Superconducting Direct Detectors for Submillimeter

Superconducting Direct Detectors for Submillimeter Astronomy John Teufel Department of Physics Yale University Yale M. Shen C. Wilson L. Frunzio D. Prober R. Schoelkopf

NASA GSFC W. Hsieh M. Li H. Moseley T. Stevenson E. Wollack

Funding from NASA ROSS, GSFC, GSRP

Goal – Cosmic Microwave Background - Measure polarization anisotropy: - Gravity waves from Big Bang; probe of inflation λ = mm to submm regime Æ in space 10-6 of 2.7 K; 100 – 600 GHz 0.1 K detector; 4.2 K antenna Methods = radio + optical 1 K = 20 GHz; mm = 15 K

vs.

Superconducting Direct Detectors for Submillimeter Astronomy

• New – RF Readout Æ Multiplexing, sensitive, fast – On-chip blackbody photon source – Measured optical NEP of 8x10-17 W/√Hz

• Applications – CMB polarization – 2x10-18 3K: 100, 200, 300 GHz

– Higher sensitivity: NEP ≤ 10-19

Types of Detectors Detectors Coherent Mixers eg: SIS Mixer Noise:

Incoherent (Direct Detectors)

Amplifiers

Bolometers

Photoconductors

HEMT

TES

STJ

Phonon Fluctuations

Counting Statistics

Quantum Fluctuations TQ = hf 2k B

NEP ~ 4kT 2G

NEP ~ 2eI dark

Future Space Missions require direct detectors with NEP ~ 10−19 − 10−21 W Hz

A Submm STJ Detector Photons with 2∆ Al ≤ hf ≤ 2∆ Nb

Nb Antenna

break single Cooper pairs Antenna impedance matched to absorber Æ good quantum efficiency

R n =50 Ω

Al Absorber

Small area junctions Æ low dark current Tunnel Junctions R J =10 kΩ

Au quasiparticle trap

Nb

Al AlOx Al

∆Nb ∆Al

V

dI 2e ≈ Responsivity = ~ 3000 A W dP hf

Au

Noise Sources & NEP nγ

I

Photoconductor ⇒ current ∝ photon rate

I = 2enγ + I dark 8

BCS predicts Idark exponential with T

Current [nA]

6 4 2 0

-2 -4 -6 -8 -400

Shot noise is the fundamental sensitivity limit:

-200

0

200

Voltage [mV] Voltage [µV]

in ~ I responsivity I ~ 1 pA → NEP ~ 10−19 W

400

NEP =

Hz

RF-STJ: Measuring MΩ’s @ GHz RSTJ − Q2 Z o Γ= R STJ + Q 2 Z o

Current [pA]

On resonance, tank transforms RSTJ to 50 Ω 10 5 0

Z o = 50Ω

Rn=4 kΩ ∆=200 µV T=260 mK T=270 mK

L RSTJ

-5 -10

-200

-100 0 100 Voltage [µV]

C

200

∆IÆ∆RSTJ Æ ∆Γ

RSTJ Q= ωo L

ωo = 1

Reflection coefficient carries same information as IV e.g.

RF-SET: Schoelkopf et al, Science 280, 1238 (1998) RF-SIN: Schmidt et al, Appl. Phys. Lett. 83, 1002 (2003)

LC

Demonstration of RF-STJ Readout Voltage

4.2 K

300 mK

HEMT

V

~

350 MHz Carrier

fo=350MHz Q=130

0.9

10

0.8

0

0.7 0.6

-10

0.5 -20 -500

0 Voltage [µV]

500

0.4

Gamma

Impedance match with tank circuit

Current [nA]

20

1-D Blackbody as a Photon Source Quasi-optical coupling efficiency:

300 mK

10

-29

10

-34

10

-39

30K

10K

10

1K

11

2

4 6

12

2

4 6

10 Frequency [Hz]

10

Detect blackbody in Wien limit

13

Submm Power [W]

10

-24

detector

Bandpass filter 10

-10

10

-11

10

-12

10

-13

10

-14

10

-15

10

-16 5 6

1

2

3

4 5 6

2

10 Blackbody Temperature [K]

3

A

4 5 6

10

-7

10

-8

10

-9

10

-10

10

-11

10

-12

100

Photo-Current [A]

Power/band [W/Hz]

Bolometer

2 λ η∼

Bolometer as a Blackbody Photon Source

140 µm

1 cm

STJ detector

Feedhorn

300 mK

•Quasi-optical coupling in place •However, initial measurements done without focusing lens Æ η < 10-4

Bandpass filter Bolometer Housing

Monolithic Si Bolometer as blackbody 3 mm

Tb

Coupling the Source & Detector Au

Nb

Vs

Al

Submillimeter Photon Source Joule heated microbridge emits broadband Johnson noise to detector

Submillimeter Photon Detector Photons break Cooper pairs in absorber strip causing decreased RF impedance of the junction

Au-SiO-Nb Overlap Capacitors 100 nm

500 nm

50Ω CPS Transmission Line

Gold Microbridge

Aluminum Junction Near unity coupling of Submm Power

Optical Microscope Picture

Microbridge as Photon Source When Le-e< L< Le-ph ,

Vs

Æ Diffusion-Cooled, Hot Electrons The Johnson noise of these hot electrons provide a submm photon source

