This work presents a comprehensive theoretical and experimental analysis of the linear and nonlinear optical responses of the chiral topological semimetal RhSi, which hosts multifold fermions. The optical conductivity displays two quasi-linear regimes linked to different multifold fermions. The circular photogalvanic effect (CPGE) shows a sign change at 0.4 eV and a large non-quantized response peak at 0.7 eV, explained by the chemical potential crossing a flat hole band. The study suggests that achieving a quantized CPGE requires increasing the chemical potential and quasiparticle lifetime. The methodology, particularly broadband terahertz emission spectroscopy, is applicable to studying photogalvanic effects in other noncentrosymmetric materials.

Zhuoliang Ni, B. Xu, M.-Á. Sánchez-Martínez, Y. Zhang, K. Manna, C. Bernhard, J. W. F. Venderbos, F. de Juan, C. Felser, A. G. Grushin, Liang Wu