Dramatic enhancement of nonlinear optical signals in distinct two-dimensional materials

With the growing demand for miniaturizing nonlinear on-chip integrated devices, enhancing the nonlinear optical responses of two-dimensional (2D) materials is essential. However, due to their atomic scale, nonlinear optical processes such as second-harmonic generation (SHG) and Raman scattering are generally weak. To enhance these nonlinear signals and improve inspection accuracy, we developed an air-gap-suspended nanocavity structure. This design effectively enhances the nonlinear optical response while minimizing substrate disturbance, facilitating the precise characterization of 2D materials. We achieved significant broadband electric field enhancement by employing optical thin-film theory and three-dimensional finite-difference time-domain (3D-FDTD) simulations to optimize the interference and cavity effects of the nanocavity. As a result, SHG and Raman signals of 2D materials were dramatically enhanced. Specifically, the SHG signals of distinct 2D materials suspended on a nanocavity were enhanced over 13 000 times, while the Raman signals were enhanced over 580 times. Moreover, polarization-resolved SHG measurements revealed a significant depolarization effect in the 2D materials after varying laser treatment durations. This observation suggests that the degree of SHG polarization anisotropy can serve as a practical indicator for assessing the quality of 2D materials. The air-gap-suspended nanocavity structure not only provides substantial signal enhancement but also serves as an excellent platform for studying the intrinsic properties of distinct 2D materials.