# Microwave Liquid Crystal Technology
## 3 Material Properties and Characterization  
The state-of-the-art setup for LC characterization at 19 GHz and 30 GHz is a precise resonator-based setup, making use of the cavity perturbation technique. In [8] the resonator based dual mode characterization technique was introduced, where a resonator is measured with and without LC inside a sample holder made of silica, see Figure 6a. Due to the dielectric properties of LC, the field distribution, and therefore, the resonance frequency of the excited mode will shift compared to the reference measurement. By this, the permittivity as well as the loss tangent of the material can be determined. Although this kind of measurement is limited to one single frequency, the dielectric properties of the LC can be determined much more precisely than with broadband measurement setups. A detailed mathematical description of this method can be found in [8]. By using a dual-mode cylindrical resonator, the permittivity and dielectric loss of LC can be measured for both parallel and perpendicular orientation in one single step, see Figure 6b. For this, two perpendicularly aligned modes are excited. The terms \"parallel\" and \"perpendicular\" define the two extreme cases of orientations of the LC molecules, where the long axes of the molecules are aligned parallel or perpendicular with respect to the applied RF field, respectively. The silica tube is placed in the center of the resonator to ensure maximum field interaction, and therefore, high sensitivity. The resonator is excited by rectangular waveguides being weakly coupled to the resonator by means of a small coupling iris, see Figure 6c. During the characterization, the LC's long axes are aligned parallel to the silica tube with the help of permanent magnets. With this characterization method, the LC mixtures could be improved in terms of loss tangent as well as relative material tunability  
\[\tau_{\text{LC}}=\frac{\varepsilon_{\parallel}-\varepsilon_{\perp}}{\varepsilon_{ \parallel}}, \tag{12}\]  
as can be seen in Figure 7. The first tunable microwave devices in the early 2000's were realized by using display LCs such as K15 or BL111. A bit later, new mixtures were specifically synthesized for the microwave range. The LC mixture being used for the components presented in this paper is the mixture GTS-26001 from Merck KGaA, Darmstadt, Germany with a permittivity between \(2.39<\varepsilon_{r}<3.27\) and dielectric losses of \(0.007>\tan\delta>0.0022\). A comparison of different LC mixtures is summarized in Table 2. A review on recent material development can be found in [9]. Figure 6: Resonator setup for dual mode LC characterization based on the cavity perturbation technique at 19 GHz. Figure 7: Development of LC’s microwave properties at 19 GHz from the early 2000’s where first display LCs have been used towards recent LC mixtures specifically synthesized for microwave applications. The mixtures were measured at room temperature. One advantage of LC compared to other technologies such as semiconductors is a decreasing dielectric loss with increasing frequency in the range above 15 GHz. Measurements up to 1.5 THz have been conducted, using a time-domain spectroscopy (TDS) THz measurement system as shown in Figure 8.  
The beam generated by a Ti-sapphire laser is split up in the beginning. While one part of the beam is used as probe beam, the other one is passing a delay station before hitting the THz emitter, e.g., a biased GaAs-based photoconductive emitter. The THz beam is guided via parabolic mirrors through the sample holder and afterwards to the ZnTe crystal for THz detection. The THz as well as the probe beams are propagating collinearly through the crystal. There, the THz beam induces a birefringence, which can be read out by the linearly polarized probe beam. By this, the linear polarization is rotated by the THz beam.