Simple method for the determination of refractive indices and loss parameters for liquid-crystal materials in the millimeter-wave region

Abstract

A reflection-type liquid crystal (LC) test cell is prepared with a rectangular waveguide for investigation of a novel method to determine refractive indices and loss parameters of nematic liquid-crystal materials. As the bottom of the test cell is sealed with a glass window and the top of the cell is capped with a metal-tipped movable reflector after the LC materials are injected, both ends of the waveguide test cell have large reflectance. Thus the reflection properties of the LC test cell can be well described by a multiple-beam interference model. A simple method for the determination of refractive indices from the reflection measurement data is proposed based on results of some investigations with the theoretical model. Commercially available LC materials have been measured with this method at a millimeter-wave frequency (50 GHz) by use of a simple experimental setup with a Gunn oscillator and a diode detector.

Measurement of Dielectric Anisotropy of Some Liquid Crystals for Microwave Applications

“…The fairly high correlation coefficients (for 3GHz and 30GHz) indicated in Figure 4 mean that, for variety of LCs, the dielectric anisotropy at microwave regime is correlated quite well with the birefringence in visible (589nm). This finding is indirectly supported by another research work in the MMW regime (Nose et al [4]). They discovered that there was strong correlation between the refractive indices for MMW and for the visible wavelengths.…”

“…They discovered that there was strong correlation between the refractive indices for MMW and for the visible wavelengths. Both the results of dielectric anisotropy (by us) and birefringence (by Nose et al [4]) confirm there is a correlation between the dielectric /optical properties in MMW and visible regions. This behavior is not only interesting but also useful.…”

“…In comparison to the existing candidates such as ferroelectric ceramic materials, liquid crystals (LCs) have drawn attention in recent years mainly because of low losses in microwave frequencies and comparatively high anisotropy [1][2][3][4][5]. For this reason, intensive research has focused on applications of LC in GHz.…”

A millimeter-wave quasi-optical grid phase shifter using liquid crystal

“…The LC material assumed in the simulation is K15 (Merck). The permittivities (ε // = 2.92, ε ⊥ = 2.62) and loss tangents (tan δ // = 0.01, tan δ ⊥ = 0.02) corresponding to the two molecular orientations of K15 measured at 50 GHz [8] were used. The thickness of the LC layer, t LC in Fig.…”

Broad spectrum measurement of the birefringence of an isothiocyanate based liquid crystal

“…Their anisotropy and sensitivity to external fields have led to numerous display and nondisplay applications. Moreover, LCs are very promising materials at microwave [1][2][3][4][5][6][7][8] and terahertz (THz) frequencies [9][10][11][12][13] due to low losses and comparatively high anisotropy. LCs are particularly suitable for operation at frequencies beyond 10 GHz as the dielectric loss decreases with increasing frequency [14], making them attractive for phase modulating devices.…”