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Single Photon Counting Detectors for Submillimeter Astrophysics: Concept and Electrical Characterization PowerPoint PPT Presentation


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Single Photon Counting Detectors for Submillimeter Astrophysics: Concept and Electrical Characterization. John Teufel Department of Physics Yale University. Yale: Minghao Shen Andrew Szymkowiak Konrad Lehnert Daniel Prober Rob Schoelkopf. NASA/GSFC Thomas Stevenson Carl Stahle

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Single Photon Counting Detectors for Submillimeter Astrophysics: Concept and Electrical Characterization

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Single Photon Counting Detectors for Submillimeter Astrophysics:Concept and Electrical Characterization

John Teufel

Department of Physics

Yale University

Yale:

Minghao Shen

Andrew Szymkowiak

Konrad Lehnert

Daniel Prober

Rob Schoelkopf

NASA/GSFC

Thomas Stevenson

Carl Stahle

Ed Wollack

Harvey Moseley

Funding from NASA Explorer Tech., JPL, GSFC


Overview

Types of detectors

Noise and sensitivity in detectors

What is the Submillimeter?

The “SQPC” – a high-sensitivity sub-mm detector

Dark currents and predicted sensitivities of SQPC

Time scales and saturation effects

Future Work


Coherent

Measures Amplitude & Phase

For Narrow-band Signals

Sensitivity given in Noise Temperature [K]

Adds a 1/2 photon of noise per mode

Minimum Noise Temperature:TQ=hf/2k

Example: a mixer

Incoherent

Measures only Amplitude

For Broad-band Signals

Sensitivity given by NEP [W/rt(Hz)]

No fundamental noise limit on detector

Ideally limited only by photon statistics of signal or background

Example: a photomultiplier

Types of Detectors


Raleigh-Jeans

Wien

When to Use an Incoherent Detector

  • Average occupancy

  • per mode

  • In the Wien limit:

  • 1/2 photon per mode of

  • noise is unacceptable!

bb


n=Rate of

incoming photons

Ntot =

Photon Counting in Optical

Background Radiation

Signal Source

Photons

PMT

nbackground+

nsource

ndark

Rate of detector false counts

Ntot=(n + ndark)• t


Direct Detection with Photoconductor

+

Signal Source

Photons

-

V

+

Background Radiation, e.g. CMB, Atmosphere...

Bandpass Filter, B

-

Typical


What is the Sub-Millimeter?

Infrared


How Many Photons in the Sub-mm “Dark?”

3 K blackbody

10 % BW

single-mode

Photon-counting (background) limit:

NEP ~ h(n )1/2

Future NASA projects need NEP’s < 10-19 W/rt(Hz) in sub-mm !

see e.g. SPECS mission concept, Mather et al., astro-ph/9812454


Antenna-coupled Superconducting Tunnel Junction (STJ)

Photoconductor direct detector

Each Photon with excites 2 quasiparticles

Nb

Al

Al

Au

AlOx

The SQPC: Single Quasiparticle Photon Counter

Nb antenna

Al absorber

(Au)

m

~

1

STJ detector

junction

sub-mm

photon

Responsivity = 2e/photon = e/ = 5000A/W


What is measured

  • Incident photons converted to current

    Lower Idark=> Higher sensitivity

Nb antenna

Photocurrent

Dark current

(Au)

sub-mm

Current readout should not add noise to measurement

FET or RF-SET should have noise

RF-SET is fast and scalable

photon

STJ detector

junction

V

Ultimate Sensitivity


Integration of RF Circuits, SETs, and sub-mm Detectors

one of four e-beam fields, with SETs and SQPC detectors, and bow-tie antenna

16 lithographic tank circuits on one chip


Sensitivity and Charge Sharing with Amplifier

Q ~ 1000 e-

CSET ~ 1/2 fF

CSTJ ~ 250 fF

RF-SET(30 nV, ½ fF)

FET(2SK152; 1.1 nV, 20 pF)

0.15 e/rt(Hz)

