Terahertz spectroscopy of thin superconducting films

Abstract: Far-infrared techniques played a crucial role in the experimental validation of Bardeen–Cooper–Schrieffer (BCS) theory and they continue to provide fundamental information about condensate, quasiparticle and vortex behaviour in both equilibrium and non-equilibrium states of superconductors.
In this talk, we will focus on the response of thin superconducting NbN films. We confirmed that their equilibrium conductivity follows the conventional BCS theory. Application of a dc magnetic field induces vortices in the films, which leads to qualitatively different responses, depending on the mutual orientation of the film, applied magnetic field and probing terahertz electric field.
Excitation by intense femtosecond laser pulses destroys the superconductivity in the films. The following recovery dynamics towards the equilibrium superconducting state occurs via a growth of superconducting islands in the normal-state environment. The spectral weight of the non-equilibrium response of the superconducting phase is found to be shifted compared to a BCS conductivity in thermal equilibrium.