Measurement of dielectric properties of liquid crystals in the THz range

A simple and efficient method is proposed for measuring the dielectric properties of liquid crystals (LCs) in the millimeter-wave band. This method is based on measuring the minimum of the reflection coefficient and the frequency corresponding to this minimum for a structure consisting of two plane-parallel quartz plates and a layer of a liquid crystal sandwiched between these plates. The method was applied to measuring the characteristics of 5CB and 8CB LCs in the frequency range from 58 to 118 GHz. The measurements have shown that the real part of the complex refractive index of 5CB LQ is somewhat less than its asymptotic value of 1.69 measured at frequency of 5 GHz and is close to the value of 1.65, which was measured at frequency of 250 GHz. As regards the imaginary part of the refractive index, the results obtained at frequencies of 0.65 and 250 GHz are greater than those obtained in our experiments by a factor of 3–4. We also measured the temperature dependence of the birefringence of both LCs.