QUANTUM CASCADE LASERS: COMPACT WIDELY TAILORABLE LIGHT SOURCES FROM THE MID-INFRARED TO THE FAR INFRARED FEDERICO CAPASSO School of Engineering and Applied Sciences Harvard University capasso@seas.harvard.edu http://www.seas.harvard.edu/capasso
Acknowledgements Harvard University P. Rauter R. Blanchard T. Mansuripur S. Menzel B. Gokden EOS Photonics L. Diehl C. Pfluegl MIT Lincoln Laboratory A. K. Goyal C. A. Wang Georgia Institute of Technology Y. Huang J.-H. Ryou R. D. Dupuis Hamamatsu Photonics - T. Edamura; M. Yamanishi, K. Fujita
QCLs: First Lasers to provide broad wavelength coverage for a largely underdeveloped spectral region Electronics up to ~1 THz (λ=300µm) Quantum Cascade Lasers (QCLs) (λ=3-300µm) Diode lasers ~ 3-0.3 µm First demonstration: 1994 J. Faist, F. Capasso, D.L. Sivco, C. Sirtori, A.L. Hutchinson, A.Y. Cho, Science 264, 553, (1994)
Region Mid-IR: Molecular Fingerprint Region Microwaves THz Mid-IR Near -IR UV Mid-Infrared: Every molecule has a unique absorption fingerprint chemical sensing with high sensitivity and selectivity Applications Industrial process control and Pharma -In line process control; Compliance testing of tablet, capsules, powders -Quality control of chemical processes from reagents to products Homeland security & DOD: standoff detection of explosives and hazardous gases Medical: breath analysis, tissue imaging Environment / Energy: pollution monitoring, atmospheric chemistry
A Quantum Cascade Laser vs. Diode laser Unipolar vs Bipolar laser ELIMINATION OF BAND-GAP SLAVERY: USES STATE OF THE ART InP BASED and GaAs BASED EPITAXIAL GROWTH PLATFORMS OF PHOTONICS AN ELECTRONINC
Designer Infrared Material In 0.53 Ga 0.47 As/Al 0.48 In 0.52 As/InP GaAs/Al x Ga 1-x As
ELIMINATION OF BAND-GAP SLAVERY: USES STATE OF THE ART InP BASED and GaAs BASED EPITAXIAL GROWTH PLATFORMS OF PHOTONICS AND ELECTRONICS: MOVPE and MBE
High power/ high Power Efficiency CW QCLs at RT Several watts/> 10% Electroplated Au n + InGaAs layer InP cladding re-grown Fe:InP Active re-grown Fe:InP InP cladding InP cladding InP substrate Page 1 Commercially available
High Power CW Room Temperature Operation λ= 4.6 µm A. Lyakh, C. Pflügl, et al., APL 92, 111110 (2008)
www.pranalytica.com
2013: Commercialization in full swing High performance QCL by both MBE and MOVPE
Quantum Cascade Laser: compact, cryogenic-free and bright laser source in the Mid-IR
Chemical Sensing and Spectroscopy Single mode DFB lasers: established technology, limited to specific applications Broadly Tunable single mode QCLs: DFB laser array external-cavity QCLs Stand-off detection Collimation Broadband Fabry-Perot QCLs in conjunction with FTIR spectrometers - FP QCLs are easy to fabricate, high output power - leverages well-established FTIR platform
AlInAs/InGaAs on InP grown by MBE and MOVPE
Distributed feedback laser : Single mode selection by 1 st order grating placed above active region: λ/2n) = Λ
ATMOSPHERIC (Troposphere & Stratosphere) TRACE GAS MEASUREMENTS WITH QCLs DUAL-LASER INSTRUMENT DESIGN LIGHTWEIGHT MULTIPASS CELL (76m) ABSORPTION SPECTRUM CH 4 1270.785 LASER 1 N 2 O 1271.078 CO 2179.772 LASER 2 TRACE GAS cm-1 std dev 1s ppb 76 m path LoD ppb 100 s NH 3 967 0.2 0.06 C 2 H 4 960 1 0.5 O 3 1050 1.5 0.6 CH 4 1270 1 0.4 N 2 O 1270 0.4 0.2 H 2 O 2 1267 3 1 SO 2 1370 1 0.5 NO 2 1600 0.2 0.1 HONO 1700 0.6 0.3 HNO 3 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 N 2 O 2240 0.2 0.1 13 CO 2 / 12 CO 2 2311 0.5 0.1
