1. I. Milostnaya, A. Korneev, M. Tarkhov, A. Divochiy, O. Minaeva, V. Seleznev, N. Kaurova, B. Voronov, O. Okunev, G. Chulkova, K. Smirnov, and G. Gol’tsman Moscow State Pedagogical University, Moscow 119992, Russia Overview Single Photon Superconducting Detectors (SSPDs) ● Developed in collaboration of the MSPU (Prof. Gregory Gol’tsman group) and University of Rochester (Prof. Roman Sobolewski group) 1-4, fabricated at MSPU ● Based on superconducting nanowires made from ultra-thin superconducting films. ● Operation is based on photon-induced resistive hotspot formation in the current-carrying superconducting nanowire 2. ● Operated at temperatures 2 - 4.2 K well below T c at a bias current I b close to I c. ● Capable of single photon counting in the VIS and IR waveranges. ● Further improvement of the SSPD performance in the IR and THz ranges is expected by implementation of superconducting materials with a lower Tc having a narrow energy gap. ● First MoRe SSPDs were fabricated and tested. Superconducting Single Photon Nanowire Detectors Development for IR and THz applications State-of-the-art NbN SSPDs L08 At operational temperature4 K2 K Quantum Efficiency, % at VIS>10>30 at NIR (1.3–1.55 μm)>6> 20 at MIR (6 μm)~0.5 Dark Counts, s -1 <1<10 - 4 NEP, W/Hz 1/2 at VIS and NIR~10 -17 ~10 -21 at MIR (5 μm)~10 -18 Wavelength range, μm0.5 - 6 Device performances

2. SSPDs Fabrication at MSPU The mature technology is developed for NbN SSPDs. Deposition of 4-nm-thick NbN film on Sapphire substrate by DC magnetron sputtering of Nb target in in Ar+N 2 mixture Patterning of stripe windows by direct e-beam lithography Formation of Au contacts by optical lithography Formation of meander-shaped structure with reactive ion etching NbN films properties: Thickness 4 nm Critical temperature Tc=10-11K Transition width Δ Tc=0.3K Critical current density j c =7x10 6 A/cm 2 ’Standard’ Device Design SSPDs are designed as meander-shaped nanowires connected to Au contact pads intended for connection to a coplanar waveguide. Drawback: response time is significantly affected by a nanowire kinetic inductance. Response of a single 500-µm- long nanowire SEM image of the 10  m x 10  m SSPD Advanced SSPD Design Multisection SSPDs with lower kinetic inductance are designed as several meander nanowires connected in parallel Subnanosecond photoresponse time achieved Oscillograms of the response for 2-section (a) and 5-section (b) SSPDs Spectral dependence of QE for a NbN SSPD at 3K and 5K at bias current of 0.94 Ic. Meander size 10x10μm 2 Nanowire width 80 - 100 nm Line pitch 200 nm Nanowire length up to 500 μm

3. MoRe SSPD The use of a material with the narrow energy gap and low quasiparticles diffusivity should shift the detectors sensitivity towards longer wavelengths. Motivation. ‘Standard’ SSPD nanowire meander structures of good quality were fabricated and tested. Ultrathin (4-10 nm thick) MoRe films were deposited by DC magnetron sputtering of a Mo 60 /Re 40 target. Oscillogram of the single-photon response of a 200-nm-wide MoRe nanowire Dependence of a counting rate on a bias current First single-photon response was obtained for 1.26  m photons at operational temperature of 4.2K. Technology of ultra-thin MoRe films suitable for SSPD was developed. Best MoRe films properties: Thickness 4 nm 10 nm Critical temperature Tc=7.7 K 9.7K Transition width Δ Tc=0.1K 0.2K Critical current density at 4.2K j c =1.1x10 6 A/cm 2 Sheet resistance Rs=65-75  / □

4. References [ 1] G. Gol'tsman at al., Appl. Phys. Lett. 79 (2001), 705. [2] A. Semenov at al., Physica C, 352 (2001), 349. [3] G. Gol’tsman et al., IEEE Trans. on Appl. Supercond. 13(2) (2003), 192. [4] A. Korneev et al., Appl. Phys. Lett. 84 (2004), 5338. [5] A. Korneev et al., SPIE Europe Int. Congress on Optics and Optoelectronics, Prague, Czech Republic, 16 - 19 April 2007.