Potential applications of nematic liquid crystal materials in the millimeter wave region

Large tunable properties induced by the change of LC molecular orientation states can be expected for wider frequency region except for visible rays. Focusing on the millimeter wave (MMW) region, a novel measurement method for LC materials is investigated using the rectangular waveguide test cell and several commercially available nematic LC materials have been actually evaluated for V-band (50GHz-75GHz) and W-band (65GHz-110GHz) regions. Refractive indices for the direction parallel to the LC molecular orientation in the MMW region are almost the same with those in the visible rays, on the other hand the indices for the direction perpendicular to the LC orientation show a little larger values. Consequently, refractive index anisotropy which is the most important for the tunable LC devices reduces to be 1/2 through 1/3 of that in the visible rays. Although the larger refractive index anisotropy is desired for the better performance, the anisotropy of the usual LC materials is still large and the loss properties are fairly good. Based on the materials data, we have demonstrated the CPW type LC devices combining with an ITO glass substrate on it. Phase and amplitude changes of the transmitted MMW by the LC driving are investigated considering the influences of some device parameters.