The realization of high-Q resonances in a silicon metasurface with various broken-symmetry blocks is reported. Theoretical analysis reveals that the sharp resonances in the metasurfaces originate from symmetry-protected bound in the continuum (BIC) and the magnetic dipole dominates these peculiar states. A smaller size of the defect in the broken-symmetry block gives rise to the resonance with a larger Q factor. Importantly, this relationship can be tuned by changing the structural parameter, resulting from the modulation of the topological configuration of BICs. Consequently, a Q factor of more than 3,000 can be easily achieved by optimizing dimensions of the nanostructure. At this sharp resonance, the intensity of the third harmonic generation signal in the patterned structure can be 368 times larger than that of the flat silicon film. The proposed strategy and underlying theory can open up new avenues to realize ultrasharp resonances, which may promote the development of the potential meta-devices for nonlinearity, lasing action, and sensing.

We report an indium phosphide nanowire (NW)-induced cavity in a silicon planar photonic crystal (PPC) waveguide to improve the light–NW coupling. The integration of NW shifts the transmission band of the PPC waveguide into the mode gap of the bare waveguide, which gives rise to a microcavity located on the NW section. Resonant modes with Q factors exceeding 103 are obtained. Leveraging on the high density of the electric field in the microcavity, the light–NW interaction is enhanced strongly for efficient nonlinear frequency conversion. Second-harmonic generation and sum-frequency generation in the NW are realized with a continuous-wave pump laser in a power level of tens of microwatts, showing a cavity-enhancement factor of 112. The hybrid integration structure of NW-PPC waveguide and the self-formed microcavity not only opens a simple strategy to effectively enhance light–NW interactions, but also provides a compact platform to construct NW-based on-chip active devices.

The position-dependent mode couplings between a semiconductor nanowire (NW) and a planar photonic crystal (PPC) nanocavity are studied. By scanning an NW across a PPC nanocavity along the hexagonal lattice’s Γ – M and M – K directions, the variations of resonant wavelengths, quality factors, and mode volumes in both fundamental and second-order resonant modes are calculated, implying optimal configurations for strong mode-NW couplings and light-NW interactions. For the fundamental (second-order) resonant mode, scanning an NW along the M – K (Γ – M) direction is preferred, which supports stronger light-NW interactions with larger NW-position tolerances and higher quality factors simultaneously. The simulation results are confirmed experimentally with good agreements.