NSF HIAPER Pole-to-Pole Observations (HIPPO of Carbon Cycle and Greenhouse Gases Gulf Stream V Aircraft QCLs for CO 2, CO, CH 4, N 2 O LATITUDE AND ALTITUDE PROFILES OF TRACERS FOR GLOBAL CIRCULATION MODELS PRECISION: (MIXING RATIO) CO 2 30 ppb (340 ppm) CO 0.2 ppb (80 ppb) CH 4 0.8 ppb (1800 ppb) N 2 O 0.1 ppb (320 ppb) ALTITUDE PROFILES PI: STEVEN WOFSY, HAVARD U. Free tropospher e Lower stratosphere CO CH 4 N 2 O CO 2
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 gain QCLs Basis of MIR broadband source: Broadband gain QCL material Electroluminescence width of >600 cm -1 demonstrated C. Gmachl et al., Nature 415, 883 (2001). R. Maulini et al., Appl. Phys. Lett. 84, 1659 (2004). K. Fujita et al., Appl. Phys. Lett. 98, 231102 (2011).
Broadband multistack external cavity quantum cascade laser Jerome Faist Group, ETH Grating coupled external cavity Continuous wave: 2 active regions, 201cm -1 tuning (8.0µm 9.6µm) 135 mw average Power Pulsed operation: 5 active regions, 432 cm -1 tuning (7.5µm 11.4µm) 1 Wpeak power
Broadband QCL spectrometer on a CHIP L 9.25µm 9.2µm 9.15µm 9.1µm 9.05µm 9.0µm Gain spectrum to detector ~3mm ~3mm pulser multiplexer controller DFB laser array fluid cell detector Broadband mid-ir QCL material (8-10 µm) Small-foot print Rugged, portable and robust: no movable parts Fast electronic tuning Computer control B. Lee, M. Belkin et al., APL 91, 231101 (2007)
DFB QCL array Multi-wavelengthsources, single-modeoperation Wavenumber spacing 3 cm -1 Temperature tuning over 5 cm -1 Continuous tuning possible isopropanol methanol acetone
Beam combining of DFB QCL array Requesition for stand-off detection: Collinear output of array elements Beam combining using diffraction gratings Excellent overlap of individual beams FW 1/e 2 A. Goyal et al., Opt. Express 19, 26729 (2011).
Stand-off detection MIR: Fingerprint region for chemicals Microwaves THz Mid-IR VIS UV (from Daylight Solutions Inc.) Powerful single-mode MIR source: Quantum cascade lasers Stand-off detection by multispectral MIR imaging, using EC QCL TNT K. Degreif et al., Proc. of SPIE 7945, 79450P (2011).
Stand-off deetction II Requirements for stand-off spectroscopy: high power good beam quality mechanical tuning, bulky Stand-off detection and spectroscopy: External cavity QCLs A. Hugi et al., Semicond. Sci. Technol. 25, 083001 (2010). Arrays of high-power, single mode QCLsas a powerful alternative to external cavity QCLs, no mechanical components
Master Oscillator Power Amplifier: QCL Single mode DFB QCLs: limited peak power. Wide facet deteriorates beam quality Alternative to large-area DFB devices: Master-Oscillator Power Amplifier QCLs MOPA: Monolithic two-section device Seeding of single mode by low-power DFB QCL Amplification of injected mode by tapered single-pass Drawing of a MOPA QCL optical amplifier Monolithic twosection device
MOPA QCLs Far-field: single-lobed, narrow intensity distribution in chip plane high peak power + MOPAs: excellent beam quality + single-mode spectrum 1.5 W peak power Single-mode spectrum(smsr > 20 db) S. Menzel et al., Opt. Express 19, 16229 (2011).
