Tunable terahertz chiral absorber with graphene–metal hybrid metasurfaces

Terahertz (THz) chiral absorbers, capable of selectively manipulating circularly polarized (CP) waves, hold significant promise for applications in THz spectroscopy and biomedicine. However, most existing designs exhibit fixed optical responses post-fabrication, limiting their practical utility. Here, we present a dynamically tunable THz chiral absorber based on a graphene–metal hybrid metasurface, whose chiral response can be actively modulated via tuning the graphene Fermi level. The metasurface consists of orthogonal, asymmetric C-shaped graphene-metal coupled resonators placed atop a thin polyimide spacer, backed by a reflective metallic ground plane to suppress transmission. This hybrid structure supports both symmetric and asymmetric resonant modes, enabling spin-selective excitation by CP waves and effectively breaking the spin-conjugation symmetry. The differential coupling strengths of these modes for left- and right-handed CP waves result in pronounced circular dichroism (CD). By tuning the graphene’s chemical potential from 0.01 eV to 0.7 eV, the resonance strengths can be modulated, leading to dynamic control of the CD response over a broad range of incident angles and frequencies. Note that a maximum CD of 0.7 at 0.385 THz can be achieved when the chemical potential reaches 0.7 eV. Furthermore, we show the application of information encryption through combining the design of an encoded hybrid unit cell with spatial encoding. This tunable chiral THz metasurface offers a new avenue for flexible polarization control, high-sensitivity sensing, and secure information encryption in next-generation THz photonic systems.