THz QCL sources based on intracavity difference-frequency mixing

Transcription

1 THz QCL sources based on intracavity difference-frequency mixing Mikhail Belkin Department of Electrical and Computer Engineering The University of Texas at Austin IQCLSW, Sept. 3, 218

2 Problems with traditional THz QCLs The maximum operating temperature of THz QCLs reported to date vs operating frequency Timeline for the maximum operating temperature achieved by THz QCLs in pulsed mode T max, K Emission frequency, THz T max, K Year B.S. Williams, Nat. Photon. 1, (27) M.A. Belkin and F. Capasso, Phys. Scr. 9, 1182 (215)

3 What happens with THz QCLs at higher temperatures Upper laser state lifetime vs temperature [1,2] J th vs temperature for THz QCLs at different frequencies [1,3] Gain does not go to zero even at 3K Frankie et al. APL 112, 2114 (218) [1] B.S. Williams, Nat. Photon. 1, (27) [2] M.A. Belkin et al., IEEE J. Sel. Top. Quantum Electron. 15, 952 (29) [3] Y. Chassagneux et al., IEEE Trans. Terahertz Sci. Technol. 2, 83 (212)

4 Alternative: DFG in mid-ir QCL Pumps Signal ω 1 ω 2 ω THz 1 2 THz = 1-2 Take: 2 THz I THz I I l 3 2n n n c (2) eff I MW THz l mm pm V 2 (2) 1 / cm, THz 2 3, eff 1, 1 I I THz

5 M.A. Belkin, F. Capasso, A. Belyanin et al., Nat. Photon. 1, 288 (27) Design concept: How to get giant (2) in a QCL Take the state-of-the-art mid-infrared QCL design and split the lower laser state State-of-the-art mid-infrared QCL design ( 2 phonon QCL): e - Reduce barrier thickness: Upper laser state E LO E LO e Lower laser state THz Active region Injector

6 M. Belkin, F. Capasso, A. Belyanin, et al. APL 92, 2111 (28) (2) estimate 3 cm Laser gain =8.9 m Assuming 1 mev 1 N e 1 15 cm THz =8.9 m 2 =1.5 m, mev (2) 15, pm/v THz =6 m (5 THz)

7 Other active region designs with giant DFG (2) Dual-upper-state (Hamamatsu) Strain-balanced strong-coupled design (Razeghi) (2) 25, pm/v K. Fujita et al., APL 16, (215) (2) 25, pm/v Q. Lu et al., Sci. Rep. 6, (216)

8 M. Belkin, F. Capasso, A. Belyanin, et al. APL 92, 2111 (28) Refractive index Refractive index Edge-emitting THz DFG-QCLs z x 1 m Mode H x 2, a.u =8.9 m 2 =1.5 m stages of each QCL structure z y x 1.8 mm 2-3 m.2 mm 6 m Mode H x 2, a.u z, m THz =6 m z, m l eff =7 m

9 M. Belkin, F. Capasso, A. Belyanin, et al. APL 92, 2111 (28) Edge-emitting THz DFG-QCLs Intensity, a.u W 1 W 7 W 3K 25K 8K Frequency, THz Conversion efficienty, W/W Temperature, K Conversion efficiency W THz /(W 1 W 2 ) 5 W/W 2 (Theoretical estimates: W THz /(W 1 W 2 ) ~ 3 W/W 2 )

10 THz extraction efficiency Top Contact Active Region Substrate 2mm 5mm THz 1 2 labs THz m THz THz l abs Absorption, cm InP, n=2x1 16 cm -3 InP, n=5x1 16 cm Frequency, THz Need to extract THz radiation along the waveguide: - surface-emitting scheme -.. or something else Active region Substrate THz DFG

