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

The electronic nonlinear response χ⁽³⁾_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 χ⁽³⁾_ee, the presence of the energy gap in the spectrum of states.