MOPA QCL array: Design key points 2 mm DFB 2 mm tapered amplifier Optical amplifier: No gain clamping Mitigate gain saturation by mode expansion Single-mode master-oscillator, low power Single-pass power-amplifier, 1.3 taper angle Conserve excellent beam quality of seeded mode during amplification adiabatic mode expansion 110 µmwide output facet Narrow in-plane intenstiy distribution in far-field Preserves single-mode output spectrum
MOPA QCL array: Characterization FTIR or bolometer I PA Pulse Generator 1 I MO Pulse Generator 2 Duty cycle 0.025% Output spectrum monitored by FTIR Successively increase I PA and I MO, until side-mode suppression ratio < 20 db is compromised by multi-mode operation Maximum single mode power measured by calibrated bolometer
MOPA QCL array: Light/amplifier-current characteristics Typical optical amplifier characteristics Exponentialpower increase for low DFB currents modal gain coefficient gγ= 4.5 cm/ka Gain saturation for high currents
MOPA QCL array: Spectra and peak power 14 different wavelengths between 9.2 and 9.8 µm Single-mode peak power between 2.7 W and 10 W High output power, excellent spectral quality! Side-mode suppression ratio 20 db P. Rauter et al., Appl. Phys. Lett. 101, 261117 (2012). P. Rauter et al., Opt. Express, accepted(2013).
MOPA QCL array: Beam quality MOPA 2 Far field MOPA 15 Measured by HgCdTe detector on rotating arm Narrow FWHM angle between 6.8 and 8.2 in chip plane Diffraction limited FWHM of 5.2 predicted by theory
MOPA QCL array: Beam quality Excellent beam quality at maximum output power Minor contributions of higher order lateral modes (5 MOPAs) No higher order contributions for 9 devices M 2 -values around 1.7 M2 = 4πσ θ σ 0 /λ σ 0 = 0.18w w...facet width σ θ...ang. std. dev. [rad] B = P/(λ 2 M 2 ) P...peak power Brightness between 1.6 MWcm -2 sr -1 and 5.5 MWcm -2 sr -1
MOPA QCL array: Summary Multi-wavelength MOPA QCL array as a single-mode source of 14 different wavelengths between 9.2 and 9.8 No mechanical components for wavelength selection High-power single mode operation with peak powers between 2.7 and 10 W Excellent beam quality, narrow in-plane far-field distribution Array highly suitable for stand-off spectroscopy systems
Divergence of semiconductor lasers SEM image of the laser facet Measured far-field mode profile Hamamatsu MOCVD-grown buried heterostructure device: λ=8.06 µm FWHM divergence angles: θ =74 o θ =42 o
Simulation: E 2 at 100 nm above surface 2D Collimation: Design
Tapered QCLs with plasmonic collimators Θ= 1 tapering angle Al 2 O 3 coating on front facet back facet front facet Gold layer applied, structured by focused ion beam: Outcoupling slit exposing waveguide region Plasmonic grating in vertical direction for collimation R. Blanchard et al., manuscript in preparation High-reflectivity coating on back facet
Tapered QCLs with plasmonic collimators Angle (degree) 10 7.5 5 2.5 0-2.5-5 -7.5-10 -15-12.5-10 -7.5-5 -2.5 0 2.5 5 7.5 10 12.5 15 Angle (degree) 1 0.8 0.6 0.4 0.2 0 Beam divergence in vertical direction drastically reduced However, still relatively strong uncollimated background High output power unchanged by application of collimator Normalized Intensity 1.0 0.8 0.6 0.4 0.2 0.0 Slow axis (horizontal) Fast axis (vertical) I = 3.5 I th -15-10 -5 0 5 10 15 Peak power (W) 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Original laser Laser with collimator 0.0 0 2 4 6 8 10 Current (A) Angle (degrees)
