Efficient second harmonic generation at quasi-bound states in the continuum in hybrid nanostructures of 2D plasmonic array and waveguide

We numerically investigate the second harmonic generation (SHG) in hybrid plasmonic-photonic structures consisting of an array of periodic gold nanodisks covered by a nonlinear dielectric layer. In the designed hybrid systems, symmetry-protected bound states in the continuum (BICs) at normal incidence, and Friedrich- Wintgen BICs due to the coupling between localized surface plasmon resonances (LSPRs) and waveguide modes at oblique incidence, are observed under both transverse-electric (TE) and transverse-magnetic (TM) polarized illuminations. SHG near the BICs is studied, and the dramatically enhanced nonlinear optical response at quasi-BICs is obtained. Such enhancement is ascribed to the enhanced local field due to hybrid modes of LSPRs and photonic modes. The results indicate that hybrid plasmonic-dielectric systems supporting BICs at both TE-and TM-polarized incidences are promising platforms for linear and nonlinear photonic devices.