Transcript Education and Outreach Program

Dark Current Measurements of a
Submillimeter Photon Detector
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
•The “SQPC” – a high-sensitivity sub-mm detector
•Contributions to dark currents
•Aluminum Junction Characterization
•Implications on overall sensitivity
•Future Work
Motivation
• Sub-mm: ~ 100 mm -- 1 mm
•Sub-mm detectors today:
17
NEP ~ 10
W
Hz
•Future NASA missions will require :
20
NEP ~ 10
•Detectors should also be:
–Fast
–High quantum efficiency
–Scalable for arrays
W
Hz
The SQPC: Single Quasiparticle Photon Counter
Nb antenna
Al absorber
(Au)
•Antenna-coupled Superconducting
Tunnel Junction (STJ)
•Photoconductor direct detector
•Each Photon with 2 Al    2 Nb
excites 2 quasiparticles
~ 1 m
STJ detector
junction
sub-mm
photon
Nb
Al
Al Au
AlOx
Responsivity=2e/photon=e/D=5000A/W
Electrical Circuit Model and Noise
Rb
V
en
SQPC
I 
2
2eI dark
en
4kT



2
Rb
Z
Shot
Noise
Johnson Amplifier
Noise
Noise
Sensitivity  I
2eI dark
2eI dark
Contributions to the Dark Current
Supercurrent
Temperature
•Cooper pair tunneling affects
the subgap current both at zero
and finite voltages
•Thermal energy breaks Cooper
pairs and excites quasiparticles
•Due to rectification of the AC
Jospephson Effect:
I c2 ( B) Re Z ( J )
I dark ( B) 
2Vbias
•BCS Predicts:
I dark (T ) e

k BT
Experimental Set-up and Testing
•Small area junctions fabricated
using double angle evaporation
Bow Tie Antenna
Detector
140 µm
1
µ
m
•Device mounted in pumped
He3 cryostat (T~250mK)
SQUID w/ Rtot = 44 kW
Al/AlOx/Al Junctions: ~ 60 x 100 nm
Current [nA]
Supercurrent Suppression
40
20
0
-20
-40
BX
-400-200 0 200 400
4
2
0
-2
Current [pA]
Current [nA]
Voltage [mV]
2
0
-2
-4
0.0
1.0
2.0
Magnetic Field [mT]
250
260
270
280
Magnetic Field [mT]
Thermal Dark Current Measurements
BCS Predicts: I dark (T ) e

k BT
BCS (357mK)
357 mK
256 mK
10
10
I/500
Current [A]
Current [pA]
20
10
10
10
10
0
-9
Tc =1.4 K
I @ 50 V
-10
-11
Rn= 13.1 kW
Rn= 9 kW
Rn= 47 kW
-12
-13
2
3
4
5
6
7 8 9
1
0
200
400
Temperature [K]
Voltage [µV]
Imin ~
1
19
2
pA  NEP  10 W
Hz
Future Work: Detecting Photon
Use a low thermal
conductance blackbody
to shine photons on
detector
Conclusions
•SQPC: a sub-mm detector capable
of NEP <
19
10
W
Hz
•Characterized two sources dark current
•Measured dark current < 0.5 pA
•Tests with Blackbody source underway