Suppression of second-harmonic generation from linear bands in the topological multifold semimetal RhSi

TL;DR

This study investigates second-harmonic generation in the topological semimetal RhSi, revealing that linear dispersions of multifold fermions contribute less to SHG than flatter band regions, guiding future enhancement strategies.

Contribution

It provides the first comprehensive analysis of SHG in RhSi, demonstrating the suppression of topological multifold fermion contributions and highlighting the role of flat bands in SHG.

Findings

Multifold fermions contribute subdominantly to SHG due to their linear dispersion.

Flat bands significantly enhance SHG responses.

Results suggest pathways to increase SHG in topological semimetals.

Abstract

Recent experiments in the topological Weyl semimetal TaAs have observed record-breaking second-harmonic generation, a non-linear optical response at $2\omega$ generated by an incoming light source at $\omega$. However, whether second-harmonic generation is enhanced in topological semimetals in general is a challenging open question because their band structure entangles the contributions arising from trivial bands and topological band crossings. In this work, we circumvent this problem by studying RhSi, a chiral topological semimetal with a simple band structure with topological multifold fermions close to the Fermi energy. We measure second-harmonic generation (SHG) in a wide frequency window, $\omega\in [0.27,1.5]$eV and, using first principle calculations, we establish that, due to their linear dispersion, the contribution of multifold fermions to SHG is subdominant as compared with other regions in the Brillouin zone. Our calculations suggest that parts of the bands where the dispersion is relatively flat contribute significantly to SHG. As a whole, our results suggest avenues to enhance SHG responses.