Second-Harmonic-Generation Effect and Giant Optical Birefringence in the Weyl Material CaAgAs.

Nonlinear-optical (NLO) materials suitable for the "8-14 μm" atmospheric transparent window are in urgent need. Arsenide is one of the most promising material systems for such an application. However, the second-harmonic-generation (SHG) effect of arsenide is difficult to characterize using the conventional powder SHG technique with a 2 μm fundamental laser. To overcome this problem, we focused on a novel arsenide Weyl material, CaAgAs, with a zero band gap and proposed a modified powder SHG measurement method for narrow-band-gap materials. We successfully observed the SHG signal of CaAgAs, which was approximately 0.7 times that of CdGeAs 2 . Moreover, on the basis of first-principles calculations, the largest SHG coefficient for CaAgAs is equal to 236 pm/V, and the ionic bonds in the [Ca 3 As 13 ] motif play an indispensable role for the SHG effect because of the distorted Kagome lattice pattern. CaAgAs is predicted to have strong optical anisotropy with a giant optical birefringence of 0.52.