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2nd order nonlinear optical responses become allowed In Weyl semimetals (WSMs) that break inversion (I) symmetry. Electric fields at frequency ω generate currents at dc and 2ω (phenomena known as photogalvanism and 2nd harmonic generation, respectively). We discovered that the WSM, TaAs, exhibits the largest 2nd harmonic susceptibility of any known crystal. In a subsequent paper, we traced the origin of giant response to the highly polar nature of TaAs and derived a new sum rule relating the frequency-integrated nonlinear susceptibility to quantum geometry.
In pursuit of a clear signature of Weyl monopole singularities, we measured their photogalvanic response. We tested the predictionthat the circular photogalvanic effect (CPGE) is quantized in WSMs that exhibit structural chirality. The CPGE is the component of photogalvanic current whose direction switches with reversal of photon helicity; the CPGE associated with a Weyl node is an emergent property whose amplitude can be directly linked to the quantization of topological charge in a Weyl material. Using far-field coherent THz detection, we performed the first measurements of the spectrum of CPGE in the chiral Weyl semimetal, RhSi