Enhancement of second-harmonic generation in silicon-rich nitride using photonic bound states in the continuum

Silicon-rich nitride (SRN) grown by plasma-enhanced chemical vapor deposition (PECVD) was used as a second-order nonlinear material platform for strongly enhanced second-harmonic generation (SHG) using photonic quasi-bound states in the continuum (QBIC) excited under nearly normal-incidence. The metasurface, which supports a symmetry-protected BIC close to the fundamental frequency of our infrared excitation laser (1030 nm wavelength), is a 2D periodic array consisting of a square lattice of nanocylinders on top of a thin foot layer. PECVD-grown SRN provides a sizeable second-order bulk nonlinearity with the main tensor component along the out-of-plane direction. We engineered the QBIC to enhance the optical near-field in the same direction, maximizing SHG conversion efficiency and leading to SHG enhancement factors of four orders of magnitude compared to an unstructured SRN film. Therefore, SRN metasurfaces utilizing photonic BICs offer a CMOS-compatible and highly efficient platform for frequency conversion applications.