Circular dichroism in second- and third-harmonic generation in chiral topological semimetal CoSi

TL;DR

This paper theoretically studies circular dichroism in second- and third-harmonic generation in chiral topological semimetal CoSi, revealing that THG-CD is robust against spectral broadening while SHG-CD is sensitive to dissipation.

Contribution

It uncovers the different origins of SHG and THG circular dichroism and proposes high-harmonic spectroscopy as a robust probe of electronic chirality.

Findings

THG-CD remains robust over a wide frequency range despite spectral broadening.

SHG-CD is strongly suppressed by dissipation, unlike THG-CD.

Non-monotonic frequency dependence and sensitivity to polarization and orientation are observed.

Abstract

We theoretically investigate circular dichroism (CD) in second- and third-harmonic generation (SHG and THG) in the chiral topological semimetal CoSi. We demonstrate that both SHG and THG exhibit dichroic responses of order unity, while their robustness against spectral broadening is strikingly different. Specifically, while SHG-CD is strongly suppressed by dissipation, THG-CD remains robust over a wide frequency range. We show that this qualitative difference originates from the phase structure of the nonlinear current, where SHG-CD arises from subleading interference processes that are sensitive to dephasing, whereas THG-CD emerges already at the leading nonlinear order and is therefore protected against spectral broadening. As a result, THG-CD provides a robust probe of chirality encoded in nonequilibrium electronic dynamics. We further reveal non-monotonic frequency dependences and pronounced sensitivity of harmonic emission to the polarization state and crystallographic orientation of the driving field. Our results uncover a general mechanism for robust nonlinear chiroptical responses in noncentrosymmetric quantum materials and establish high-harmonic spectroscopy as a powerful probe of phase-resolved electronic dynamics.