Efficient enhancement of photoluminescence and second-harmonic generation of few-layer InSe coupled with surface-plasmonic Ag prism array

Two-dimensional (2D) materials with exotic photophysical effects and prominent optical performance are considered suitable nanobuilding blocks for optoelectronic device applications. Among these materials, indium selenide (InSe) has attracted enormous research attention owing to its unique structure, high layer-tunable optical response in the visible to near-infrared regions, and substantial second-order nonlinear optical properties. However, InSe features an extremely weak response to normal incident light (in-plane polarization) because its band-edge excitons are oriented out-of-plane, which considerably limits its practical applications. Herein, by integrating few-layer InSe with an Ag nanoprism array, we substantially improved the optical response of InSe in linear and nonlinear regimes. The enhanced response is attributable to the synergy between two mechanisms. First, the resonant excitation of the local surface plasma enhances the coupling efficiency of InSe with the incident light; second, the unique bending structure increases the out-of-plane polarization component of the incident light, resulting in enhanced light absorption and scattering by the out-of-plane excitons at the InSe band edge. We further achieved a two-order magnitude enhancement in the photoluminescence and second-harmonic generation intensities by tuning the InSe thickness. This paper proposes an effective approach for improving the optical properties of InSe, contributing to its practical applications in optoelectronic devices.