Electrophysical properties of nematic liquid crystals under stationary and microwave electric and magnetic fields in the millimeter waveband

It is known that dielectric non-linearity of liquid crystals (LC) is based on reorientation of their molecules (which are characterized by geometric anisometry, accompanied by anisotropy of dielectric permittivity ε and magnetic susceptibility χ) under external electroand magnetostatic fields. The effective values of ε measured in specific experimental conditions can vary within the range limited by two extreme values corresponding to molecular orientations parallel and perpendicular to the external field (dielectric anisotropy Δε - ε,, - ει). Correspondingly, birefringence An - ft, - nι is observed in the optical range. These properties are widely used in LC displays and light modulators. However, manifestations of dielectric anisotropy in the centimeter and millimeter wavelength range, as well as their possible applications, have been studied insufficiently. Thus, tunable microwave varactors, filters, phase shifters that could be used as components of phase-locked aerial arrays were reported [1-4]. It was noted that variation of Δε caused by changes in the external field strength could be used for phase changes of about 127dB. For certain LC with positive dielectric anisotropy such phase change could reach 50°/dB. Similar results were observed in the case of waveguide structures filled with nematic LC [5]. Thus, at frequencies of 22-40 GHz a single-mode waveguide filled by a thin (3 mm) layer of nematic 5CB ensured phase shifts up to 60° upon application of static magnetic fields up to 7150 Oe.