Leaky modes in multilayer InSe flakes

Periodic fringes of both the second and third harmonic generations are observed near the edge of multilayer InSe flakes exfoliated on PMMA/Si substrates. Combined experimental and theoretical investigations show that these fringes are induced by the propagation of in-plane leaky modes of the fundamental wave launched by the flake edges. The leaky mode becomes fully guided as the InSe flakes are transferred on a free-standing 30 nm SiN membrane. Using multiphoton microscopy, the propagation of leaky- or guided-mode in 2D materials can be mapped in a fast and non-perturbative manner, demonstrating an effective approach in probing the optical and electronic properties of van der Waals structures. Since InSe is associated with an ultra-high second-order nonlinear susceptibility, a nanophotonic InSe rib waveguide is numerically proposed based on our findings to realize highly-efficient second harmonic generation with perfect phase-matching. We will also discuss how the exfoliated InSe flakes can be tailored to form nanoantennas or metasurfaces in sensing and optoelectronic applications.