1. 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

2. 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 ≈ THz
S. Seliverstov t al. IEEE Trans. On Appl. Supercond., 25, 3 (2015) 2

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

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

5. 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

6. 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

7. 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, (R) ( 2007) 7

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

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

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

11. MM-HEB: modeling and simulation
Electric field
Currents 11

12. HEBs: resistive hotspotN 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)

13. 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

14. 2d MM-HEB closer view
1 μm
0.2 μm 14

15. 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

16. 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

17. 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)
 7pW/Hz1/2
The measured photocurrent is
δI≈ 40 nA and
Optical responsivity
SI ≈ 3 A/W 17

18. MM-HEB in 2D array: response to black body source
CLEO QELS
Pulse tube
chopper
300K/77K
Optical  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

19. 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

20. MM-HEB single pixel: response to black body source
Optical  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

21. MM-HEB single pixel: response to black body source
Optical  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

22. 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

23. MM-HEB single pixel: response to black body source

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

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

26. 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

27. 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)

28. Thanks for your attention!

29. Experimental setup

31. AC-HEB noise equivalent power
@Vbias=2.6 mV
 7pW/Hz1/2 31