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

Abstract

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.

Liquid Crystals for Advanced Smart Devices with Microwave and Millimeter‐Wave Applications: Recent Progress for Next‐Generation Communications

“…Reproduced under the terms of the Creative Commons Attribution 4.0 International license (CC BY 4.0). [ 80 ] Copyright 2022, published by IEEE. f) Relaxation time τ off for the Sb 2 O 5 particle with PDLC composite; τ off decreases with increasing the polymer concentration and nanoparticle volume fraction.…”

“…e) Schematics of transmissive and reflective RIS cells with LC/polymer composite for improving the response time. Reproduced under the terms of the Creative Commons Attribution 4.0 International license (CC BY 4.0) [80]. Copyright 2022, published by IEEE.…”

Characterization and Application of Dual Frequency Liquid Crystal mixtures in mm-Wave Reflectarray Cells to improve their Temporal Response

“…These were designed to operate at the optical frequencies and have never been characterized in RF, where the use of this technology is novel. The electromagnetic measurements have been compared to full-wave electromagnetic simulations (CST Studio [33] under a periodic unit cell configuration in order to consider mutual coupling) with different complex permittivities to extract, through an iterative process, the ε || and ε ⊥ permittivities and the tanδ || and tanδ ⊥ losses, similarly to [17]. More specifically, the reflection coefficient (S 11 parameter) of such structures has been measured in a free-space quasi-optical bench setup.…”

“…Different strategies, summarized in Table I, have been proposed to tackle the decay time limitation. For instance, Polymer Network LC (PNLC) mixtures have shown to reduce the decay time of LC molecules up to a 50x factor, achieving decay times in the order of 200 ms in these frequencies [17], and in optical devices, this factor can be even higher [18], [19]. However, the dielectric tunability of PNLCs is dramatically reduced with respect to conventional LCs, given that the polymer network cannot be tuned, which in practice seriously diminishes the achievable phase shift range.…”

Review Paper on Hardware of Reconfigurable Intelligent Surfaces

“…52 shows the reflectarray for the performance evaluations. Liquid crystal is very promising to design RIS [151,152].…”