1 x 10-4 e/rt(Hz)

Collects all charge

Collects CSET/CSTJ ~ 0.2%

still ~ 3 times better

Either FET or SET can readout STJ @ Fano limit,

But only SET is scalable for > 50-100 readouts


Bow Tie Antenna

Detector

140 µm

1 µm

Experimental Set-up and Testing

  • Small area junctions fabricated using double angle evaporation

Device mounted in pumped He3 cryostat (T~250mK)


Fig. 2. (a) SQPC detector strip and tunnel junctions are located between two halves of a niobium bow-tie antenna for coupling to submillimeter radiation. A gold quasiparticle trap is included here in the wiring to just one of two dual detector SQUIDs. (b) Close-up view of detector strip and tunnel junctions made by double-angle deposition of aluminum through a resist mask patterned by electron beam lithography. Pairs of junctions form dc SQUIDs, and critical currents can be suppressed with an appropriately tuned external magnetic field.

quasiparticle trap

SQUID

loop

1 µm

junction

antenna

antenna

detector strip


X

B

Supercurrent Suppression

Detector Junctions form a SQUID

Al/AlOx/Al Junctions: ~ 60 x 100 nm


Supercurrent

Cooper pair tunneling affects the subgap current both at zero and finite voltages

DC Josephson effect:

AC Josephson effect:

V

Supercurrent Contributions to Dark Current

DC Power

RF Power

Zen

Zen

Ic sin(J t)

SQPC

*

*Holst et al, PRL 1994


Magnetic Field Dependence of Sub-gap Current


 {

} eV

BCS Predictions for Dark Current

T=1.6 K

T=250 mK


Thermal Dark Current Measurements

BCS Predicts:

Tc =1.4 K

I @ 50 mV

Current [pA]

Voltage [µV]


x-ray

Vabs

1000 mm3

0.01 mm3

½ W

50 kW

RN

ttunnel

2 ms

2 ms

sub-mm

Recombination and Tunneling Times

Vabs

ttunnel ~ VabsRN

lead

(large

volume)

g

thermal trecomb ~ 100 ms

@ 0.26 K

absorber

at low power:

ttunnel << trecomb

so quantum efficiency

is high

False count rate = Idark/e = 3 MHz for ½ pA


Saturation: Recombination vs. Tunneling

Current

Noise

I ~ P1/2

Absorber gap reduced by excess q.p.’s

trec ~ ttunn

I ~ P

NEP ~ P1/4

Idark

NEP ~ P1/2

Power (P)

(or photon rate, Ng)

Ng~ Id/e

Nsat ~ (tth/ttun) Id/e

Psat~ 0.02 pW; scales as 1/RN


Demonstration of an RF-SET Transimpedance Amplifier

Input gate

0.5 fF

Trim gate


Rb

V

en

SQPC

Shot Noise

Johnson Noise

Amplifier Noise

Electrical Circuit Model and Noise


Future Work: Detecting Photons

Problem: Need to couple known amount of sub-mm radiation to detector

Solution: Use blackbody radiation from a heat source in the cryostat


V

1 cm

Cryogenic Blackbody as Sub-mm Photon Source

Hopping conduction thermistor

Micro-machined Si for low thermal conduction


T= 1-10K

Coming Soon: Photoresponse Measurement

Si Chip with SQPC

Quartz Window

T= 250 mK


Advantages of SQPC

Fundamental limit on noise = shot noise of dark current

Low dark currents imply NEP’s < 10-19 W / rt.Hz

High quantum efficiency – absorber matched to antenna

High speed – limited by tunneling time ~ msec

Can read out with FET, but SET might resolve single g’s

Small size and power (few mm2 and pW/channel)

Scalable for arrays w/ integrated readout


Summary

When hf>kTbb, a photon counter is preferred

In the sub-mm, no such detector exists

The SQPC would be a sub-mm detector with unprecedented sensitivity

Contributions to detector noise have been measured and are well-understood

Photocurrent measurements in near future


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