Design and applications of a metasurface supporting chiral quasi-bound state in the continuum: Refractive index sensing and nonlinear harmonic generation

Metasurfaces empowered by quasi-BICs (q-BICs) have been widely employed to enhance chiral optical responses and enable sensing; however, studies that integrate both functionalities within a single design remain limited. In this work, we design a planar q-BIC chiral metasurface consisting of tilted TiO₂ bars with off-center inner holes, placed on a SiO₂ substrate and coated with polymethyl methacrylate (PMMA). Numerical simulations demonstrate that this design presents near-perfect circular dichroism (CD>0.99). Beyond exhibiting strong chirality, refractive index sensing with a sensitivity of 75.8 nm/RIU and a remarkable nonlinear CD approaching 1 are achieved with the same metasurface. These findings may provide a versatile platform for applications such as chiral laser generation, precision chiral sensing, and nonlinear optical filtering.