11 Leaky THz mode extraction 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 device length

12 Cherenkov THz DFG-QCLs Cherenkov emission P THz ~ χ (2) E 1 E 2 e i(β 1 -β 2 )z x z n MIR g THz MIR 1 2, ng c k sub (ω THz ) Semi-insulating InP 3.37 Index of semi-insulating InP n THz c cos -1 (n g /n sub ) θ C K. Vijayraghavan et al., APL 1, (212) Broadband THz DFG extraction

13 First demonstration Side current extraction V Facet polishing Upper clad Lower cladding and current extraction S.I. InP ω 1, χ (2) ω 2, χ (2) K. Vijayraghavan et al., APL 1, (212) Relative Intensity, a.u. 1 μm Regular device 2 Exit Facet 3 Exit Facet Angle, degrees

14 Far field Vertical Hamamatsu Photonics ~1 o FWHM 1.7 mm X 25 m device Horizontal Angle (degrees)

15 Optimization of the Cherenkov DFG waveguide Analytical solution: THz polarization in the waveguide Phase distribution goes as z n g THz /c Emission in confined in the waveguide or emitted into the S.I. InP substrate 1 W ~, k, 3 cm W w/ g 2 2 abs k 2 rad C 2 2 rad abs 2 abs rad abs L ~, max k 2 2 for 1 InP, 1.5x1 16 cm -3, 4.5 THz AR, 4x1 16 cm -3, 3 m InP, 1.5x1 16 cm -3, 4.5 m InGaAs, 1x1 18 cm -3,.2 m S.I. InP K. Vijayraghavan et al., Nature Comm. 4, 221 Position, m Double-metal waveguide Cherenkov with current-inj. layer Cherenkovw/o current-inj. layer H 2, a.u.

16 Typical performance for DFB Cherenkov devices Frequency (THz) Intenity (a.u.) Wavenumber (cm -1 ) 1 Intensity (a.u.) K. Vijayraghavan et al., Nature Comm. 4, 221 (213) Frequency (THz)

17 Typical performance for DFB Cherenkov devices THz Peak Power ( W) THz Pk Power ( W) Current Density (ka/cm 2 ) K. Vijayraghavan et al., Nature Comm. 4, 221 (213).6 mw/w W x W (W 2 ) 3 mw/w 2 Theoretically 1.7mm-long 25μm-wide device MIR Peak Power (W)

18 Timeline THz DFG-QCLs on S. I. InP THz peak power (mw) 1 power Edge emission Cherenkov on SI InP Conversion eff Year MIR-to-THz conversion efficiency (mw/w 2 )

19 CW operation of Cherenkov DFG-QCLs at RT Q. Lu et al., Sci. Rep.6, (216)

20 Emission linewidth of CW DFG-QCLs Beat note between THz emission of a DFG-QCL and a reference THz FC The HEB beat note signal is sent to a real-time FFT Spectrum Analyzer DFG-QCL operated continuous-wave at 8K at 2.5 THz Free-running device, no active frequency stabilization THz DFG-QCL emission linewidth <1 MHz L. Consolino et al., Sci. Adv. 3, e16331 (217) Typical beat note signal of the DFG-QCL with 2 ms integ. time

21 Single-mode CW DFG-QCL at 2.5 THz 1 µm Buried-heterostructure waveguide W=12 µm, L=2 mm Back-facet with HR coating Buried dual-wavelength DFB gratings for 2.5 THz to be within HEB detector BW Mounted episide-up on Cu heatsink

22 Linewidth vs. observation time [1] [2] 125 µs int. time 1. Vitiello et al., Nature Photon. 6, (212) 2. Bartalini et al., Phys. Rev. Lett. 14, 1 4 (21) L. Consolino et al., Sci. Adv. 3, e16331 (217)

23 Broadly-tunable THz output 2.3-mm-long 22-μm-wide I 4A Y. Jiang et al., J. Opt. 16, 942 (214).

24 Dielectric properties of SI InP Frequency (THz) ''/ '/1 2 Permittivity '' InP, T=3 K (cm -1 ) Wavenumber (cm -1 ) Alyabyeva et al., Sci. Rep. 7, 736 (217) Absorption coefficient (cm -1 )

