The study of nonlinear responses in the chiral Weyl semimetal CoSi*

The study of nonlinear optical and optoelectronic properties in Weyl semimetals has attracted much interest since the first observation of giant anisotropic second harmonic generation (SHG) in the type-I Weyl semimetal TaAs. The recently discovered class of chiral Weyl semimetals adds a unique perspective to this research because the lack of mirror symmetries allows for the energy separation of paired Weyl points. This separation allows light of micron-scale wavelengths to be used to directly probe one of the two paired Weyl cones, which is not possible in the more familiar type-I and type-II Weyl semimetals. In this talk, I will present our experimental study of two second order nonlinear effects, SHG and photocurrent generation, in an archetypal chiral Weyl semimetal, CoSi. I will quantify these effects and compare their strengths to observations of previously studied nonlinear crystals. Finally, I will apply field-symmetry-based arguments to distinguish between the AC effect of SHG and the DC effects of the shift current and circular photogalvanic effect.