We propose a new spectroscopical method for the investigation of ultrafast phenomena in metallic and high-Tc superconductors by using tunable laser beams, without requiring femtosecond laser pulses. The method is based on the self-diffraction of a laser beam from a laser-induced moving grating. The theory of the method has been developed in detail for the case of electron-phonon (EP) interaction. Our theoretical predictions are in good agreement with a recent experiment. Using experimental data we estimated the value of the EP coupling constant for the YBa2Cu3O7 film.