25 THz peak power ( W) THz loss in SI InP vs THz DFG-QCL performance SI InP absorption coefficient (cm -1 ) Y. Jiang et al., J. Opt. 16, 942 (214) 2.3-mm-long device Frequency (THz)

26 QCL on SI InP THz power lost in the substrate C 2 t 3% W out / W tot.1 =5 cm -1 =1 cm -1 =2 cm -1 xcos c W t I e dx out L =4 cm Cavity length (mm)

27 Cherenkov DFG-QCL on Si substrate Si InP 1 Cherenkov angle: θ C = cos -1 (n g /n sub ) Refractive index Absorption (cm -1 ) θ C_InP ~2 t 3% n g Frequency (THz).1 1E Frequency (THz) Advantages of Si over InP substrate High THz transmission Long THz extraction length Facet polishing free THz out-coupling No beam steering for THz tuning P Si / THz PInP THz InP =2 cm -1 InP =1 cm InP =5 cm Cavity length (mm) θ C_Si ~8 t 45%

28 Device dimension 2.1mm 2.1mm InP θ P Si S. Jung et al., Optica 4, (217) Param. InP device Si device Width(μm) Length(mm) T sub (μm) 66 1 θ P ( ) 3 15 T SU8 (nm) n/a 1 9 % transmission

29 Mid-IR and THz spectra Bias: 15 khz, 4 ns Heat sink temp.: 2 C 116 cm THz S. Jung et al., Optica 4, (217)

30 Mid-IR power Bias: 15 khz, 4 ns Heat sink temp.: 2 C MIR peak power (W) InP device long short Current density (ka/cm 2 ) Si device Voltage (V) Current (A) S. Jung et al., Optica 4, (217)

31 Bias: 15 khz, 4 ns Heat sink temp.: 2 C THz power and conversion efficiency THz peak power ( W) Current density (ka/cm 2 ) closed-si device open-inp device Current (A) Device η slope-thz (µw/a) THz Power (µw) η conv (mw/w 2 ) InP InP Si Si Si S. Jung et al., Optica 4, (217) conv (mw/w 2 ) η conv = W THz W 1 W 2

32 Beam-steering free EC-DFG-QCL on Si

33 DFG-QCLs with grating outcouplers P THz ~ χ (2) E 1 E 2 e i(β 1 -β 2 )z THz L abs ~1 μm x z Doped InP substrate

34 Choice of grating outcoupler Outcoupler gratings in THz QCLs provide both outcoupling and feedback In DFG-QCLs, the outcoupler grating only provides outcoupling the phase of the THz wave w.r.t. grating position is difficult to control Implementation of the second-order grating for DFG-QCLs is challenging outcoupling of the THz wave in forward direction was implemented 1. k gr = k DFG - k air 2. k gr = k DFG + k air

35 Laser heterostructure design is based on K. Fujita et al. Opt. Expr. 24, (216). Grating optimization at 1.9 THz Electric field 1.9THz 24 o (2) Direction of P THz Power distribution 67 % higher power output vs Cherenkov THz DFG-QCLs Λ = μm (24 o beam angle) d.c. = 7%

36 Fabricated devices Facet view Top view High quality metal-metal bonding Grating period : 36.9 μm, duty cycle 7% 28- m-wide ridge, 1.5-mm-long waveguide

37 Y. Jiang et al. Sci. Rep. 6, (216) Testing

38 Device performance cm cm -1 5kHz, 2ns RT 63.4 cm -1 (1.9 THz) k gr = k DFG + k air cos(θ) 15 μw/w μw J.-H. Kim, under review (218)

39 Summary Room-temperature alternative to criogenic THz QCLs mw-level power output at room-temperature Broad tunability in 1-6 THz range Narrow emission linewidth in CW operation Significant design space for further improvement Funding