Asymmetrically Layered Unit Cell Topology to Reduce Cell Gap of Liquid Crystals for 5G Millimeter-Wave Transmissive Reconfigurable Intelligent Surface Applications

This study presents transmissive reconfigurable intelligent surface (TRIS) unit cells (UCs) with slimmed liquid crystals (LCs), where the effective cell gap is electromagnetically tailored to control the LC using empirically practical voltage values. Prior LC-based transmissive UCs utilize thickened cell gaps at millimeter-wave frequencies due to limited parametric degrees of freedom, aiming for enhanced performance, but this approach is unfavorable in terms of LC controllability. Therefore, a coupled resonator-loaded UC featuring a dumbbell-shaped complementary loop with intrinsically reduced coupling strength is proposed, despite the close proximity due to the slimmed cell gap. Moreover, a cleverly designed asymmetrical topology is proposed, where the resonators are positioned apart from each other in the perpendicular direction of the coupled dominant current. The designed UC achieves a simulated phase tuning range (PTR) of 123° by employing only a single-layered LC with a cell gap of 0.21 mm, which is improved more than two times than that of the previously studied counterpart. In addition, an operating range of 27.9-29 GHz with reasonable loss sustainable for entire range of LC dielectric constant is attained successfully, contrary to previous works where low loss is maintained only for a single tone (ST) frequency.