Active tunable bidirectional terahertz absorber based on dual-metasurface hybrid liquid-crystal metamaterials

Terahertz (THz) metamaterial absorbers are widely studied for communication, sensing, and stealth applications. Most conventional designs use a single metasurface with a metallic ground plane that enhances absorption but blocks reverse coupling, leading to a unidirectional response and limiting dual-sided or reconfigurable THz applications. Here, we propose an actively tunable bidirectional THz absorber based on a dual-metasurface architecture integrating hybrid liquid-crystal (LC) metamaterials. The device comprises two orthogonally rotated quarter-ring gold resonator layers separated by a voltage-controlled LC interlayer, which enables bidirectional absorption without the need for a metallic backplane. Numerical simulations reveal peak absorption efficiencies of 98.1% at 1.136 THz for forward incidence and 99.7% at 1.123 THz for backward incidence. By exploiting the electrically induced birefringence of the LC layer, the resonance frequency can be continuously tuned from 1.136 THz to 1.088 THz under an external bias. The electric-field analysis confirms that the bidirectional response arises from plasmonic coupling between the two metasurfaces separated by the LC spacer. The proposed design provides a compact, reconfigurable platform for dynamically tunable THz absorbers, offering new possibilities for two-sided sensing, adaptive communication, and stealth technologies.