Single photon counting detector for THz radioastronomy.

D.Morozov1,2, M.Tarkhov1, P.Mauskopf2, N.Kaurova1, O.Minaeva1, V.Seleznev1, B.Voronov1 and Gregory Gol’tsman1

1Department of Physics, Moscow State Pedagogical University,Moscow 119992, Russia

2Cardiff University, Cardiff, CF24 3YB, Wales, UK

Outline

Introduction and motivation

Operation mechanisms of superconducting single-photon detectors (SSPD)

Performance and experimental results for NbN SSPD

Prospective Superconducting material for terahertz single-photon detector

Infrared single-photon detector comparison table

Terahertz Receivers

input signal

Signal

Signal

Signal

N photons

t

p

N

n

N* N*+N

n=N minn) = 1 min (N) = 1/

n

t

Amplifier+integratingdetector

Amplifier+countingdetector

HotElectronBolometer

SinglePhotonCounter

Satellite dish

Energy Relaxation Process

e-e interactionPhoton h

Debyephonons

Cooperpairs

e-e interaction

Quasi particles2

kbT

10-3

10-1

100

eV

Schematic description of relaxation process in an optically excited superconducting thin film.

Mechanism of SSPD Photon Detection

G. Gol'tsman et al, Applied Physics Letters 79 (2001): 705-707A. Semenov et al, Physica C, 352 (2001) pp. 349-356

IV-curves of the 4-nm thick film devices at 4.2 K

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50

5

10

15

20

25C

urr

ent

, A

Voltage, mV

50 load line

B

A

Superconducting state

Metastable region

Resistive state

Mechanism of elliptic spot formation

j=0 => gap equals Δ>ε => qps diffusion is blocked by Andreev reflection

Consider an average quasi-particles (qps) energy ε: T<ε<Δ(T). In the absence of j they would be trapped due to Andreev reflection. Existence of j flowing around the spot makes the gap spatially nonuniform.

j~jc => minimal gap equals Δ-pFvs<ε => qps diffuse in that regions

Schematic gap profile across the spot

vw vw

vL

|vw|>|vL|

Scanning electron microscope image of one of the current SSPDs

Fabrication:• DC reactive magnetron

sputtering of 4-nm-thick NbN film

• Patterning of meander-shaped structure by direct e-beam lithography.

• Formation of Au contacts with optical lithography.

Korneev A. et al, Appl. Phys. Lett. 84 (2004) 5338

Image of new SSPD design(in electron resist before etching process)

52 nm

120 nm

Stripe width 68 nm, spacing 120 nm

Image of new SSPD design(in electron resist before etching process)

Stripe width: 54 nmSpacing: 41 nm

41 nm

Narrower stripeNarrower spacing

We expect:- better light coupling-higher QEWider wavelength range

Resistance vs Temperature Curves for Sputtered NbN Film 4 nm Thick and for SSPD Device

Direct electron beam lithography and reactive ion etching process

Experimental quantum efficiency and dark counts rate vs. normalized bias current at 2 K

0.4 0.5 0.6 0.7 0.8 0.9 1.010-3

10-2

10-1

100

101

102

10-10

10-8

10-6

10-4

10-2

100

102

104

106

1.26 m

0.94 m

1.55 m

0.56 m

QE

, %

Ib/I

c

Dar

k co

unts

, cps

Experimental data for QE (open symbols) and the dark count rate (closed symbols) vs. the bias current measured for 1.55-μm photons

and different temperatures

10 12 14 16 18 20 2210-5

10-4

10-3

10-2

10-1

100

101

102

10-2

10-1

100

101

102

103

104

105

106

107

Dar

k co

unts

, s-1

QE

, %

Ib, A

, T=4.2 K, Ic=16.9A

, T=3.2 K, Ic=19.5A

, T=2.2 K, Ic=21.5A

INFRAREDSPECTROMETER

LiquidHelium

to Pump

Vacuum

RoomTemperature

FilterColdFilter

SuperconductingSingle Photon

Detector (SSPD)

