Single photon counting detector for THz radioastronomy. D.Morozov 1,2, M.Tarkhov 1, P.Mauskopf 2, N.Kaurova 1, O.Minaeva 1, V.Seleznev 1, B.Voronov 1 and Gregory Gol’tsman 1 1 Department of Physics, Moscow State Pedagogical University, Moscow , Russia 2 Cardiff University, Cardiff, CF24 3YB, Wales, UK Outline Introduction and motivation Operation mechanisms of superconducting single-photon detectors (SSPD) Performance and experimental results for NbN SSPD Prospective Superconducting material for terahertz single- photon detector

Infrared single-photon detector comparison table

Terahertz Receivers

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

Mechanism of SSPD Photon Detection G. Gol'tsman et al, Applied Physics Letters 79 (2001): A. Semenov et al, Physica C, 352 (2001) pp

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

Mechanism of elliptic spot formation j=0 => gap equals Δ>ε => qps diffusion is blocked by Andreev reflection 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~j c => minimal gap equals Δ-p F v s <ε => qps diffuse in that regions Schematic gap profile across the spot vwvw vwvw vLvL |v w |>|v L |

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. Korneev A. et al, Appl. Phys. Lett. 84 (2004) 5338

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

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

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

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

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

NbN SSPD spectral sensitivity at 3 K temperature I c =29.7  A at 3 K

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

Experimental data for count per second vs. the temperature measured for 3-μm photons and constant normalized bias current.

1.7 K insert for liquid helium storage dewar

Experimental data for QE vs. the bias current measured for 5-μm photons and different temperatures

Experimental data for QE vs. the temperature measured for 5-μm photons and different normalized current.

NbN SSPD noise equivalent power (NEP) at different radiation wavelengths at 1.7K temperature

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

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.

Width=200 nm Length=10 m m 1.72D (cm 2 /s) (1-5)*10 6 j c (А/cm 2 ) 170 – 125I c (µА)  µ Ω *cm  R s ( Ω /□) ,2R 300 /R 20 ~0.1  T c (К) – 5.2Т с (К) 4 nm3 nm2 nmThickness of the film Prospective materials for superconducting single- photon detector: MoRe on sapphire substrate

Conclusions Our best NbN SSPD exhibit at 1.7 K temperature: - QE~30% at near infrared (  m) - QE~0.25% at 5  m - extremely low dark counts rate provides NEP about 5x W/Hz 1/2 at near infrared and ~ W/Hz 1/2 at 5  m. MoRe Prospective material for THz SSPD are: –200-nm-wide and 10-  m-long bridge made from 4-nm- thick MoRe film exhibited single-photon counting capability

Experimental Setup 300mK