Resonance enhanced nonlinear optical conductivity in Weyl semimetal TaAs

Weyl semimetals are a class of materials with nontrivial band structure geometry, and have recently been shown to host a variety of exotic nonlinear optical properties, including the largest measured second-order optical susceptibility χ⁽²⁾. Here we report the measurement of the spectrum of nonlinear optical susceptibility χ⁽²⁾(ω) in the visible and near-infrared spectral range from photon energies of 0.5eV to 1.5eV. We observe that χ⁽²⁾(ω) has a sharp peak in amplitude at 0.7eV, and that the peak value is over four orders of magnitude higher than that of a standard nonlinear optical semiconductor, GaAs. The spectral lineshape of χ⁽²⁾(ω) is consistent with the prediction from a theoretical model of optical nonlinearity in 1D Rice-Mele chains and the magnitude of the frequency-integrated response saturates a universal bound that emerges from the model.