Oscilloscope

BroadbandAmplifier

DC BiasSource

CRYOSTAT

PulseCounter

Bias T

Filament

DiffractionGrating

Filter

EntranceSlit

SphericalMirror

NbN SSPD spectral sensitivity at 3 K temperature

1 2 3 4 5 610-6

10-5

10-4

10-3

10-2

10-1

100

101

I

b/I

c=0.94

Ib/I

c=0.88

Ib/I

c=0.82

Ib/I

c=0.78

T=3K

QE

,%

,μm

Ic =29.7A at 3 K

Spectral dependences of QE for normalized bias currents Ib/Ic>0.9 measured at 4.9 K and 2.9 K

110-5

10-4

10-3

10-2

10-1

100

101

765432

Ib/I

c=0.99

Ib/I

c=0.97

Ib/I

c=0.94

Ib/I

c=0.91

QE

, %

Wavelength, m

10-5

10-4

10-3

10-2

10-1

100

101

102

10.6 0.8 432

QE

, %

Wavelength, m

T=2.9 K, Ib/I

c=0.91

T=4.9 K, Ib/I

c=0.94

T=4.9K

0.5 0.75 1 2.5

101

102

103

= 3

I=16.7 uA

I=19.1 uA

I=19.5 uA

I=19.3 uA

I=19.7 uA

I=17.7 uA

cou

nt

per

sec

on

d

T,K

Experimental data for count per second vs. the temperature measured for 3-μm photons and constant normalized bias

current.

1.7 K insert for liquid helium storage dewar

He pumpvacuum pump

He filter

Device holder

He 1.6 - 4.2 K

He 4.2 K

vacuum volume

capillary with SSPD and LED 3u and 5u

fiber

DC bias connectorfiber connector

RF output connector

22 24 26 28 30 32 34 36

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

= 5

T=4.9K T=4.2K T=3K T=1.7K

QE

%

Ib, A

Experimental data for QE vs. the bias current measured for 5-μm photons and different temperatures

0.8 0.9 2 3 4 5

1E-5

1E-4

1E-3

0.01

0.1

1 = 5

QE

%

T, K

0.95 0.93 0.89 0.86

Experimental data for QE vs. the temperature measured for 5-μm photons and different normalized current.

NbN SSPD noise equivalent power (NEP) at different radiation wavelengths at 1.7K temperature

0.88 0.90 0.92 0.94 0.96 0.98 1.00

10-21

10-20

10-19

10-18

10-17

NE

P, W

/Hz

1/2

Normalized bias current

= 5 = 3 = 1.55 = 1.26

SSPD integrated with optical cavities

SiO2 Au contactAu contact

Sapphire substrate

NbNmeander NbN layer

Metallicmirror layer

Incidentradiation

The design of advanced SSPD structure consists of a quarter-wave dielectric layer, combined with a metallic mirror.

Spectral sensitivity of SSPD integrated with optical cavities

1.0 1.2 1.4 1.6 1.8 2.0 2.210-1

100

101

QE2()

QE1()

QE

, %

, m

SSPD with /4 cavity SSPD without /4 cavity

1.0 1.2 1.4 1.6 1.8 2.0 2.20.0

0.5

1.0

1.5

2.0

2.5

3.0QE

1()/QE

2()

No

rmal

ized

QE

,m

experiment calculated

T~3-3.5K

Tests performed on relatively low-QE devices integrated with microcavities, showed that the QE value at the resonator maximum was of the factor up to 2-3 higher than that for a nonresonant SSPD.

Width=200 nm

Length=10 m

2cmA6104.8(4.2K)cj

1.72D (cm2/s)

(1-5)*106 jc (А/cm2)

170 – 125Ic (µА)

21-2823-3524-38µΩ*cm

52-6977-117120-190Rs (Ω/)

1.38-1.491,2R300/R20

~0.1 ~0.1 ~0.1 Tc (К)

5.17-7.224.4-6.534.2 – 5.2Тс (К)

4 nm3 nm2 nmThickness of the film

Prospective materials for superconducting single-photon detector: MoRe on sapphire substrate

Conclusions

• Our best NbN SSPD exhibit at 1.7 K temperature:• - QE~30% at near infrared (1.3-1.55 m)• - QE~0.25% at 5 m • - extremely low dark counts rate provides NEP about

5x10-21 W/Hz1/2 at near infrared and ~10-19 W/Hz1/2 at 5 m.• MoRe Prospective material for THz SSPD are:

– 200-nm-wide and 10- m-long bridge made from 4-nm-thick MoRe film exhibited single-photon counting capability

Experimental Setup300mK