Scalable mm-Wave Liquid Crystal Reconfigurable Intelligent Surfaces based on the Delay Line Architecture

This paper presents the design, fabrication, and characterization of broadband liquid crystal (LC) reconfigurable intelligent surfaces (RIS) operating around 60 GHz and scaling up to 750 radiating elements. The RISs employ a delay line architecture (DLA) that decouples the phase shifting and radiating layer, enabling wide bandwidth, continuous phase control exceeding 360°, and fast response times with a micrometer-thin LC layer of 4.6 micrometer. Two prototypes with 120 and 750 elements are realized using identical unit cells and column-wise biasing. Measurements demonstrate beam steering over +-60° and -3 dB bandwidths exceeding 9% for both apertures, confirming the scalability of the proposed architecture. On top of a measured nanowatt power consumption per unit cell, aperture efficiencies above 20% are predicted by simulations. While the measured efficiencies are reduced to 9.2% and 2.6%, a detailed analysis verifies that this reduction can be attributed to technological challenges in a laboratory environment. Finally, a comprehensive comparison between the applied DLA-based LC-RIS and a conventional approach highlights the superior potential of applied architecture.