Dynamically tunable circular dichroism and optical activity of bi-layer four-fold graphene chiral metasurfaces

In this manuscript, we propose a bi-layer fourfold J-shaped graphene-based chiral metasurface structure to achieve tunable circular dichroism and optical activity without oblique incidence in the mid-infrared region. For our four-fold graphene structure, there is a giant circular dichroism of 16.13% at the resonance wavelength. And the pure rotations of polarization azimuth of 10.7° and − 15.18° respect to incident waves can be obtained at the wavelength of 21.05 and 21.4 µm, respectively. The physical mechanism of chiral nature is attributed to the induced surface current with parallel and antiparallel distributions, which can form magnetic and electric dipoles along specific directions. Besides, the circular dichroism and optical activity can be tuned by changing the Fermi energy and be affected by intrinsic relaxation time. All simulations are conducted by the finite element method. We believe our research on bi-layer four-fold graphene chiral metasurfaces will deepen the understanding of the interaction between light and matter and can be applied to design innovative polarization converters, polarization analysers and polarization-controlled devices.