Nonlinear optics with quantum-engineered intersubband metamaterials Mikhail Belkin Department of Electrical and Computer Engineering The University of Texas at Austin 1
Mid-infrared and THz photonics Electronics Radiowaves, microwaves, etc. THz & Mid-IR Near-IR/ Vis/UV Photonics Deep UV, X-rays, -rays Hz Mostly undeveloped area (compared to near-ir/visible photonics) Important applications: spectroscopy and sensing, thermal vision, communications, THz imaging Very interesting science: intersubband transitions, novel 2D materials, metamaterials and plasmonics
Group research THz sources and photonic systems Conversion efficiency(w/w ) THz peak power (W) 600 Conversion efficiency (W/W ) 1 2 3 4 5 6 Frequency(THz) Recent selected publications: 1. S. Jung et al., Nature Comm., accepted (2014). 2. Vijayraghavan et al., Nature Comm. 4, 2021 (2013). 3. Vijaraghavan et al., APL 100, 251104 (2012). 400 200 0 Mid-IR sources and photonic systems 80 60 40 20 0 THz peak power(w) Nanospectroscopy Molecular force signal, a.u. 2 1 0 1350 1300 1250 1200 1150 Wavenumber, cm -1 Recent selected publications: 1. Lu et al., Nature Photon. 8, 307 (2014). 2. Lu and Belkin, Optics Express 19, 19942 (2011). Plasmonics and metamaterials Recent selected publications: 1. Suchalkin et al., APL 103, 041120 (2013). 2. Jang et al., IEEE JQE 49, 60 (2013). 3. Jang et al., APL 97, 141103 (2010). Recent selected publications: 1. Lee, et al., Nature, accepted (2014) 2. Lee and Belkin, APL 103, 181115 (2013). 3. Zhao et al., Nature Comm. 3, 870 (2012) 3
Intersubband transitions N-doped quantum wells 0 0 Key features: Control of energy levels positions, transition dipole moments, lifetime Tailored linear and nonlinear optical properties for TM-polarized light Various materials can be used AlInAs/InGaAs/InP, GaAs/AlGaAs, Structures can be mass-produced at existing diode laser foundries Foundation of quantum cascade lasers
Quantum cascade lasers Current Z Band offset Z Excellent performance in mid-ir 27% WPE @ RT T max, K but not so great in THz 200 175 150 125 100 75 50 25 0 1 2 3 4 5 Emission frequency, THz
THz QCL sources via nonlinear optics Use intra-cavity DFG in mid-ir QCLs I 1 2 THz = 1-2 Pumps (2) 2 I I THz 1 2 l coh THz QCL source based on intra-cavity DFG Dual-frequency mid-infrared QCLs with giant (2) Coherent THz output at room temperature THz output tunable over the entire 1-5 THz range 1 2 THz 2
(2) with population inversion 1 2 3 1 2... THz 3 e 2 i i i THz 23 1 13 12 2 Laser action instead of absorption! (2) 210 4 pm/v Active region design z z z ( 2) 12 23 31 1 Ne 0 1 Section 1, (2) and 1 Section 2, (2) and 2 1 2 THz
Laser with (2) design Design concept: 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 - Simply thin-down this barrier: Upper laser state E LO E LO e - Lower laser state THz Active region Injector Belkin et al. Nature Photon. 1, 288 (2007)
Cherenkov DFG emission D 2 (2) i 1 2 P ~ E E e THz 1 2 ( THz ) k 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 c If k THz sub 1 2 then k THz cos sub ngroup THz 1 2, ng 3.3 ( group index ) c Undoped InP has n THz 3.6 c 20 o c 1 2
DFB devices Frequency (THz) 29 30 31 32 33 34 Intenity (a.u.) 950 1000 1050 1100 1150 Wavenumber (cm -1 ) Vijayraghavan et al., Nature Comm. 4, 2021 (2013)
DFB devices Intensity (a.u.) 2 3 4 5 6 Frequency (THz) Vijayraghavan et al., Nature Comm. 4, 2021 (2013)
Power output and conversion efficiency THz Peak Power (W) 120 90 60 THz Pk Power (W) 120 100 80 60 40 20 0.6 mw/w 2 0 0.0 0.1 0.2 0.3 0.4 W x W (W 2 ) 30 0.2 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 ) Vijayraghavan et al., Nature Comm. 4, 2021 (2013) 0.8 0.6 0.4 MIR Peak Power (W)
