mm-Wave Reconfigurable Intelligent Surfaces Based on Thin Planar Liquid Crystal Phase Shifters

This work investigates liquid crystal (LC)-based passive reconfigurable intelligent surfaces (RISs) for future wireless communication systems, particularly those operating in the mm-Wave (24 GHz-300 GHz) range. RISs offer a promising solution to overcome the propagation challenges inherent to the high-frequency operation, e.g., by improving communication reliability in cluttered dynamic environments. Comprised of periodically arranged individually tunable unit cells, RISs are envisioned to form electrically large apertures for wave collection and controlled redirection. To enable reconfigurability, nematic LC is considered as the tuning technology for RISs, mainly due to the associated cost- and energy-efficient scalability to large apertures. Unlike conventional LC-RIS architectures based on resonant element detuning, this work reports, to the best of the author’s knowledge, the first successful realizations of a delay line architecture relying on tunability via integrated planar LC phase shifters. In this context, LC phase shifter designs are proposed and experimentally verified at 28 GHz and 60 GHz, outperforming state-of-the-art prototypes across multiple metrics. Key highlights include designs featuring i) high compactness up to more than 1300 °/λ₀, ii) high figure of merit (FoM) reaching values above 120 °/dB, and iii) thin LC layer thicknesses down to 2 µm for fast response times. Building on these outcomes, frameworks for computationally efficient simulation and experimental characterization of LC-RISs are proposed and validated through two fabricated 60 GHz prototypes with 120 and 750 unit cells, demonstrating the enhanced capabilities of the delay line architecture. On top of just 10 Volts required for control and nanowatt-level static power consumption per unit cell, the micrometer-thin LC layer enables millisecond-range reconfigurability. Further, the prototypes provide more than 360° phase tuning, a 120° scan range of the reflected beam, and up to 17 % relative bandwidth. Measured aperture efficiencies reach 9 %, while simulations with novel low-loss materials predict efficiencies exceeding 40 %. Summarizing, this work provides the basis for the design, prototyping, and characterization of LC-RISs based on thin-film phase shifters and verifies their promise for future mm-Wave systems.