Strong Mid-Infrared Optical Harmonic Processes Mediated by Excitons in Layered Indium Selenide

Nonlinear optics in 2D materials has attracted significant attention due to their potential in various optoelectronic applications such as nonlinear optical detection, bioimaging, and nonlinear optical modulation. Optimising the nonlinear optical response in the infrared, especially the mid-infrared (MIR) region, is particularly desirable for various applications including tunable coherent sources, high-order harmonic generation, supercontinuum light generation, and frequency-comb-based spectroscopy. However, reports on MIR nonlinear optical responses in 2D materials are limited due to a lack of materials with suitable nonlinear optical properties. Van der Waals layered γ-phase indium selenide (γ-InSe) featuring broken inversion symmetry can serve as a promising candidate for MIR nonlinear devices. Beyond the noncentrosymmetric crystal structure, γ-InSe possesses unique out-of-plane dipoles, distinguishing it from other 2D materials such as black phosphorus (BP) and transition metal dichalcogenides (TMDs). Herein, multiphoton harmonic generation processes including second (SHG), third (THG), fourth (FHG) and fifth (FIHG) harmonic generation are investigated in γ-InSe in the MIR region. Notably, γ- InSe shows strong second and higher order nonlinearities. A metal-insulator-metal (MIM) plasmonic metasurface is further designed with large SHG and THG enhancement factors reaching 8400 and 32000 times, respectively. These results demonstrate the great potential of γ-InSe for next-generation MIR nonlinear photonic devices.