Recently, high-order topological photonic crystals (PhCs) have attracted huge research attention due to their novel physics mechanism and the application potential in integrated photonics. Based on the two-dimensional Su-Schrieffer-Heeger model, we construct and study the mutual coupling between the high-order corner states in 2D dielectric PhCs. Simulation results show that the Q-factor of such corner-localized quasi-bound states in the continuum (BICs) could be enhanced following mutual coupling in finite size. Furthermore, we study the side-coupled structure based on defect-hybrid waveguides and the edge state microring, the quasi-BIC microcavity. The refractive index sensing application based on corner-localized quasi-BICs shows outstanding simulated sensitivity (312.8 nm/RIU) and figure of merit (∼103 1/RIU). The robustness against fabrication errors due to its topologically protected nature makes it competitive compared with other quasi-BICs sensors.
Lithium niobate (LN) is a good nonlinear material with a large second-order nonlinear polarization coefficient and low optical loss in the UV to mid-infrared spectral regions. By combining bound states in the continuum (BICs) with chirality, a high conversion efficiency and strong second-harmonic circular dichroism (SHG-CD) can be achieved. The simulated results show that the SHG conversion efficiency can reach 1.35 × 10 −2 % for a peak-pump intensity of ≈5.3 GW cm −2 in the near-infrared, and the SHG-CD can reach 0.98 by introducing a two-layer structure. Quasi-BICs are introduced to study nonlinear CD, which provide new insights for the further implementation of spin-dependent nonlinear applications.
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