Giant second harmonic generation from polar van der Waals Bismuth tellurohalide semiconductors

Rashba materials from the Bismuth tellurohalide family of polar layered van der Waals (vdW) semiconductors hold great promise for nonlinear optical (NLO) applications, since their broken spatial inversion symmetry leads to a large second-order nonlinear optical polarizability, χ⁽²⁾. Here, we report the first second harmonic generation (SHG) studies of the bulk polar semiconductors BiTeBr and BiTeI. Our results reveal that BiTeBr, in particular, hosts a large SHG response, comparable to that of archetypal semiconductors and larger than other vdW materials. In contrast to BiTeBr, the response of BiTeI is substantially smaller, suggesting that their relative halide polarity plays a key role in the dramatically different nonlinear optical response. Furthermore, we compared our results to the nonlinear optical response of the Weyl semimetal TaAs, which has previously been shown to produce extremely large SHG, under identical conditions and observe that BiTeBr has nearly half the nonlinear conversion efficiency of TaAs, despite the absence of any known topological properties. This suggests that the BiTeX family of compounds, particularly the BiTeBr compound, are ideal candidates for NLO applications.