Wide-range manipulation of photonic spin Hall effect via quasi-bound states in the continuum in a compound grating structure

The photonic spin Hall effect (PSHE), manifesting itself as the spin-dependent displacement of a light beam, holds potential applications in optical sensing and spin-based nanophotonic devices. Thus, achieving effective manipulation and enhancement of PSHE is highly meaningful. Previous works have indicated that the PSHE can be enhanced by considering the quasi-bound state in the continuum (q-BIC), which relies on the generation of Fano resonances with near-zero reflectivity. However, some limitations exist, such as the enhanced PSHE based on q-BIC is either constrained to a narrow range of incident angles a specific optical wavelength, or the quality factor is not considered. In light of the above issues, this work proposes a composite grating structure, which regulates the enhanced PSHE based on q-BIC within a wide incident angle range (approximately 10 degrees) and possesses a high quality factor. We find that the modulation of enhanced PSHE based on q-BIC can be achieved by adjusting the wavelength, the grating width, and the temperature variations of the Vanadium Dioxide (VO₂) grating. The physical mechanism is that, under the influence of surface plasmon, this structure can achieve q-BIC with near-zero reflectivity over a wide range of incident angles to enhance PSHE. Moreover, we design a PSHE refractive index sensor based on this q-BIC metasurface with a high sensitivity of Sn = 88.66^°/RIU.