High performance THz quantum cascade lasers Karl Unterrainer M. Kainz, S. Schönhuber, C. Deutsch, D. Bachmann, J. Darmo, H. Detz, A.M. Andrews, W. Schrenk, G. Strasser
THz QCL performance High output power Electrically pumped DFG = Difference frequency/parametric conversion techniques BWO = Backward wave oscillator Multiplier = Solid state frequency multiplier chains RTD = Resonant tunneling diode Compact adapted from Tonouchi, Nat. Photonics 1 (2007)
Terahertz Quantum Cascade Laser Performance (155 K) ETHZ/TU Wien GaAs/AlGaAs almost exclusively used Performance limited due to temperature-activated phonon scattering
Outline Novel active region materials InGaAs/GaAsSb, InGaAs/InAlAs Novel active region designs symmetric active region, barrier heights scaling Wafer-bonded high performance devices Random laser Coupled cavities - switching Talk by J. Darmo, friday
Strategies for Higher Performance New active region designs New waveguide concepts Other material systems Large optical phonon energies Low effective masses: higher gain E. Benveniste et al, APL 93, 131108 Lateral confinement operation of THz QCLs up to 225 K under a magnetic field A. Wade et al., Nature Photonics 3, 41 (2009) growth of quantum dot or nanowire QCL Saturation intensity II ssssss = ωω σσ 21 ττ 21 200 nm M. Krall et al., Proc. SPIE, 8640, 864018 (2013)
Importance of Material Properties Effective electron mass 3 2 gg mm e Benveniste et al., Appl. Phys. Lett. 93, 131108 (2008) Longitudinal-optical phonon energy TT ττ ee (ħωω LLLL ħωω) kk BB Beeler et al., Semicond. Sci. Technol. 28, 074022 (2013) InAs/AlSb InGaAs/InAlAs GaAs/Al 0.45 Ga 0.55 As Conduction band offset 135 mev 4.6 nm GaAs/Al 0.15 Ga 0.85 A s 360 mev Deutsch et al., Appl. Phys. Lett. 101, 211117 (2012) Material system m* (m 0 ) 3.0 nm In 0.53 Ga 0.47 As/GaAs 0.51 Sb 6 CBO (mev) GaN/AlGaN 0.200 0-1700 92 GaAs/Al 0.15 Ga 0.85 As 0.067 / 0.080 120 36 InGaAs/InAlAs (InP) 0.043 / 0.075 520 34 InGaAs/GaAsSb (InP) 0.043 / 0.044 360 34 InGaAs/InAlGaAs (InP) 0.043 0-520 34 InAs/AlAsSb 0.023 / 0.120 2200 30 0.49 520 mev 1.8 nm In 0.53 Ga 0.47 As/In 0.52 Al 0.48 A s ħωω LLLL (mev)
Injector Barrier Design C. Deutsch et al, Appl. Phys. Lett. 101, 211117 (2012)
InGaAs/GaAsSb THz QCLs C. Deutsch et al, Appl. Phys. Lett. 97, 261110 (2010) C. Deutsch et al, Appl. Phys. Lett. 101, 211117 (2012) Three-well phonon depletion scheme
InGaAs/GaAsSb THz QCL C. Deutsch et al, Appl. Phys. Lett. 101, 211117 (2012) D. Turcinkova et al, Appl. Phys. Lett. 99, 191104 (2011) 700 GHz Lasing around 3.6 THz Continuous, broadband lasing over 700 GHz, f/f 0 =18% Already competitive with broadband GaAs/AlGaAs THz QCLs Attractive for mode-locking and tuning applications Large bandwidth not favorable for high T operation Origin of broadening? Interface roughness, asymmetry?