Off-axis collimator Multi-beam capability SEM images Experimental far-fields Modeled far-fields
THz QCL performance T max, K 300 275 250 225 T max, K 200 175 150 125 100 Timeline of T max achieved by THz QCLs 75 50 25 200 175 150 125 100 75 50 25 0 1 2 3 4 5 Emission frequency, THz 2002 2004 2006 2008 2010 2012 Year 40
Difference Frequency Generation (DFG) Use intra-cavity DFG in 3-15 µm QCLs to create roomtemperature sources in 60-300 µm (1-5 THz) range ω 1 ω 2 ω THz =ω 1 -ω 2 Pumps THz QCL source based on intra-cavity DFG Dual-frequency mid-infrared QCLs with giant χ (2) Coherent THz output at room temperature THz output tunable over the entire 1-5 THz rangeω 1 ω 2 ω THz
How much THz power? 2 ( ) (2) ( ) ( ) 2 ω χ I ω I ω I THz 1 2 l eff l eff = ( ( ) ) 2 2 1 2 k + αthz 2, where k = k1 k2 kthz Solid-state laser systems QCLs I(ω 1 ), I(ω 2 ) 100 MW/cm 2 10 MW/cm 2 l eff χ (2) 5 mm 0.1mm 100 pm/v need 10 4 pm/v
Giant χ (2) in quantum wells 1 ω 1 ω 2 2 ω 3 THz χ z z ( ω + iγ) ( + iγ) ( + iγ) THz ω23 ω1 ω13 ω12 ω2 Giant χ (2) and giant absorption! z 3 ( 2) e 12 23 31 1 1 = Ne + 2 h ε0 χ (2) 10 6 pm V C. Sirtori et al., Appl. Phys. Lett. 65,445 (1994)
χ (2) with population inversion 1 2 3 ω 1 ω 2... ω THz χ z z z 3 ( 2) e 12 23 31 1 1 = Ne + 2 h ε0 ( ω + iγ) ( + iγ) ( + iγ) THz ω23 ω1 ω13 ω12 ω2 Laser action instead of absorption! Active region design Section 1, χ (2) and ω 1 Section 2, χ (2) and ω 2 ω 1 ω 2 ω THz
Leaky THz waveguide Upper cladding Dual-color MIR pump ω 1, ω 2, χ (2) Undoped substrate THz THz leaky mode propagate into the substrate at an angle θ. Benefits: Directional THz emission Efficient THz extraction Works for any THz frequency THz power scales with length
Cherenkov DFG emission D ( 2 ) (2) i( 1 2 P ~ E E e β β ) THz χ 1 2 ( THz) k ω sub z θ c If the P (2) wave propagates faster than THz radiation in the substrate, THz radiation is emitted into the substrate at the Cherenkov angle θ c If ( ) k ω sub THz β β > 1 2 then ( ) k ω sub THz cos ngroupωthz β1 β2, ng 3.4 ( group index) c UndopedInP has n THz 3.6 θ c 20 o θ = β β c 1 2
Device design V InP cladding ω 1, χ (2) InP cladding, 10 17 cm -3 ω 2 Semi-insulating InP Substrate Side Contact Biasing Manual polishing 50 µm 100 µm
Three out-coupling configurations Unpolished W THz Straight-facet emission θ= 20 θ= 30 W THz W THz Emission normal to the polished plane Emission refracts normal to laser facet
DFB devices Device processing steps; SEM images of processed device Λ Λ 1 Λ 2 Au Heavily-doped InP cladding InP cladding InP substrate ω 1, Active region χ (2) (2) ω 2 χ (2)
Emission spectra
Power output and conversion efficiency THz Peak Power (µw) 80 60 40 20 THz Pk Power (µw) 80 60 40 20 0.4 mw/w 2 0 0.0 0.1 0.2 0.3 0.4 W 1 x W 2 (W 2 ) 4 THz 3 THz 2 THz 1.70mm-long 25μm-wide device 0 0.0 0.0 2.0 4.0 6.0 8.0 10.0 Current Density (ka/cm 2 ) 0.8 0.6 0.4 0.2 MIR Peak Power (W)
Far field@4thz XZPlane XYPlane z 1.70mm-long 25μm-wide device x y -20-10 0 10 20 Angle wrt laser facet normal (degrees)
Summary: THz QCL sources with record performance at RT W THz THz η η η 82µ W@4THz emission from1.2thz (@4THz) (@3THz) 0.42mW W 0.13mW W ( ) 2 @2THz 0.07mW W 2 2 to 4.5THz Significant design space for further improvement
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 Higher performance at short wavelength ( down to 3 microns) QCL at telecom wavelengths? High temperature (T 0 = 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