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

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 beta-WP2. Although beta-WP2 exhibits extremely high conductivity and a high carrier density (approximate to 10(21) 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 beta-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 beta-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.