Nonlinear microwave response of clean superconducting films

TL;DR

This paper develops a gauge-invariant semiclassical method to analyze the nonlinear microwave response of clean superconducting films, revealing charge conservation constraints and polarization-dependent effects in photoinduced currents.

Contribution

It introduces a novel gauge-invariant semiclassical approach to study nonlinear responses in superconductors, specifically addressing the behavior under THz radiation in clean, quasi-two-dimensional films.

Findings

Charge conservation forbids nonlinear response to uniform fields in clean superconductors.

Photoinduced current strongly depends on radiation polarization and incidence angle.

Explicit expression for photoinduced current in quasi-two-dimensional films is derived.

Abstract

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.