Abstract This paper presents an electrically tunable hybrid copper-graphene reflective metasurface for the simultaneous dynamic generation and independent steering of multiple orbital angular momentum (OAM) vortex beams, targeting reconfigurable intelligent surface (RIS) applications in future terahertz wireless systems. The reflective metasurface consists of a 25 × 25 array of graphene-loaded patch unit cells operating at 2 THz, with overall dimensions of 800 × 800 × 12 μm³ \: \: (5. 3\: \: ₀\: 5. 3\: \: ₀\: 0. 08\: \: ₀). Each unit cell integrates a copper patch loaded with tunable graphene stubs on a thin hafnium oxide HfO 2 gate dielectric and polysilicon layer on a silicon dioxide SiO 2 grounded substrate, enabling low-voltage control (0. 1–4. 3 V). A resonant copper patch on a grounded SiO 2 substrate is loaded with graphene stubs to overcome the inherent loss limitations of standalone graphene elements, and a comprehensive parametric study is performed on dimensions and the effect of graphene chemical potential (\: {\: }₂), relaxation time (\: \: ), and temperature (\: T) on material response characteristics for phase tuning performance to achieve the highest continuous phase coverage. The optimized unit cell is then arranged in a 25 × 25 reflectarray to synthesize the required phase distribution via full-aperture phase superposition for simultaneous multi-beam OAM vortex generation. At \: \: =3\: ps\: and \: T=300\: K, variation of \: {\: }₂ from 0. 05 eV to 1. 0 eV yields reflection magnitude ranging from − 6 dB to − 0. 12 dB with 350° phase coverage. Under focused wave illumination, the metasurface supports vortex beam steering over elevation angles up to \: \: 60^\: with full \: 360^\: azimuthal coverage, achieving a maximum gain of 13. 6 dB. Unlike aperture-partitioning approaches, the design employs phase superposition to simultaneously generate multiple orbital angular momentum (OAM) beams (\: l=1, dual, triple and quadruple configurations) with independent directional control, and enhanced miniaturization. Mode purity reaches 99. 5%, 96. 1%, 92. 6%, and 79. 8% for \: \: l = 1, 2, 3, and 4, respectively, surpassing comparable tunable THz metasurface designs. The demonstrated combination of full-aperture phase superposition multi-beam generation, small footprint, high mode purity, ultra-low bias voltage, and wide-angle steering establishes the proposed architecture as a high-performance platform for THz RIS-enabled communication and sensing systems.
Elhenawy et al. (Wed,) studied this question.