What limits performance? C. Deutsch et al, Optics Express 21, 7209 (2013) R. M. Feenstra et al, Phys. Rev. Lett. 72, 2749 (1994) How to probe the effect of asymmetric interface roughness and doping migration? Study symmetric devices
Symmetric Active Region C. Deutsch et al, Optics Express 21, 7209 (2013)
Transport direction-dependent performance of InGaAs/GaAsSb Bidirectional lasing (same frequency) Performance asymmetry J th /J th+ =0.57 T max /T max+ =(124 K/103 K)=1.2 P peak /P peak+ =2.38 (dp/di) /(dp/di) + =1.21 Negative polarity beneficial Electron transport in growth direction Reason: Interface asymmetry and interface roughness scattering C. Deutsch et al, Optics Express 21, 7209 (2013)
Polarity-dependent Interface Roughness Scattering Different overlap with the two sides of the barrier Problem: Input parameters for interface roughness Reasonable assumption realistic? n =0.15 nm, i =0.3 nm, Λ n =Λ i =10 nm C. Deutsch et al, Optics Express 21, 7209 (2013)
Polarity-dependent Parasitic Leakage Weak coupling regime Injector lifetime, phonon emission time and intrasubband scattering have influence on the dephasing time τ II More complicated to model Coherence effects have a large influence Simple model fails NEGF based transport codes C. Deutsch et al, Optics Express 21, 7209 (2013) T. Kubis et al, Phys. Rev. B 79, 195323 (2009)
Symmetric GaAs/AlGaAs THz QCLs 3-well phonon depletion GaAs/AlGaAs InGaAs/GaAsSb Opposite asymmetry of LIVs (J th, P peak ) Interface asymmetry is not main asymmetry in this device Negative polarity transport sees normal, sharper interfaces (AlGaAs on GaAs) Another growth-related asymmetry C. Deutsch et al, Appl. Phys. Lett. 102, 201102 (2013)
Dopant Migration Dopant migration in growth direction overlaps with upper laser level for negative polarity Polarity-dependent Different upper laser level lifetimes τ 4+, τ 4 and/or parasitic currents J par+, J par Doping profile can be manipulated Ultimate proof: set back doping profile C. Deutsch et al, Appl. Phys. Lett. 102, 201102 (2013)
Compensation of Dopant Migration Narrower doping profile, 5 nm setback
InGaAs/GaAsSb Design Scaling In 0.53 Ga 0.47 As / GaAs 0.51 Sb 0.49 In 0.53 Ga 0.47 As / In 0.52 Al 0.15 Ga 0.33 As 14.1 13.2 2.9 1.0 2.9 23.8 matched @ 8.8 kv/cm 11.1 11.1 6.0 3.1 6.0 20.6 1 n-1 4 n 3 n 2 n 1 n 4 n+1 8.8 kv/cm 57.9 nm ΔE C 360 mev 1 n-1 4 n 3 n 2 n 1 n 4 n+1 8.8 kv/cm 57.9 nm ΔE C 120 mev three-well resonant-phonon design phonon transition Reference design Deutsch et al., Appl. Phys. Lett. 101, 211117 (2012) optical transition extraction injection
InGaAs/GaAsSb Design Scaling Results In 0.53 Ga 0.47 As / GaAs 0.51 Sb 0.49 In 0.53 Ga 0.47 As / In 0.52 Al 0.15 Ga 0.33 As Reference design Deutsch et al., Appl. Phys. Lett. 101, 211117 (2012) Influence of conduction band offset thinner barriers are not limiting the temperature performance 19
InGaAs/InAlAs Design Scaling In 0.53 Ga 0.47 As / In 0.52 Al 0.48 As 13.3 13.3 1.8 0.6 1.8 24.0 In 0.53 Ga 0.47 As / In 0.52 Al 0.18 Ga 0.30 As matched @ 10.0 kv/cm In 0.53 Ga 0.47 As / In 0.52 Al 0.15 Ga 0.33 As matched @ 10.0 kv/cm 11.9 10.6 4.7 1.5 4.6 21.5 11.8 9.8 5.4 1.7 5.3 20.8 1 n-1 4 n 3 n 2 n 1 n 4 n+1 1 n-1 4 n 3 n 2 n 1 n 4 n+1 1 n-1 4 n 3 n 2 n 1 n 4 n+1 10 kv/cm 54.8 nm 10 kv/cm 54.8 nm 10 kv/cm 54.8 nm ΔE C 520 mev ΔE C 150 mev ΔE C 120 mev Reference design Symmetric three-well resonant-phonon design phonon transition optical transition phonon transition optical transition C. Deutsch, PhD thesis, TU Wien (2013) extraction injection extraction injection M. Krall, PhD thesis, TU Wien (2016)
InGaAs/InAlAs Design Scaling Results In 0.53 Ga 0.47 As / In 0.52 Al 0.48 As In 0.53 Ga 0.47 As / In 0.52 Al 0.18 Ga 0.30 As In 0.53 Ga 0.47 As / In 0.52 Al 0.15 Ga 0.33 As Reference design C. Deutsch, PhD thesis, TU Wien (2013) Influence of conduction band offset thin barriers are not limiting the temperature performance M. Krall, PhD thesis, TU Wien (2016)
Study InAlAs barrier devices InGaAs/InAlAs is state of the art for midinfrared QCLs 2 monolayer barrier If interface asymmetry is present the influence is much larger ~E CBO 2 Study a nominally symmetric structure
