Dynamic Modeling of Liquid Crystal-Based Metasurfaces and Its Application to Reducing Reconfigurability Times

Abstract

This paper describes and validates for the first time the dynamic modelling of Liquid Crystal (LC)-based planar multi-resonant cells, as well as its use as bias signals synthesis tool to improve their reconfigurability time. The dynamic LC director equation is solved in the longitudinal direction through the finite elements method, which provides the z-and time-dependent inhomogeneous permittivity tensor used in an electromagnetic simulator to evaluate the cells behaviour. The proposed model has been experimentally validated using reflective cells for phase control (reflectarray) and measuring the transient phase, both in excitation and relaxation regimes. It is shown how a very reduced number of stratified layers are needed to model the material inhomogeneity, and that even an homogeneous effective tensor can be used in most of the cases, which allows a model simplification suitable for design procedures without losing accuracy. Consequently, a novel bias signal design tool is proposed to significantly reduce the transition times of LC cells, and hence, of electrically large antennas composed of them. These tools, similar to those used in optical displays, are experimentally validated for the first time at mm-and sub-mm wave frequencies in this work, obtaining an improvement of orders of magnitude.

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

“…Since in RF the phase shift is created by the resonances, and ε r determines the resonances frequency shift upon external bias excitation, a larger dielectric anisotropy facilitates designing structures with larger phase range and bandwidth. The value of n(r, t) for a conventional nematic LC with uniaxial molecules can be obtained by solving the Ericksen-Leslie equation [17], [30], which considers the elastic properties of the material and predicts the LC molecule rotation for a given excitation.…”

“…This, in turn, adds losses, which limits the minimum thickness of the LC cavities. In conventional LCs, that is, pure nematic uniaxial LC materials that have not been mixed or altered, their molecules respond in micro to milliseconds to an externally applied electric field excitation, hereby the molecule rise time may be reduced using overdrive techniques in the bias signals [16], [17]. However, the LC relaxation response, when the excitation is removed, is governed exclusively by the material elastic constants and surface anchoring and not by an external driving force.…”

“…Although a PNLC model based on effective scalar permittivities (isotropic model) is used in optic ranges, it is not suitable in resonant structures, where the electromagnetic fields are more complex, and a more sophisticated model is needed to include the boundary conditions imposed by the device. In [17] and [29], an uniaxial model was demonstrated to be appropriated to accurately describe both steady and dynamic models, for conventional LC mm-wave metasurfaces and biasing sequences. However, these models can not be applied to PNLCs, as their molecules are not really uniaxial (sometimes could be assumed as biaxial) and the elastic constants of the new mixture are unknown.…”

“…Estimated considering a 90%-10% RF phase change from the exponential time constant expressions found in[45]. OD indicates overdrive[17] 2. Declared 200ms optical response.8 VOLUME 4, 2016 This article has been accepted for publication in IEEE Access.…”

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

“…This can accelerate the rising transition by several hundred times. However, in the opposite case of accelerating a decaying transition from a higher voltage to a lower voltage (UD), the benefit is limited to 2x-3x for conventional nematic mixtures, as the minimum elastic energy that can be provided is at zero volts [21]. With the aforementioned decay times, it is clear that research in other technologies must be done to accelerate the decay time into the range of the milliseconds.…”