Picosecond four-photon spectroscopy of kinetics of nonequilibrium state of HTSC

We propose new and promising procedure for nonlinear spectroscopy of high-Tc superconductive thin films. It enables us to determine the phase transition temperature, superconducting energy gap width, interaction constants, etc. For Y-Ba-Cu-O thin films, we present our results in testing of this procedure including investigation of relaxation kinetics after picosecond optical excitation of the film.