Possible photon instability of high-temperature nonequilibrium superconductor

Possibility of electromagnetic wave amplification at recombination frequencies in nonequilibrium superconductors is analyzed. It is shown that electromagnetic wave may be amplified at inverse function of electron distribution in superconductors with rather low concentration of elastic scattering centers. This situation takes place in standard superconductors during funnel injection from massive sample to thin film. High-temperature superconductors provide for new possibilities, due to the fact that pair binding energy is comparable with boundary phonon frequency. Recombination, related with electron-phonon interaction, will be of multiphonon character and possess lower intensity, as compared to relaxation. Photon instability is possible in the case of inverse occupation in high-temperature superconductors. Absence of inverse occupation (under the condition of energy exclusion for single-phonon recombination processes) and accompanying effects can testify to nonphonon mechanism of pairing