mm-Wave Metasurface Unit Cells Achieving Millisecond Response through Polymer Network Liquid Crystals

The slow response time of planar Liquid Crystal (LC)-based phase-shift metasurface and Reconfigurable Intelligent Surfaces (RIS) cells is addressed in this paper by introducing a polymer network LC (PNLC) mixture suitable at mm-wave bands. Since the conventional effective isotropic model used in optical cells for describing the PNLC is not suitable in RF, an effective anisotropic and uniaxial model for such mixture is provided and experimentally validated at 100 GHz for the first time. In order to compare the temporal performance and tunability of the PNLC, transmissive and reflective cells, containing conventional LC and PNLC, have been manufactured and measured at optical and mm-wave frequencies. The temporal responses of PNLC are also compared for both RF and optical cells, obtaining relevant differences between their improvement factors, which are also discussed. Specifically, a 50 fold response time improvement is attained in cells designed to work at 100 GHz, although at the expense of a 3X tunability reduction. The model, which is robust to varying angle of incidence and cell dimensions, has been experimentally validated by designing and manufacturing a PNLC reflectarray cell of a different geometry. The cell shows reconfigurability times of 210ms, representing a significant improvement with respect to state-of-the-art response time in mm-wave cells, which are in the order of several seconds.