PHOTORESPONSE MECHANISMS OF THIN SUPERCONDUCTING FILMS AND SUPERCONDUCTING DETECTORS

The photoresponse of ordinary and high-T-c superconductors depends cri tically on the hierarchy of relaxation times, such as the electron-pho non and phonon-electron scattering times, the time of phonon escape fr om a superconducting film and also the phonon return time. For thin fi lms of cuprates, close to the superconducting transition the following components of transient response are identified. The picosecond photo response is attributed to the dynamics of nonequilibrium quasiparticle s and Cooper pairs. The nanosecond response is described by the therma l boundary resistance (the Kapitza resistance) between a superconducti ng film and a substrate. The microsecond response is associated with t he phonon diffusion in the substrate. Using experimental results, we d educe the characteristic time of electron-phonon relaxation and parame ters of the film-substrate interface. The kinetic inductance photoresp onse of superconductors with s- and d-wave pairing far below the super conducting transition is also calculated. We study parameters (respons ivity, operating speed and noise equivalent power) of a nonequilibrium detector, in which only electron states are changed under the radiati on, while the film phonons stay in thermodynamic equilibrium with the substrate. Our analysis demonstrates that the nonequilibrium supercond ucting detectors have essential advantages compared to superconducting bolometers and other detectors.