Middle-Infrared to visible-light superconducting single-photon detector.

1. MSPU Gregory Gol’tsman Department of Physics, Moscow State Pedagogical University, Moscow 119992, Russia Middle-Infrared to visible-light superconducting single-photon detector. Outline Introduction and motivation Operation mechanisms of superconducting single-photon detectors (SSPD) Performance and experimental results for NbN SSPD Prospective Superconducting materials for terahertz single-photon detector

2. The research is funded by INTAS project #03-51-4145"Superconducting Hot Electron Single-Photon Counter for Terahertz Radioastronomy" MSPU Goal: To develop a novel concept of ultra-sensitive superconducting quantum detector for terahertz application in radioastronomy. Participants: Space Research Organization Netherlands, The Netherlands (coordinator) Moscow State Pedagogical University, Russia DLR Institute of Space Sensor Technology, Germany Cardiff University, United Kingdom Institute of Applied Physics, Russia Institute for Low Temperature Physics and Engineering, Ukraine

3. Optical single-photon detector comparison table

4. Semiconducting vs. superconducting single-photon detectors for 1.3-1.5 m wavelength Superconductors One optical photon creates ~100–1000 excited electrons (superconducting gap ~ 2 meV for NbN). Relaxation times are picosecond. Extremely low dark counts. No gating required, simple biasing source. Semiconductors One optical photon creates only one electron-hole pair (typical bandgap 1-2 eV). Room temperature and cryogenic operation. Large dark counts. Complicated biasing schemes. Low temperature environment reduces background noise and thermal fluctuations responsible for dark counts.

5. Energy Relaxation Process Schematic description of relaxation process in an optically excited superconducting thin film.

6. Mechanism of SSPD Photon Detection G. Gol'tsmanet al, Applied Physics Letters 79 (2001): 705-70 A. Semenov et al, Physica C, 352 (2001) pp. 349-356

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

8. Mechanism of elliptic spot formation 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=0 => gap equals Δ>ε => qps diffusion is blocked by Andreev reflection j~jc => minimal gap equals Δ-pFvs<ε => qps diffuse in that regions vw vw |vw|>|vL| vL Schematic gap profile across the spot

9. Alternative way of resistive state formation mean pair velocity vs'in case of reduction of Cooper-pair density: if vs' exceeds critical velocity vc the resistance appears without formation of normal state in the hotspot The grey shaded volume is a cloud with reduced Cooper-pair density ns − ns. If the density is low enough such the mean pair velocity v′s exceeds the critical velocity vс in a cross-section with longitudinal extension  or larger, this cross-section switches into the normal conducting state. A. Semenov, A.Engel, H.-W. Hubers, K. Il'in, M. Siegel. The European Physical Journal B, 47(4) 2005, pp.495-501

10. Probable vortex contribution on dark counting Existence of vortices in a narrow strip (w<<) : Vortex motion below TBKT. Likharev criterion w > 4.4(T) is satisfied => External magnethic field H=0 Resistance may be caused by vortex motion events (Berezinskii–Kosterlitz– Thouless - BKT) FL Ib VAPs below TBKT free vortices above TBKT TBKT: FL Source of free vortices below TBKT: Vortex- antivortex Pair (VAP) A pair of free vortices Current assisted thermal unbinding (CATU) A. Engel,A.D. Semenov, H.-W. Hubers, K. Il’in,M.Siegel "Fluctuation effects in superconducting nanostrips"; arXiv:cond-at/0411033

11. 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. Gol'tsman G. et al, Appl. Phys. Lett. 79 (2001) 705 Korneev A. et al, Appl. Phys. Lett. 84 (2004) 5338

12. Fabrication of SSPD Using Direct Electron Beam Lithography and Reactive Ion Etching Process

13. Fabrication of SSPD Using Direct Electron Beam Lithography and Reactive Ion Etching Process

14. Image of new SSPD design(in electron resist before etching process) 120 nm 52 nm Stripe width 68 nm, spacing 120 nm

15. Image of new SSPD design (in electron resist before etching process) • Narrower stripe • Narrower spacing • We expect: • - better light coupling • higher QE Wider wavelength range 41 nm Stripe width: 54 nm Spacing: 41 nm

16. Resistance vs Temperature Curves for Sputtered NbN Film 4 nm Thick and for SSPD Device Direct electron beam lithography and reactive ion etching process

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

18. 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

19. NbN SSPD noise equivalent power (NEP) at different radiation wavelengths at 2K temperature

20. 1 GHz-rate photoresponse train (real-time oscilloscope picture) for 3.5 nm thick NbN film 10x10 m2 device and 1.55 m photons T = 4.2 K Detector photoresponse speed is limited by its kinetic inductance A. Korneev et al, Appl. Phys. Lett., 84, 5339 (2004)

21. Time delay and jitter of SSPD signal Timing electronics jitter – 20 ps J. Zhang, W. Slysz, A. Verevkin, R. Sobolewski, O. Okunev, and G. N. Gol’tsman, "Time Delay of the Resistive State Formation in Superconducting NbN Stripes Illuminated by Single Optical Photons", Phys. Rev. B 67, No.13, pp. 132508-1-4 (2003)

