We develop an explicitly gauge-invariant semiclassical approach to investigate nonlinear response of superconductors to monochromatic THz radiation. We demonstrate that in clean superconductors charge conservation forbids non-linear response to a uniform field. We apply our approach to quasi-two-dimensional films and obtain an explicit expression for the photoinduced current. We find that the photoinduced current exhibits a strong dependence on polarization and incidence angle of the radiation. Our predictions may be directly verified in experiments with quasi-two-dimensional superconducting films.
@article{arxiv.2206.07121,
title = {Nonlinear microwave response of clean superconducting films},
author = {A. A. Radkevich and A. G. Semenov},
journal= {arXiv preprint arXiv:2206.07121},
year = {2022}
}