THz QCL sources for operation above cryogenic temperatures Mikhail Belkin

Transcription

1 THz QCL sources for operation above cryogenic temperatures Mikhail Belkin Department of Electrical and Computer Engineering University of Texas at Austin IQCLSW, Monte Verita, Switzerland 008

2 Need for semiconductor THz source Frequency, Hz Spectroscopy Local oscillator for THz heterodyning Remote sensing, screening, inspection, etc. THz frequency amplification

3 THz Quantum Cascade Lasers Maximum operating temperature VS frequency BS Williams, Nature Photonics, (007) data of Capasso, Colombelli, and Linfield groups

4 Upper laser state lifetime vs T LO phonon Electron lifetime in state, sec. µ 0-8. µ 0-6. µ 0-4. µ 0 -. µ 0 - k // ν=thz ν=thz ν=3thz ν=4thz ν=5thz ν=6thz LO phonon scattering of thermal electrons: τ upper (E // < E LO -hν)=0ps τ upper (E // >E LO -hν)=0.5ps Assume D Fermi distribution in state and low population in state E LO =36meV 8.7 THz Electron temperature, K

5 RT THz QCLs: very difficult Upper state lifetime <ps for temperature >00K This lifetime is similar to that in mid-ir QCLs BUT, THz QCLs have higher w/g losses and smaller injection efficiency Mid-IR QCLs THz QCLs T max (pulsed) First demonstration Year T max (pulsed) First demonstration Year Nevertheless, no fundamental limit on RT operation

6 Room-temperature THz source?. Investigate alternative designs, improve GaAs/AlGaAs AlGaAs quality, try different materials (InGaAs/AlInAs,, Si/SiGe SiGe, AlGaN/GaN), low- dimensional systems (quantum dots, wires), etc.. Develop semiconductor THz sources that do not require population inversion across the THz transition

7 Room-temperature THz source?. Investigate alternative designs, improve GaAs/AlGaAs AlGaAs quality, try different materials (InGaAs/AlInAs,, Si/SiGe SiGe, AlGaN/GaN), low- dimensional systems (quantum dots, wires), etc.. Develop semiconductor THz sources that do not require population inversion across the THz transition

8 Difference Frequency Generation Difference-frequency generation (DFG) occurs in a medium with second-order nonlinear susceptibility χ () ω ω ω THz =ω -ω Pumps THz QCL source using intra-cavity DFG Dual-frequency mid-infrared QCLs with χ () THz radiation is generated via intra-cavity DFG Widely tunable THz source at RT ω ω ω THz

9 Challenges for intra-cavity THz DFG ( ) ( ω ) χ I ( ω ) I ( ω ) l coh I THz l coh = ( ) ( ) ) k k k + α THz THz Solid-state systems QCLs I(ω ), I(ω ) MW/cm -0 MW/cm l coh t0 mm d0.mm χ () ~00 pm/v can be designed Need giant χ ()

10 Giant χ () in quantum wells χ z z z 3 ( ) e n nn' n' = Ne + h ε0 n, n' nn' nn' n' n' n n ( ω ω + iγ ) ( ω + ω + iγ ) ( ω ω + iγ ) Sirtori et al., APL (994)

11 Giant χ () in quantum wells ω ω ω 3 THz χ z z z 3 ( ) e n nn' n' = Ne + h ε0 n, n' nn' nn' n' n' n n ( ω ω + iγ ) ( ω + ω + iγ ) ( ω ω + iγ ) Giant absorption!

12 χ () with population inversion 3 ω ω... ω THz χ z z z 3 ( ) e n nn' n' = Ne + h ε0 n, n' nn' nn' n' n' n n ( ω ω + iγ ) ( ω + ω + iγ ) ( ω ω + iγ ) Laser action instead of absorption! Gmachl, Belyanin, et al., JQE (003)

13 χ () with population inversion 3 ω ω... ω THz χ z ( ω ω + iγ ) ( ω + ω + iγ ) ( ω ω + iγ ) Laser action instead of absorption! Active region design z z 3 ( ) e n nn' n' = Ne + h ε0 n, n' nn' nn' n' n' n n Section, χ () and ω Section, ω ω ω ω THz

14 χ () () -section design Bound-to-continuum active region for λ 9µm [Faist et al. IEEE JQE (00)] Injection Extraction

15 χ () () -section design

16 χ () () -section design -4 Laser gain Assuming ÑΓ=0meV λ =8.9 µm Ñω, (mev) 3 4 5

17 χ () () -section design -4 Laser gain Assuming ÑΓ=0meV λ =8.9 µm Ñω, (mev) 3 4 5

18 χ () () -section design cm Laser gain Assuming ÑΓ=0meV N e 0 5 cm -3 λ =8.9 µm Ñω, (mev) 3 4 5

19 χ () () -section design cm Laser gain Assuming ÑΓ=0meV N e 0 5 cm -3 λ =8.9 µm Ñω, (mev) ω THz λ =8.9 µm λ THz =60 µm λ =0.5µm χ () pm/v

20 Waveguide design y x z 0µm Mode H x, a.u λ =8.9 µm λ =0.5 µm Refractive index stages of each QCL structure z y x 0. mm.8 mm 0-30 µm 60 µm Mode H x, a.u z, µm λ THz =60 µm Refractive index z, µm 0 l coh =70µm

21 Device performance: mid-ir Spectra Power (combined) Intensity, a.u. 300K 50K 50K Peak power, W K 50K 00K 50K 300K 80K Wavenumbers, cm Peak current, A µm-wide, tapered to 50-µm-wide, -mm-long, back facet HR coating. Testing in pulsed mode (60ns pulses at 50kHz).

