Uniaxial Strain Dependence on Optical Second Harmonic Generation from Indium Selenide Few-Layers

Since the Van der Waals materials can be mechanically exfoliated and transferred to any substrate including bendable polymeric or plastic substrates, studies of 2D materials also benefit the development of flexible devices. Indium selenide (InSe) is an emerging star of the Van der Waals semiconductors due to its superior properties such as ultrahigh mobility and large elastic deformability. It was also applied to bendable photodetectors with high performance and broad spectral response. For application of flexible devices, it is important to understand how strain affects the physical properties. Recently, optical second harmonic generation (SHG) techniques has been demonstrated to be a powerful tool for strain mapping. In addition to the intensity distribution of strain, the direction of the strain distribution can also be mapped by analyzing angle-resolved SHG patterns. For this application, it relies on fundamental understanding of strain effect on the SHG of InSe. In this work, we studied angle-resolved second harmonic generation pattern of InSe few-layers under uniaxial strain both experimentally and theoretically. We used the exfoliation method to fabricate InSe flakes on bendable and transparent substrate, and then measured the SHG images. We found the SHG intensity of InSe decreased while the compressive strain increased. By using first-principles electronic structure theory, we calculated how strain alters the susceptibilities and angle-resolved SHG pattern. The theoretical results agreed with experimental results qualitatively.