THz non-linear optical response in cuprates: predominance of the BCS response over the Higgs mode

TL;DR

This paper provides a detailed theoretical analysis of the non-linear THz optical response in cuprate superconductors, showing the dominance of the BCS quasiparticle response over the Higgs mode and matching experimental polarization dependence.

Contribution

It offers the first comprehensive quantitative model for the non-linear THz response in cuprates, emphasizing the BCS quasiparticle contribution and its polarization dependence.

Findings

BCS quasiparticle response dominates the non-linear THz response in cuprates.

The model accurately reproduces third-harmonic generation polarization dependence.

Additional effects may influence the Kerr effect beyond the BCS quasiparticle contribution.

Abstract

Recent experiments with strong THz fields in unconventional cuprates superconductors have clearly evidenced an increase of the non-linear optical response below the superconducting critical temperature Tc. As in the case of conventional superconductors, a theoretical estimate of the various effects contributing to the non-linear response is needed in order to interpret the experimental findings. Here we report a detailed quantitative analysis of the non-linear THz optical kernel in cuprates within a realistic model, accounting for the band structure and disorder level appropriate for these systems. We show that the BCS quasiparticle response is the dominant contribution for cuprates, and its polarization dependence accounts very well for the third-harmonic generation measurements. On the other hand, the polarization dependence of the THz Kerr effect is only partly captured by our calculations, suggesting the presence of additional effects when the system is probed using light pulses with different central frequencies.