L

2 •Fast: τ −1 ≈ π 2D ⇒ GHz

Te

L

Te

•Calculable: x L f = 2∆ h

Absorbed by Al

⎛ hf ⎞ = ∫ ⎜ hf ⎟⎟ df ⎜ kT 2∆ h ⎝ e −1⎠ ∞

Psubmm Reflected by Al

π Vs

Te ≈

8

L

Mattis-Bardeen Calculations Normalized Conductivity

1.0 0.8

σ = σ 1 − iσ 2

sigma1 (Real) sigma2 (Imag)

0.6 0.4 0.2 0.0 0

1

2

3

4

5

6

7

8

9

hf/2D

Impedance

60

Rn Z= (σ 1 − iσ 2 )

R (real) X (imag)

40

20

0 0

2

4

6

8

f/fg 1.0

Coupling = 1 − Γ

Coupling

0.8 0.6 0.4 0.2 0.0 0

2

4

6 f/fg

8

Z − Zo Γ= Z + Zo

2

Measurement Results But chopping @ 10 kHz gives only photocurrent modulation

DC IV curves have response from both photons & phonons Subgap Current & reflected RF signal respond to source heating as expected

1st measurements of detector responsivity

Responsivity = 1700 A

W

Noise is white to lowest measured frequencies Æno 1/f noise

NEP = 8 x10

−17

W Hz

PhotoCurrent [pA]

Detector, Source & Readout are all fast τ tunnel ∼ µ s

1000 800 600 400 200 0 0

200

400

600

Coupled Photon Power [fW]

What if the microbridge is too short? Vs

If L< Lee : •Electrons do not thermalize on the bridge L

•Now there is Shot Noise •For diffusive, mesoscopic conductor: η = 1 3

⎛ ⎛ eV + hν S I = ηG ⎜ ( eV + hν ) coth ⎜ ⎝ 2kT ⎝

⎞ ⎛ eV − hν eV h ν coth + − ( ) ⎟ ⎜ ⎠ ⎝ 2kT

⎞ ⎛ hν ⎞ ⎞ h ν 4 coth + ⎟ ⎜ ⎟⎟ ⎠ ⎝ 2kT ⎠ ⎠

If kT>>eV, hv Æ Johnson Noise

S I → 4kTG If eV>>kT, hv Æ Shot Noise

S I → η 2e I If hv>>eV, kT Æ “Quantum” Noise

S I → 2hν G Schoelkopf et al: PRL ‘97

Comparison of Thermal & Shot Noise When Le-e< L< Le-ph Æ Diffusion Cooled Hot-Electrons Sub-mm photon source: Johnson Noise

Pν =

hν e

hν

kT

−1

When L< Le-e , Le-ph Æ Electron do not thermalize Sub-mm photon source: Shot Noise Pν =

η⎛

4 ⎜⎝

eV + hν ⎝ 2kT

( eV + hν ) coth ⎛⎜

⎞ ⎛ eV − hν ⎟ + ( eV − hν ) coth ⎜ ⎠ ⎝ 2kT

⎞ ⎛ hν ⎞ ⎞ ⎟ − 2hν coth ⎜ ⎟⎟ ⎠ ⎝ 2kT ⎠ ⎠

Summary Conclusions

Future Work

•

Antenna-coupled STJ as a submm detector

•

Improve quasi-optical coupling, measure responsivity, efficiency

•

Dark current well understood & very small: Idark as low as 1 fA

•

Develop higher Q tank circuits for better matching / lower noise for smaller junctions

•

Developed & demonstrated novel RF readout with large BW and low noise

•

Use RF readouts to measure quasiparticle dynamics

•

Integrated on-chip blackbody source

•

Mattis-Bardeen modeling of Al absorber

•

Couple to waveguide probe

•

Microstrip bandpass filters

•

Demonstrated optical NEP using on-chip blackbody

Frequency Dependence Power/band [W/Hz]

10 10 10 10 10 10 10 10

-23

hν

-24 -25

Pν =

Tbb=1K Tbb=2K Tbb=3K

-26 -27 -28 -29 -30 2

3

4 5 6 7

0.01

2

3

4 5 6 7

2

0.1

3

4 5 6 7

e

hν

kT

−1

1

Coupled Power/ band [W/Hz]

f/fg

10 10 10 10 10 10

-24 -25 -26 -27 -28 -29

0

2

4

6

8

Coupled Power/ band [10

-24

W/Hz]

f/fg

35 30

4

x10−3

25 20 15

3

10 5 0 0.8

2

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

f/fg

1 0 0

2

4

6 f/fg

(

Pcoupled = 1 − Γ

5

8

2

)P

ν

Frequency Dependence of Responsivity

Coupled Power/ band [10

-24

W/Hz]

12

10 Tbb= 3 K Unity coupling Calculated coupling Calculated coupling & frequency dependent responsivity

8

6

4

2

0 0

2

4

6

8

f/fg

2e Responsivity[ A W ] = hν