Optical coupling of SC nanosensors for THz frequencies S. Cibella, P. Carelli, M. G Castellano, F. Chiarello, A. Gaggero, E. Giovine, G. Torrioli and R. Leoni IFN-CNR, Roma, Italy G. Scalari Institute of Quantum Elecronics, ETH Zürich, Switzerland

Antenna coupled AC-HEB The conventional way to couple the THz radiation to the hotspot in the NbN bridge 𝜈≈1−10𝑇𝐻𝑧 ( 𝜆≈300−30𝜇𝑚) 0.2 um 2 um Deposition on silicon of 4-5 nm NbN ( temperature sensor) NbN etch by RIE Two steps of electron beam lithography : Define pad antenna and alignment markers Define HSQ mask NbN etch by RIE Estimated optical bandwidth of the log-spiral antenna BW ≈ 0.4 -30 THz S. Seliverstov t al. IEEE Trans. On Appl. Supercond., 25, 3 (2015) 2

Motivation Change from a broadband antenna to a narrow-band, high field enhancement split ring resonator.

Ongoing direction The goal is to realize an on-chip THz spectrometer based on HEBs coupled to narrow-band LC resonators .

Superconducting hot electron bolometers (HEB) - our approach CLEO QELS Outline Superconducting hot electron bolometers (HEB) - our approach Phonon cooled AC-HEB and MM-HEB Basic principles Detector fabrication Readout Circuit Optical characterization: Black body source QCL emitters Conclusions

Optical Coupling?? MM- or AC- HEB N S HEB consists of two thick contact metal pads connected by a small superconducting bridge N S Optical Coupling?? S =NbN nano bridge: 4-5 nm thick ~0.2 um long ~1 um wide 6

MM- HEB Different approach of coupling the NbN sensitive element to THz radiation: array of split ring resonators (SRR) w p l g 0.2μm NbN sensor l(um) w( um) g ( um) p( um) fr( THz) Sample 1 15 4 1.5 18 2.2 Sample 2 16 2.45 Sample 3 13 2.5 2.7 Sample 4 12 2.9 Padilla et al. Physical Review B 75, 041102 (R) ( 2007) 7

MM-HEB: schematic view We studied 2D planar array HEB Linear array Single element 8

MM-HEB: modeling and simulation LC mode E field Dipolar mode E field

MM-HEB: modeling and simulation LC mode E field Electric field Currents 10

MM-HEB: modeling and simulation Electric field Currents 11

HEBs: resistive hotspot N LH Antenna L TC Lithographic antenna or a metamaterial is electrically coupled to a temperature sensor (NbN bridge). Formation of a normal-state ( T>Tc) hot spot at the center of the superconducting bridge Input power modulates the current through the bridge Current ( mA) Superconducting region Transition region Normal region Modulates the volume of the hot spot Modulation of R E. Bründerman, H. W. Hübers, M. F. Kimmit “Terahertz Techniques” Springer 2012 Voltage ( mV)

HEB(MM-AC) L Au NbN Substrate Two steps of electron beam lithography : fabrication process Substrate NbN Au L Two steps of electron beam lithography : Define pad antenna and alignment markers Define HSQ mask NbN etched by RIE Deposition on silicon of 4-5 nm NbN (temperature sensor) by DC-Magnetron sputtering NbN etching by RIE

2d MM-HEB closer view 1 μm 0.2 μm 14

Electronic readout and optical setup First proposed by J.S. Pentilla et al. SUSTE (2006) S.Cibella et al. J. Low. Temp. Phys. 154 ( 2009) Detector mounted in a pulse tube cooler with cold filters To≈5K Rfb Vout HEB Vb - + AD797 I T3K Pulse tube with detector Optical window with filter chopper Free space coupled A current sensitive transimpedance amplifier provides: a constant Voltage bias an output related to the bolometer current ETF Tydex cold Filter

I-V and R-T characteristics of a AC- HEB T3K NbN-Au Contact pads NbN bridge Contact pads Contact pads Tc2 Tc1 16

I-V and R-T characteristics of a AC- HEB The input power Pi, due to blackbody emission and assuming to have a single spatial mode is Pi = P1 + P2= kB (300K – 77K)BW ≈ 14 nW BW≈ 4.5 THz (cold filter Tydex) NEP(@10kHz)  7pW/Hz1/2 The measured photocurrent is δI≈ 40 nA and Optical responsivity SI ≈ 3 A/W 17

MM-HEB in 2D array: response to black body source CLEO QELS Pulse tube chopper 300K/77K Optical NEP(@10kHz)  4.7 pW/Hz1/2 @Vo = 1.27 mV The input power Pi, due to blackbody emission and assuming to have a single spatial mode is Pi = P1 + P2=kB (300K – 77K)BW ≈ 4.5 nW BW≈ 1.5 THz (MM determined) With the same BB source the measured photocurrent is δI≈ 200 nA @ Vo=1.27mV and Optical responsivity SI ≈ 40 A/W

MM-HEB in linear array: response to black body source With the same BB source the measured photocurrent is δI≈ 150 nA @ Vo≈1mV and Optical responsivity SI ≈ 33 A/W

MM-HEB single pixel: response to black body source Optical NEP(@10kHz)  24pW/Hz1/2 @Vo = 1.20 mV With the same BB source the measured photocurrent is δI≈ 200 nA @ Vo≈1.20mV and Optical responsivity SI ≈ 63 A/W

MM-HEB single pixel: response to black body source Optical NEP(@10kHz)  24pW/Hz1/2 @Vo = 1.20 mV With the same BB source the measured photocurrent is δI≈ 200 nA @ Vo≈1.20mV and Optical responsivity SI ≈ 63 A/W

MM-HEB single pixel: response to black body source With the same BB source the measured photocurrent is δI≈ 200 nA @ Vo≈1.20mV and Optical responsivity SI ≈ 63 A/W

MM-HEB single pixel: response to black body source

HEB response to a QCL rise time  250 ns and BW  1.4 MHz HEB response to a QCL @ 2.9 THz HEB QCL rise time  250 ns and BW  1.4 MHz Limited by electronic readout (intrinsic speed C/G =50ps) 24

MM-HEB spectral characterization: FT-IR spectrometer QCL source : QCL at 2.5-3 THz FTIR spectral resolution : 0.075 cm-1 5 scans at 40 kHz scanning speed 25

MM-HEB spectral characterization: FT-IR spectrometer Globar MIR source HEB Source : Globar Resolution: 10 cm-1 1800 scans T5K V0=1.27 mV

CLEO QELS Conclusions We have demonstrated metamaterial coupled Hot electron bolometers We measured both laser coupled and blackbody source through FTIR We found MM-HEBs show an optical responisivity larger than AC-HEB as in agreement with Palaferri et al. Acoll = 3λ²/4π Ongoing directions: on chip THZ spectrometers based on MM-HEBs Palaferri et al. New Journal of Physics 18( 2016)

Thanks for your attention!

Experimental setup

AC-HEB noise equivalent power @Vbias=2.6 mV NEP(@10kHz)  7pW/Hz1/2 31