Giant nonlinear optical response in transition metal monopnictide Weyl semimetals*

The second-order nonlinear conductivity σ⁽²⁾ describes the current generated in proportion to the square of the applied electric field and is equal to zero unless the medium breaks inversion symmetry. Recently, we reported a giant, anisotropic σ⁽²⁾ at photon energy 1.5 eV in TaAs, a polar Weyl semimetal, that is larger than previously measured in any crystal. Subsequently, we measured the spectrum of σ⁽²⁾(ω) from 0.4 to 1.6 eV and found that the response at 1.5 eV is, in fact, the high-energy tail of a sharp resonance at 0.7 eV. Our discovery of a giant anisotropic σ⁽²⁾(ω) in TaAs raises the following questions: what is special about TaAs and/or polar metals that accounts for large resonant optical nonlinearity, and, is there a fundamental upper bound on σ⁽²⁾(ω) in such inversion breaking crystals? After describing the experimental findings, I will describe a simple model based on the band-geometric theory of nonlinear optical response that addresses these questions. The model is relevant to applications that attempt to use intrinsic inversion breaking to convert optical power to electrical current.