Sum-Frequency Generation Spectroscopic studies on Multiple-Domain Weyl Semimetal CoSi (0.2 – 1eV)

With the spin-orbit coupling and the inversion symmetry breaking, the states near the Weyl node are spin-polarized with spin-texture highly correlated to the chirality of the Weyl node. It leads to unusual nonlinear optical responses of Weyl semimetals, such as strong second harmonic generation and photogalvanic effect, as recently found in TaAs and CoSi. These discoveries offer new opportunities for a variety of applications, such as high-sensitivity terahertz photodetector and high-frequency microwave rectification. With a single Weyl node near the Fermi level, CoSi serves as an ideal platform to study nonlinear optical properties associated with the Berry curvature near the node. Here we aim to reveal the second-order optical susceptibility related to the interband electronic transition across a single Weyl cone. We applied a broadband heterodyne-detected sum-frequency-generation spectroscopy to characterize CoSi crystals with an incoming photon energy of 0.2 –1 eV, corresponding to electron transition across single or double Weyl cone(s). With polarization-dependent experiments, we separately determined the two independent complex elements in the nonlinear optical susceptibility tensor. Our quantitative analysis was executed and confirmed via SHG microscopic study through a crystal-orientational analysis on different domains of a multi-domain CoSi crystal. We shall present our spectrum interpretation with helps of a density functional calculation in the talk.