of the array

20 cm

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 Spectroscopy

Methanol Absorption Isopropanol Absorption (3 sec accumulation time) (3 sec accumulation time) 1.2 3 QCL spectrometer QCL spectrometer 1 Commercial spectrometer 2.5 Commercial spectrometer 0.8 2 0.6 1.5

0.4 1 absorbance absorbance 0.2 0.5 0 0 1040 1070 1100 1130 1160 1050 1080 1110 1140 wavenumber (1/cm) wavenumber (1/cm)

Acetone Absorption (3 sec accumulation time) 0.6 QCL spectrometer Commercial spectrometer 0.5 0.4 0.3 0.2

absorbance 0.1 0 1040 1070 1100 1130 1160 wavenumber (1/cm)

Important spectral regions “Mid-infrared” region, =3-25 m

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) “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) Astronomy • Applications: – Medical imaging – Local oscillators – Spectroscopy

• Needed: THz imaging – Expand wavelength coverage – Raise operating temperature – Mode control, tunability far-infrared Gas Lasers: discrete frequencies, not tunable, large, power hungry, etc Waveguide design

Metal-metal plasmon Semi-insulating surface plasmon

Mode intensity Mode intensity

e 10 m

Doped GaAs Undoped GaAs substrate substrate

0 0 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

THz Difference Frequency Generation

)

1

2 THz= 1- 2 Pumps

THz QCL source using intra-cavity DFG

• Dual-frequency mid-infrared QCLs with (2)

• THz radiation is generated via intra-cavity DFG 1 • Widely tunable THz source at RT 2 THz

Mid-IR QCLs help solve the problem of THz QCLs! Giant (2) with population inversion

1 3 (2) e z1n znn' zn'1 1 1 Ne 2  0 n,n' nn' i nn' 1 n'1 i n'1 2 n1 i n1 2 1 Laser action instead of absorption! 2 3 THz . . . Active region design

(2) Section 1, and 1

Section 2, 2 1 2 THz Terahertz output at different T

2.5 2.0 0.3 W 300K • Peak positions agree with

1.5 mid-IR data

1.0 • Red-shift with temperature 0.5 can also be observed in mid- 0.0 8 250K IR data 1 W

6 • THz DFG signal observed

4 up to room temperature 2

0 Intensity, a.u. Intensity, 60 80K

7 W

40 20 0 2 3 4 5 6 7 8 9 10 11 12 Frequency, THz

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). THE FUTURE 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

Population Inversion in THz QCLs

Optical phonon

Unfavorable ratio Use selective depopulation of lower of lifetimes State by resonant tunneling Temperature effects

1

2

LO phonon

k//

Nonradiative decay rates grow quickly with temperature in THz QCLs 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

SEM image of the laser facet Measured far-field mode profile

FWHM divergence angles: Hamamatsu MOCVD-grown buried =74o heterostructure QCL: =8.06 m o ||=42 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

Apertur grating groove groov radius of e size period width w e the first FWHM divergence angles: w w ( m) depth groove r1 1 2 o ( m2) ( m) d ( m) =2.7 (reduction by a factor of ~30) ( m) o ||=3.7 (reduction by a factor of ~10) 2.1 1.9 7.8 0.6 1.0 6.0