The exploration of multiferroic materials and their interaction with light at the nanoscale presents a captivating frontier in materials science. Bismuth Ferrite (BiFeO₃, BFO), a standout among these materials, exhibits room-temperature ferroelectric and antiferromagnetic behaviour and magnetoelectric coupling. Of particular interest is the phenomenon of photostriction, the light-induced deformation of crystal structures, which enhances the prospect for device functionality based on these materials. Understanding and harnessing multiferroic phenomena holds significant promise in various technological applications, from optoelectronics to energy storage. The orientation of the ferroelectric axis is an important design parameter for devices formed from multiferroic materials. Determining its orientation in the laboratory frame of reference usually requires knowing multiple wavevector transfer (Q-Vector) directions, which can be challenging to establish due to the need for extensive reciprocal-space searches. Our study demonstrates a method to identify the ferroelectric axis orientation using Bragg Coherent X-ray Diffraction Imaging (BCDI) measurements at a single Q-vector direction. This method involves applying photostriction-inducing laser illumination across various laser polarisations. Our findings reveal that photostriction primarily occurs as a surface phenomenon at the nanoscale. Moreover, a photo-induced crystal length change ranging from 30 to 60 nm was observed, consistent with earlier findings on bulk material.

对多铁性材料及其在纳米尺度上与光的相互作用的探索呈现出材料科学的一个迷人前沿。铁酸铋（BiFeO ₃，BFO）是这些材料中的佼佼者，表现出室温铁电和反铁磁行为以及磁电耦合。特别令人感兴趣的是光致伸缩现象，即光引起的晶体结构变形，这增强了基于这些材料的器件功能的前景。理解和利用多铁性现象在从光电子到能源存储的各种技术应用中具有重大前景。铁电轴的方向是由多铁性材料形成的器件的重要设计参数。在实验室参考系中确定其方向通常需要了解多个波矢量传递（Q-Vector）方向，由于需要广泛的倒易空间搜索，这可能很难确定。我们的研究展示了一种使用布拉格相干 X 射线衍射成像 (BCDI) 测量在单个 Q 向量方向上识别铁电轴方向的方法。该方法涉及在各种激光偏振上应用光致伸缩诱导激光照明。我们的研究结果表明，光致伸缩主要作为纳米尺度的表面现象发生。此外，还观察到光致晶体长度变化范围为 30 至 60 nm，这与早期对块体材料的发现一致。