Strong Nonlinear Optical Response and Transient Symmetry Switch in Type‐II Weyl Semimetal β‐WP₂

Abstract The topological Weyl semimetals (WSMs) with peculiar band structures exhibit novel nonlinear optical enhancement phenomena, even for light at optical wavelengths. While many intriguing nonlinear optical effects are constantly uncovered in type‐I WSMs, few experimental works focuse on basic nonlinear optical properties in type‐II WSMs. Here a static and time‐resolved second harmonic generation (SHG) is performed on the 3D type‐II WSM candidate β‐WP₂. Although β‐WP₂ exhibits extremely high conductivity and a high carrier density (≈10²¹ cm⁻³), the SHG is unscreened by conduction electrons, and a rather strong SHG response is observed, comparing with non‐topological polar metals. Additionally, the time‐resolved SHG experiment traces ultrafast symmetry switch and reveals that polar metal β‐WP₂ tends to form an inversion symmetric metastable state after photo‐excitation. Intense femtosecond laser pulse can optically drive symmetry switch and tune nonlinear optical response on ultrafast timescales although the interlayer coupling of β‐WP₂ is very strong. These findings are illuminating for the polar metal nonlinear optics and provide a new perspective for future ultrafast topological optoelectronic devices.