This paper presents a graphene-based reconfigurable intelligent surface (CP-GRIS) for circularly polarized beam scanning in the sub-THz range. The 1-bit coding metasurface combines phase tuning and polarization conversion in a compact structure, enabling dynamic beam steering with low sidelobe levels. The structure consists of a 45 \(\times\) 45 array (\(15 \lambda \times 15 \lambda\)) of unit cells, each formed by two inverted semilunar-shaped gold patches and an embedded graphene strip that produces a controllable \(180^\circ\) phase difference via current reversal. This phase control yields two stable phase states with an approximately \(180^\circ\) phase difference over 266-332 GHz, while maintaining a reflection magnitude above 0.85. By modulating graphene’s chemical potential and thereby altering its surface conductivity, the metasurface switches between these two stable phase states. This reconfigurability enables the reported single-beam operation and full two-dimensional (2D) and three-dimensional (3D) beam-scanning capability. At 300 GHz, the proposed CP-GRIS achieves a maximum broadside gain of 24.8 dBi, a beamwidth of \(4.4^\circ\). The proposed RIS achieves beam steering with a sidelobe level 15.3 dB below the main lobe at broadside, primarily due to its large electrical aperture and phase-controlled 1-bit coding, resulting in a gain enhancement of 12.4 dBi over the feed horn antenna. Beam scanning up to \(\pm 40^\circ\) is achieved with a low scan loss of 1.2 dBi, highlighting the proposed CP-GRIS as an efficient and versatile platform for sub-THz and THz beam manipulation and emerging sixth-generation (6G) wireless systems.