22. Jitter of a NbN SSPD at 1550 nm and 778 nm wavelengths is below 18 ps 1.0 0.8 778 nm 0.6 FWHM 0.4 ~18 ps 0.2 0.0 1.0 1550 nm 0.8 0.6 FWHM ~18 ps 0.4 0.2 0.0 Oscilloscope: 50-GHz bandwidth Tektronix TDS-8000B Laser source: Pritel OptiClock 1 GHz rate, 1.6 ps pulses, 70 fs jitter, Signal amplifiers: Miteq JS3-00101800-24, 0.1-18 GHz bandwidth A. Korneev et al, Appl. Phys. Lett., 84, 5339 (2004)

23. Inductance-limited recovery of NbN nanowires Output pulses for 100 nm wide wires at T=4.2K with Ibias=11.5A. Device dimensions: (a) 10m x 10m meander, total length 500m, (b) 4m x 6m (120m), (c) 3m x 3.3m (50m), (d) 5 m-long single wire; (e) Electrical model (f) Inductance vs resistance A.Kerman, E.Dauler,W.Keicher,J.Yang,K.Berggren,G.Gol'tsman, B.Voronov, "Kinetic-inductance-limited reset time of superconducting nanowire photon counters", submitted to Appl. Phys. Lett., preprint available at http://arxiv.org/abs/physics/0510238

24. SSPD with reduced response time Ibias Lk1 Lk2 50Ω R R

25. NbN SSPD spectral sensitivity at 3 K temperature Ic =29.7A at 3 K

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

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

28. Spectral sensitivity of SSPD integrated with optical cavities 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.

29. Design of the device integrated with optical cavity and anti-reflection coating Top-down view of optical cavity Device cross-section transmission electrton micrograph Schematic design K.Rosfjord, J.Yang, E.Dauler, A.Kerman, V.Anant, B.Voronov, G.Gol'tsman, K.Berggren "Nanowire single-photon detector with an integrated optical cavity and anti-reflection coating", submitted to Optics Express

30. Histogram of the detection efficiencies for (a) bare devices after initial fabrication, (b) after addition of the cavity structure and mirror, (c) after an anti-reflection coating was added T=1.8 K =1550 nm 132 devices were tested K.Rosfjord, J.Yang, E.Dauler, A.Kerman, V.Anant, B.Voronov, G.Gol'tsman, K.Berggren "Nanowire single-photon detector with an integrated optical cavity and anti-reflection coating", submitted to Optics Express

31. Terahertz Receivers

32. Background limited Ti-based Single Photon Counter

33. Prospective materials for submillimeter single photon detector

34. Prospective materials for superconducting single-photon detector: MoRe on sapphire substrate Thickness of the film 2 nm 3 nm 4 nm Тс (К) 4.2 – 5.2 4.4-6.53 5.17-7.22 Tc (К) ~0.1 ~0.1 ~0.1 R300/R20 1,2 1.38-1.49 Rs (Ω/□) 120-190 77-117 52-69 µΩ*cm 24-38 23-35 21-28 Ic (µА) 170 – 125 jc (А/cm2) (1-5)*106 D (cm2/s) 1.72

35. Resistance vs temperature of 4 nm thick MoRe microbridge Width=200 nm Length=10 mm

36. Prospective materials for superconducting single-photon detector: Si-Mo-Si on sapphire substrate Processing Technology: Magnetron sputtering in Argon atmosphere. Ar pressure 5 mTorr. For bulk material In Collaboration with Institute for physics of microstructures Russian academy of sciences. Nizhny Novgorod. (Rogov V.V. et al). Temperature dependence of Si-Mo-Si film resistance

37. Si-Mo-SiDiffusivity

38. Critical current of structured Si-Mo-Si 100GHz waveguide mixer chip

39. Heterodyne technique for energy relaxation time measurement Experimental setup 100 GHz waveguide mixer block

40. Energy relaxation: electron-phonon time and escape time of nonequilibrium phonons T=5.2K T=1.6K N. Perrin, and C Vanneste, Phys. Rev. B 28, 5150 (1983)

41. Temperature dependence of electron-phonon relaxation time for Si-Mo-Si structure

42. Ta films parameters # d, nm Rs, Ohm/sq Tc,K Ohm*cm Ta4 10 180 - 180 Ta14 10 117 - 117 Ta15 15 50 - 75 Ta13 20 26.3 1.7 53 Ta11 40 8.88 2.68 36 Ta10 100 3.42 3.49 34 Ta9 250 1 3.95 21 Ta 8 500 0.42 4.2 21 Ta5 500 0.52 3.92 26 Superconducting transition of thin Ta films Ultrathin films Ta are not superconducting

43. Two-channel photon counter system

44. 1.5 K insert for liquid helium storage dewar

45. Conclusions • Our best NbN SSPD exhibit at 2 K temperature: • - QE~30% at near infrared (1.3-1.55 mm) • - QE~0.1% at 3 mm and QE~10-3-10-2% at 6 mm • - Although large-active-area SSPD's response time is significantly affected by the kinetic inductance, it is capable of GHz counting rate with jitter <18 ps • - extremely low dark counts rate provides NEP about 5x10-21 W/Hz1/2 at near infrared and ~10-18 W/Hz1/2 at 6 mm. • Prospective materials for THz SSPD are: • MoRe: 200-nm-wide and 10- mm-long bridge made from 4-nm-thick MoRe film exhibited single-photon counting capability • Multilayer Si-Mo-Si demonstrated Tc=3.7-6.6 K, diffusivity D=0.52 cm2/s, t esc=2.7 ns and te-ph=7.6 ns at 1.6 K • NbN SSPD was implemented in the integrated IR photon counter for quantum correlation experiment and quantum cryptography and communication