Four-photon picosecond spectroscopy of phase transition in high-Tc superconductive thin films

An effective procedure for nonlinear spectroscopy of high-Tc superconductive thin films is proposed. It enables to determine as their phase, as critical temperature, superconducting energy gap width, constants of interaction, etc. Results of its testing for Y-Ba-Cu-O thin films are presented, including results of investigation of relaxation kinetics of preliminary picosecond optical excitation.