Optical responses of chiral multifold fermions

Multifold fermions are generalizations of two-fold degenerate Weyl fermions with three-, four-, six- or eight-fold degeneracies protected by crystal symmetries, of which only the last type is necessarily non-chiral. Their low energy degrees of freedom can be described as higher spin generalizations of Weyl semimetals. In this talk I will discuss their linear and non linear optical responses including optical activity, linear optical conductivity and photogalvanic effects. These are typically enhanced compared to Weyl fermions of the same Fermi velocity and can be quantized in the case of the circular photogalvanic effect. We use these results to qualitatively predict optical responses in multifold materials such as RhSi or CoSi.