Widely-tunable external-cavity THz QCLs
Widely-tunable external-cavity THz QCL 1.70mm-long, 25μm-wide device Vijayraghavan et al., Nature Comm. 4, 2021 (2013), Y. Jiang et al., under review (2014)
Monolithic tuning THz performance: S. Jung et al., Nature Comm., accepted (2014)
Ultrathin nonlinear metasurfaces 2 d Phase-matching requirements for surface NLO: k k k in1// in2// out// satisfied automatically Phase-matching requirements for bulk NLO: k k k in1 in2 out requires special efforts
Intersubband nonlinearities (2) zzz 2 N e 3 e 2 0 2 i i 31 z 12 z 31 23 z 31 21 21 Compare with 10-1 -10-3 nm/v for traditional NLO materials
Nonlinear polaritonic metasurface 400 nm Quantum-engineered nonlinear MQW structures with giant nonlinearity + Electromagnetically-engineered modes in metallic nanostructures = Large-area ultrathin metasurfaces with record nonlinearities ω 2ω ~1 m N e 10 17 cm -3 E F 12 mev ~0.8 m SHG mirror: 8 m 4 m (LWIR MWIR)
Nonlinear polaritonic metasurface (2) eff (2) ijk zzz V E ( x, y, z) E ( x, y, z) E ( x, y, z) dv 2 z( k ) z( j) z( i) E E E V 2 k ( inc) j( inc) i( inc)
Nonlinear polaritonic metasurface (2) eff yyy 31 nm V (2) eff xxx 25 nm V (2) eff xyy 6.5 nm V (2) eff yxx 3.9 nm V
SHG generation (theory) I 2 2 (2) eff ˆ ˆ ˆ 2 e 3 2 ee 8 0c 2 2 2 I L 2 k2 // 2k // (Gaussian focal spot dia: 35 m)
Fabrication InP Thermocompression bonding InP InP Substrate removal InP Patterning Fabricated structure Top view x y Side view Reflection spectrum x y Ground plane MQW 1 μm 400 nm InP 2
Experimental setup 2 InSb detector SP ZnSe lens Polarizer W~75mW on a sample I~15 kw/cm 2 on a sample Collimating lens BS LP HWP Tunable QCL
Second-harmonic measurements SH power [W] FF intensity squared [kw 2 /cm 4 ] 0 50 100 150 200 250 0.16 yyy xxx 0.06 yxx xyy 0.12 23 W/W 2 57 W/W 2 0.04 0.08 11 W/W 2 0.04 24 W/W 2 0.02 5 W/W 2 2 W/W 2 0.00 0.00 0.0 2.0x10-3 4.0x10-3 6.0x10-3 FF power squared [W 2 ] SH inensity [W/cm 2 ] SH power [nw] 120 yyy 100 80 60 40 20 0.05 0.04 0.03 0.02 0.01 0 0.00 1160 1200 1240 1280 1320 FF wavenumber [cm -1 ] SH intensity [W/cm 2 ] (2) eff yyy 55 nm V (2) eff xxx 36 nm V (2) eff yxx 16 nm V (2) eff xyy 10 By far the highest nonlinear response from a metasurface! ( (2) is 10 3-10 4 times larger than that reported previously) Lee et al., Nature, accepted (2014) nm V
Conversion efficiency and spectra Conversion efficiency [%] FF intensity [kw/cm 2 ] 0 3 6 9 12 15 2.0x10-4 1.5x10-4 1.0x10-4 5.0x10-5 0.0 0 10 20 30 40 50 60 70 FF power [mw] Intensity [a.u.] 1000 800 600 400 200 Intensity [a.u] 1.0 0.8 0.6 0.4 0.2 Bare MQW surface 0.0 2300 2400 2500 2600 2700 Wavenumber [cm -1 ] 0 2300 2400 2500 2600 2700 2800 SH wavenumber [cm -1 ] 2 10-6 conversion efficiency for 15 kw/cm 2 input intensity No nonlinear response from bare MQW surface Lee et al., Nature, accepted (2014)
Electrically-tunable metasurfaces Giant EO effect: 2 ( ) N (ez ) e 12 core i 0 12.4μm 1.5μm 1.7μm
Fabrication
Electrically-tunable polaritonic metasurfaces =6.75 m =6.75 m Lee et al., under review (2014)
Summary Mass producible THz semiconductor-laser-like sources at RT Widely-tunable (1.2-5.9 THz) THz output Highly-nonlinear metasurfaces based on combination of QM engineering of polarization transitions and EM engineering of plasmonic nanoresonators First steps towards flat nonlinear optics paradigm based on ultrathin highly-nonlinear frequency-mixing elements Ultra-fast voltage-tunable metasurface with >5% tuning range in LWIR
Acknowledgements Collaborators: Prof. Andrea Alu group at UT Austin (another AFOSR YIP) Prof. Markus Amann group at TU Munich (MBE growth)
Funding