Features of the nonlinear response of high-temperature superconductors in coherent picosecond four-photon spectroscopy with high excitation level

Assuming that the nonlinear response of high-temperature superconductors is caused by interband transitions in the electronic spectrum with a metastable energy gap, a model describing the results of experiments performed by picosecond coherent four-photon spectroscopy with a high excitation level is developed. It is shown that a jump in the dependence of the self-diffraction efficiency on the initial temperature of a sample should be observed in the vicinity of the point of superconducting phase transition. It is found that upon biharmonic pump – probing, the energy gap in the electronic spectrum can be detected by a specific two-photon resonance.