Unveiling the Fine Structural Distortion of Atomically Thin Bi 2 O 2 Se by Third‐Harmonic Generation

Bismuth oxyselenide (Bi 2 O 2 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 2 O 2 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 2 O 2 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