Accurate Modeling for Wideband Characterization of Nematic Liquid Crystals for Microwave Applications

Abstract

Liquid crystals are attractive materials for use in millimeter-wave band communications devices due to their large birefringence and reconfigurability. However, characterizing these materials is challenging at these frequencies. This work describes the modeling tools that have been developed for the accurate analysis of the liquid crystal orientation and of the wave propagation through the inhomogeneous and anisotropic liquid crystal layer. It also shows how the modeling has been used in conjunction with experimental measurement to characterize liquid crystal materials over a wide frequency range.

Accurate and Efficient Modeling to Calculate the Voltage Dependence of Liquid Crystal-Based Reflectarray Cells

“…The parameters 11 , 22 , 33 are the elastic constant associated to bend, twist and splay deformations, Δ = // − ⊥ is the LC dielectric anisotropy at the quasi-static excitation frequency and ��� is the quasi-static electric field applied. Numerical methods have been used to solve (2) for LC-devices with different geometries and the corresponding boundary conditions [10]- [11]. However, if the LC is confined by two parallel plates separated a distance = on which orienting layers are placed (orienting layers impose molecular orientations in the YZ plane), and if the LC is excited by a uniform quasi-static electric field with only z-component, ��� = (see.…”

“…This creates transversal inhomogeneity in the LC layer along and , as a consequence that the quasi-static electric field is not purely polarized in the z direction throughout the entire LC volume. Consequently, an inhomogeneous tensor along the three dimensions, � ( , , , , ), is obtained, which may be rigorously calculated by solving (2) with the corresponding quasi-static field (3D LC modeling [10]- [11]). However, if the transversal inhomogeneity effect is low, the LC can be modelled in the entire cell as an inhomogeneous anisotropic layer in one dimension, z, and the permittivity distribution � ( , ) can be obtained at each voltage (V) from the elastic problem described in Sec.…”

Variable Temperature Broadband Microwave and Millimeter-Wave Characterization of Electrochromic (WO3/LiNbO3/NiO) Thin Films

“…However, the dielectric tunability of the EC material is lower than that of the more sophisticated LC mixture (GT3-23001), about 27%, which is commercially available from Merck KGaA and specially engineered for microwave applications [19]. On the other hand, the absolute value of the extracted relative permittivity of the present EC cells is approximately an order of magnitude higher than that reported for LCs [3] - [5]. In this case, the proposed EC composition may be suitable for those applications that cannot be practical with LCs; e.g.…”

Microstrip Device for Broadband (15–65 GHz) Measurement of Dielectric Properties of Nematic Liquid Crystals

“…It should be noted that the change of the voltage changes the director direction from the default state (being in the direction parallel to electrodes along the wave propagation) to a nonuniform distribution largely perpendicular to the electrodes. A computer modeling [25] developed by the authors clearly shows this behavior for GT3-23001 (Fig. 9).…”

“…Increasing and of the top substrate increases the effect of the field at the electrode edges, widening the difference between the measured effective permittivities and the values of and . However, more accurate values can be extracted using a finite-element computer program developed by the authors [25].…”