Symmetric InGaAs/InAlAs THz QCL Dopant migration (Parasitic leakage) J th /J th+ =(0.44 ka/cm 2 /0.31 ka/cm 2 )=1.4 Interface asymmetry (Laser level lifetimes) T max /T max+ =(138 K/126 K)=1.09 P peak /P peak+ =0.87 (dp/di) /(dp/di) + =1.26
Optimized structure Optimized structure for negative bias polarity thinner injector barrier doping shifted away from optical transition Lasing up to 155 K at a central frequency of 3.7 THz C. Deutsch at al., ACS Photonics 4, 957 (2017) Collaboration with ETHZ: K. Otani, M. Rösch, G. Scalari, M. Beck, J. Faist
Lens coupled InGaAs/InAlAs devices Hyperhemisherical GaAs lens improves output power up to almost 600 mw Excellent far field Record for single layer double metal THz QCLs C. Deutsch at al., ACS Photonics 4, 957 (2017)
Laser Cavity: Double Metal Waveguide Surface plasmons at the metalsemiconductor interface High confinement factor, Γ ~ 1 Losses mainly due to absorption in the metal layers Top contact/ waveguide layer Sub-wavelength facet aperture Reflectivity depends on Facet type Frequency Waveguide thickness Carrier substrate Bottom contact/ waveguide layer
THz QCLs with Wafer-bonded Active Regions Advantages of increased active region thickness: lower waveguide loss due to increased waveguide thickness higher optical power inside the cavity due to increased active region thickness improved far-field due to larger facet aperture (double metal waveguide) -> Doubling active region thickness by wafer bonding thermo-compression bonding doubled active region see also results Univ. Leeds
Experimental Results Output power more than doubled Reduced waveguide losses Increased light generation Improved collection efficiency α W = 5.01 cm -1 Doubled threshold voltage Good electrical interface properties Temperature performance unchanged Devices are mirror loss dominated α W = 2.56 cm -1 M. Brandstetter et al., Opt. Express 20 (2012)
Far-field Measurements Lower beam divergence Reduced interference pattern from rear facet M. Brandstetter et al., Opt. Express 20 (2012)
Wafer bonded surface plasmon waveguide Advantage of SP wageguide: Lower facet reflectivity, higher output power, lower divergence No sub-wavelength confinement better far-field compared to DM WGs High power devices, > 1 W Direct wafer bonding Low confinement factor, Γ decreased temperature performance 12 300 Growth direction Plasmon layer Requirement: Symmetric QCL Intensity (a.u.) 10 8 6 4 2 0 Au waveguide layer Contact layers at bonding interface Highly doped plasmon layer Active region SI substrate 0 20 40 60 80 Growth direction (µm) 200 100 0 Refractive index
Wafer bonded SISP GaAs Devices Wafer bonded surface plasmon device Output power increased by a factor of four Two times increased light generation Two times increased confinement factor Record high single facet peak output power of 470 mw (2-facet output power: 940 mw) High maximum operation temperature of 122 K Γ = 0.288 Γ = 0.472 M. Brandstetter et al., Appl. Phys. Lett. 103 (2013)
Broadband Emission Spectrum Increased lasing bandwidth to 440 GHz M. Brandstetter et al., Appl. Phys. Lett. 103 (2013)
A novel cavity: Random Laser Broadband emission For spectroscopic and imaging applications Surface emission Tureci et al., Science 320, 643 646 (2008)
Surface Emission: Previous Concepts Single Mode Emission! θ ( ) θ ( ) θ ( ) ϕ ( ) ϕ ( ) ϕ ( ) Chassagneux et al., Nature 2009 Halioua et al., Optics Express 2015 Mujagic et al., APL 2009
Realization QCL active region Etched holes as scattering elements 100 µm
High Power Emission Higher output power for higher filling factors due to increased outcoupling losses up to 40 mw S. Schönhuber, Optica 3, 1035 (2016). Q. Wang, ACS Photonics 3, 2453 (2016)
Broadband Directional Surface Emission Broadband 400 GHz bandwidth Diffraction limited beam 7 FWHM divergence
Conclusion time Lower effective mass materials are very usefull for THz QCLs Problems with interface roughness Symmetric active region allow to study and correct growth asymmetries InGaAs/InAlAs high performance THz QCL Wafer-bonded active regions provide record output power, better far-field, bandwidth Random lasers: broadband surface emission