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.
@article{arxiv.1712.01006,
title = {Polarization dependence of the third-harmonic generation in multiband superconductors},
author = {T. Cea and P. Barone and C. Castellani and L. Benfatto},
journal= {arXiv preprint arXiv:1712.01006},
year = {2018}
}