PHOTORESPONSE MECHANISMS OF THIN SUPERCONDUCTING FILMS AND SUPERCONDUCTING DETECTORS

The photoresponse of ordinary and high-T c superconductors depends critically on the hierarchy of relaxation times, such as the electron–phonon and phonon–electron scattering times, the time of phonon escape from a superconducting film and also the phonon return time. For thin films of cuprates, close to the superconducting transition the following components of transient response are identified. The picosecond photoresponse is attributed to the dynamics of nonequilibrium quasiparticles and Cooper pairs. The nanosecond response is described by the thermal boundary resistance (the Kapitza resistance) between a superconducting film and a substrate. The microsecond response is associated with the phonon diffusion in the substrate. Using experimental results, we deduce the characteristic time of electron–phonon relaxation and parameters of the film-substrate interface. The kinetic inductance photoresponse of superconductors with s- and d-wave pairing far below the superconducting transition is also calculated. We study parameters (responsivity, operating speed and noise equivalent power) of a nonequilibrium detector, in which only electron states are changed under the radiation, while the film phonons stay in thermodynamic equilibrium with the substrate. Our analysis demonstrates that the nonequilibrium superconducting detectors have essential advantages compared to superconducting bolometers and other detectors.