Tunable and enhanced optical activity in twisted graphene metasurface bilayers.

The phenomenon about optical activity has widespread applications in polarization optics, biosensing, and analytical chemistry. The optical activity in twisted graphene metasurface bilayers (TGMBs) is studied theoretically in this paper. It is found that the large circular dichroism (CD) value can be adjusted by various physical parameters of TGMBs such as separation distance, the voltage applied to metasurfaces, and twist angle. By adjusting the twist angle of TGMB, the shapes of the CD spectra, circular birefringence spectra, and ellipticity spectra can be manipulated in the broadband range. When the twisted bilayer metasurfaces are stacked with an ultrathin spacer, it is found that there might exist the strong optical activity responses near the rotated--near-zero regime and topological transition -near-zero regime. The corresponding phenomena raise the prospect of tunable, compact, and on-chip terahertz devices with graphene metasurfaces based on optical activity.