Unveiling the Fine Structural Distortion of Atomically Thin Bi O Se by Third-Harmonic Generation.

Bismuth oxyselenide (Bi O Se), a new type of 2D material, has recently attracted increased attention due to its robust bandgap, stability under ambient conditions, and ultrahigh electron mobility. In such complex oxides, fine structural distortion tends to play a decisive role in determining the unique physical properties, such as the ferrorotational order, ferroelectricity, and magnetoelasticity. Therefore, an in-depth investigation of the fine structural symmetry of Bi O Se is necessary to exploit its potential applications. However, conventional techniques are either time consuming or requiring tedious sample treatment. Herein, a noninvasive and high-throughput approach is reported for characterizing the fine structural distortion in 2D centrosymmetric Bi O Se by polarization-dependent third-harmonic generation (THG). Unprecedentedly, the divergence between the experimental results and the theoretical prediction of the perpendicular component of polarization-dependent THG indicates a fine structural distortion, namely, a <1.4° rotation of the oxygen square in the tetragonal (Bi O ) layers. This rotation breaks the intrinsic mirror symmetry of 2D Bi O Se, eventually reducing the symmetry from the D to the C point group. The results demonstrate that THG is highly sensitive to even fine symmetry variations, thereby showing its potential to uncover hidden phase transitions and interacting polarized sublattices in novel 2D material systems.