Polarization dependence of the third-harmonic generation in multiband superconductors

TL;DR

This paper investigates how the polarization dependence of third-harmonic generation in multiband superconductors like NbN is affected by electronic response and pulse width, revealing challenges in experimental observation.

Contribution

It provides a theoretical computation of THG in NbN, highlighting the suppression of polarization dependence due to electronic effects and pulse characteristics.

Findings

Non-resonant electronic response suppresses polarization dependence

Finite pulse width significantly reduces polarization effects

Challenges in observing polarization dependence experimentally

Abstract

In a superconductor the third-harmonic generation (THG) of a strong THz pulse is enhanced below Tc by the resonant excitation of lattice-modulated charge fluctuations (LCF), which modulate the response according to the polarization of the field. Here we compute the THG within a multiband model for the prototype NbN superconductor. We show that the non-resonant contribution coming from the instantaneous electronic response and the finite width of the pulse significantly suppress the polarization dependence of the signal, challenging its observation in real systems.