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

1. 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

2. 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

3. 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

4. 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

5. 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

6. Direct Detection with Photoconductor + Signal Source Photons - V + Background Radiation, e.g. CMB, Atmosphere... Bandpass Filter, B - Typical

7. What is the Sub-Millimeter? Infrared

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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)

14. 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

15. X B Supercurrent Suppression Detector Junctions form a SQUID Al/AlOx/Al Junctions: ~ 60 x 100 nm

16. 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

17. Magnetic Field Dependence of Sub-gap Current

18.  { } eV BCS Predictions for Dark Current T=1.6 K T=250 mK

19. Thermal Dark Current Measurements BCS Predicts: Tc =1.4 K I @ 50 mV Current [pA] Voltage [µV]

20. 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

21. 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

22. Demonstration of an RF-SET Transimpedance Amplifier Input gate 0.5 fF Trim gate

23. Rb V en SQPC Shot Noise Johnson Noise Amplifier Noise Electrical Circuit Model and Noise

24. 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

25. V 1 cm Cryogenic Blackbody as Sub-mm Photon Source Hopping conduction thermistor Micro-machined Si for low thermal conduction

26. T= 1-10K Coming Soon: Photoresponse Measurement Si Chip with SQPC Quartz Window T= 250 mK

27. 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

28. 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