Giant photonic spin Hall effect in bilayer borophene metasurfaces.

We investigate theoretically the photonic spin Hall effect (PSHE) in bilayer borophene metasurfaces. Based on the combined effect of the Fabry-Perot resonance of the bilayer system and the resonant interaction of individual meta-atoms in borophene metasurface which lead to the topological transition, it is found that there exist giant PSHE shifts of the transmitted beams which can be flexibly regulated by adjusting the twist angle of metasurface bilayers, incident angle, spacer refractive index and spacer thickness. Near the topological transition of borophene metasurface the magnitude of PHSE shifts in bilayer borophene metasurfaces is generally on the order of tens of wavelengths and even on the order of hundreds of wavelengths near the epsilon-near-zero (ENZ) regions. The manipulation frequency range of the large PSHE shifts can reach hundreds of terahertz or even picohertz through adjusting the ribbon width of borophene metasurface or the electron density for borophene. It is found that in bilayer borophene metasurfaces there exist the ultrahigh sensitivity of the PSHE shifts to spacer refractive index, which can be applied to design the refractive index sensors with high performance.