22 Device performance: mid-ir Spectra Power (combined) Intensity, a.u. 300K 50K 50K Short pass filter Peak power, W K 50K 00K 50K 300K 80K Wavenumbers, cm Peak current, A µm-wide, tapered to 50-µm-wide, -mm-long, back facet HR coating. Testing in pulsed mode (60ns pulses at 50kHz).

23 Product of the pump powers ( ) ( ω ) χ W ( ω ) W ( ω ) W THz l eff W(ω )xw(ω ), W K 50K 00K 50K 300K Peak current, A 5-µm-wide, tapered to 50-µm-wide, -mm-long, back facet HR coating. Testing in pulsed mode (60ns pulses at 50kHz).

24 Terahertz emission 80K Experiment (zoom-in) Signal, a.u x N970A uppper 80K DFG signal simulation (incoherently) Wavenumber, cm - Simulations from mid-ir data Wavenumber, cm - signal wavenumber (cm-) 5-µm-wide, tapered to 50-µm-wide, -mm-long, back facet HR coating. Testing in pulsed mode (60ns pulses at 50kHz).

25 Terahertz output at different T Intensity, a.u µw 300K µw 50K 7 µw 80K Frequency, THz 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 5-µm-wide, tapered to 50-µm-wide, -mm-long, back facet HR coating, + Silicon lens Testing in pulsed mode (60ns pulses at 50kHz).

26 Conversion efficiency at different T Peak THz and mid-ir power vs T Conversion efficiency vs T THz power, µw Temperature, K W(ω )W(ω ), W Conversion efficienty, µw/w Temperature, K Conversion efficiency ~5 µw/w

27 Conversion efficiency: analysis THz eff S eff, l eff, refractive indices are known from waveguide calculations: n eff 3, l eff 70 µm, S eff 800 µm Estimate χ () using electron density in upper laser state from gain=loss condition: χ () 3x0 4 pm/v Uncertain parameters: Mid-IR lasing in higher order lateral modes THz wave out-coupling efficiency from QCL waveguide (~30%?)

28 Conversion efficiency: analysis THz eff S eff, l eff, refractive indices are known from waveguide calculations: n eff 3, l eff 70 µm, S eff 800 µm Estimate χ () using electron density in upper laser state from gain=loss condition: χ () 3x0 4 pm/v Uncertain parameters: Mid-IR lasing in higher order lateral modes THz wave out-coupling efficiency from QCL waveguide (~30%?) Theoretical efficiency: W THz /(W W ) ~ 30 µw/w Experiment (corrected for the collection efficiency): ~ 5 µw/w

29 Need for surface extraction of DFG Edge emitting THz DFG is not efficient Mid-IR pump -5 mm Active region substrate Region contributing to THz DFG output, approx. l coh ~0.mm THz DFG Only a small section ~l coh contributes to THz DFG output Surface-emitting scheme to boost THz power THz DFG Mid-IR pump Active region substrate The whole device contributes to THz DFG

30 Device design for surface emission Grating groves λ =8.9 µm 4 Mode H x, a.u λ =0.5 µm Refractive index 0.05 THz radiation is produced by P () P () (ω THz =ω -ω )~χ () E(ω )E(ω )e i(k -k )y For efficient vertical outcoupling, we need the grating k-vector k gr =k -k Mode H x, a.u z, µm Grating groves z, µm λ THz =60 µm Refractive index

31 Surface emission Edge-emission mid-ir spectrum Surface-emission THz spectrum 5-µm-wide,with second order grating for THz DFG, -mm-long, back facet HR coating. Testing in pulsed mode (60ns pulses at 50kHz) with 3.5A current pulses at 80K.

32 Power output Edge emission mid-ir power Surface emission THz power 5-µm-wide,with second order grating for THz DFG, -mm-long, back facet HR coating. Testing in pulsed mode (60ns pulses at 50kHz) with 3.5A current pulses at 80K.

33 Future improvements Improve waveguide designs for larger l coh Improve active region designs for higher χ () Improved surface emission for more outcoupling Mode-locking for higher peak intensity mw-level THz DFG QCL sources

34 People Harvard: Federico Capasso, Christian Pflügl gl,, Jonathan Fan, Qijie Wang, Laurent Diehl, Sahand Hormoz. Acknowledgements Texas A&M: Alexey Belyanin, Feng Xie University of Leeds: Edmund Linfield, Suraj Khanna,, Mohamed Lachab, Giles Davies University of Paris: Raffaele Colombelli, Yannick Chassagneux ETH-Zürich: Jérôme Faist,, Milan Fischer, Andreas Wittmann Funding AFOSR FA

35 Postdoc opportunities University of Texas, Austin City of Austin