Influence Of Sheet Resistance And Strip Width On The Detection Efficiency Saturation In Micron-Wide Superconducting Strips And Large-Area Meanders

We report our study of detection efficiency (DE) saturation in wavelength range 400-1550 nm for the NbN superconducting microstrip single-photon detectors (SMSPDs) featuring the strip width up to 3 mu m. We observe an expected decrease of the DE saturation plateau with the increase of photon wavelength and decrease of film sheet resistance. At 1.7 K temperature DE saturation can be clearly observed at 1550 nm wavelength in strip with the width up to 2 mu m when sheet resistance of the film is above 630 omega sq(-1). In such strips the length of the saturation plateau almost does not depend on the strip width. We used these films to make meander-shaped detectors with the light sensitive area from 20 x 20 mu m(2) to a circle 50 mu m in diameter. In the latter case, the detector with the strip width of 0.49 mu m demonstrates saturation of DE up to 1064 nm wavelength. Although DE at 1310 and 1550 nm is not saturated, it is as high as 60%. The response time is limited by the kinetic inductance and equals to 20 ns (by 1/e decay), timing jitter is 44 ps. When coupled to multi-mode fibre large-area meanders demonstrate significantly higher dark count rate which we attribute to thermal background photons, thus advanced filtering technique would be required for practical applications.