Spectral features of the nonlinear response of high-temperature superconductor films in methods of degenerate four-photon spectroscopy

The electronic nonlinear response {chi}{sup (3)}{sub ee} of high-temperature superconductor films in methods of degenerate four-photon spectroscopy is calculated. It is shown that the model considering interband electronic transitions in a 'real' band structure with the only fitting parameter (the electron-phonon relaxation rate) well describes all the experimental spectral features of the response, and the methods of degenerate four-photon spectroscopy themselves can reveal, due to the interference character of {chi}{sup (3)}{sub ee}, the presence of the energy gap in the spectrum of states. (laser applications and other topics in quantum electronics)