Liquid crystal compositions
Liquid crystals are rod-like molecules, that can be orientated in space by applying an external electric bias field. This feature lets to tune refelctarray's phase response electronically.
The relationship between the amplitude of the liquid crystal supply voltage and the phase variation, as presented in picture 3 obtained in the reflectarray presents a non-linear behavior, mainly for two reasons:
Intrinsic nonlinearity of liquid crystal: liquid crystal shows, by its very nature, a non-linear behavior as a function from a potential difference variation between its two surfaces. Thus, in a situation where the voltage variation is small, it does not cause a substantial change in the direction of the molecules and, therefore, in the phase variation. From a certain voltage value, specific to the type and characteristics of each Liquid crystal, small voltage changes produce large phase changes. Finally, as the voltage continues to increase, the liquid crystal goes into saturation and again shows a behavior similar to the initial one. An example of this behavior of liquid crystal molecules is visible in picture 3.
Particle design: each metamaterial particle design shows different behavior, not only from a maximum phase offset point of view, but also from the frequencies at which the offset occurs and the speed with which it occurs.
Therefore, reflectarray phase behavior is due to a combination of both factors, being difficult to characterise theoretically. Thus, by designing the control system, an empirical adjustment curves for particular liquid crystal material shall be obtained.
We are developing a new effective synthetic approach to prepare new polyaromatic compounds with the lateral substituents (chloro- or methyl groups) to provide the desired properties. We synthesised four-ring compounds with low melting point (less than 70°C), and based on them, liquid crystal compositions with large optical anisotropy and a wide range of operating temperatures were developed and optimised. The developed compositions are characterised by high values of the dielectric anisotropy in the mm-wave region (1.12−1.34) and large values of the tun-ability (0.32−0.36) that are suitable and optimized for applications in high-frequency devices.