Photonic spin Hall effect at an optical bound state in the continuum

The photonic spin Hall effect (PSHE) plays a key role in the development of efficient spin-based selective devices. However, the quality factor (Q-factor), another core indicator for stronger light-matter interaction in spin-based optoelectronic devices, has rarely been considered for enhanced PSHE. In this paper, we demonstrate the enhancement mechanism of PSHE in a continuum bound state (BIC) metasurface with a high Q-factor. By utilizing the ultraweak angular dispersion of our specially designed three-tier architecture quasi-BIC (q-BIC) metasurface, the first-order partial derivative term of the Fresnel coefficient has been suppressed at a very low level. Coupled with a large Fresnel coefficient ratio and high Q-factor, the transverse shift of PSHE is greatly enhanced. In addition, by tuning the thickness of the substrate layer, the incident angle of maximum transverse shift can be flexibly manipulated. In this paper, we uncover the physical mechanism for enhancing PSHE in a BIC structure and offer a foundation for designing high-performance spin-based optoelectronic devices.