Quantum efficiency and time-domain response of superconducting NbN hot-electron photodetectors

We report our studies on the response of ultrathin superconducting NbN hot- electron photodetectors. We have measured the photoresponse of fem-nm-thick , micron-size structures, which consisted of single and multiple microbridg es to radiation from the continuous-wave semiconductor laser and the femtos econd Ti:sapphire laser with the wavelength of 790 nm and 400 nm, respectiv ely. The maximum responsivity was observed near the film's superconducting transition with the device optimally current-biased in the resistive state. The responsivity of the detector, normalized to its illuminated area and t he coupling factor, was 220 A/W (3 x 10(4) V/W), which corresponded to a qu antum efficiency of 340. The responsivity was wavelength independent from t he far infrared to the ultraviolet range, and was at least two orders of ma gnitude higher than comparable semiconductor optical detectors. The time co nstant of the photoresponse signal was 45 ps, when was measured at 2.15 K i n the resistive (switched) state using a cryogenic electro-optical sampling technique with subpicosecond resolution, The obtained results agree very w ell with our calculations, performed using a two-temperature model of the e lectron heating in thin superconducting films.