Highly birefringent nematic mixtures at room temperature for microwave applications

Liquid crystals (LCs) are widely used in optics, in particular for displays and other electro-optical devices. Of particular relevance here, LCs are tunable materials that can achieve a high figure of merit (i.e., efficiency) for ubiquitous microwave devices such as phase shifters. As a result, there is tremendous interest in studying LC properties in the microwave region up to terahertz frequencies. In this range, performance comparable to that of LCs is generally not possible using typical inorganic tunable materials such as barium strontium titanate (Ba_1−xSr_xTiO₃, BST). Although the switching speed of BST is faster, LCs show lower losses in the microwave region. Furthermore, LCs permit low bias voltages, up to 40V, depending on the line topology (e.g., microstrips or waveguides), which means less power consumption.

Thus far, however, not much is known about the behavior of nematic LCs (the type most frequently used in practical applications) in the microwave frequency region. The few results published deal with commercial mixtures of unknown composition and shed little light on the relationship between the properties of LCs and absorption processes in that portion of the spectrum.

Accordingly, we decided to approach the problem from a very basic point of view.^1–4 We asked the question: which molecular properties are responsible for low losses in the microwave region and why? First, we considered the influence of enlarged optical birefringence in the context of reduced losses. Because nematic LCs are optically uniaxial, the optical birefringence Δn=n_e−n_o (n_e being the extraordinary and n_o the ordinary refractivity) depends on the specific electrical dipolar structure of a chemical compound. Enlarged conjugated aromatic molecules, molecules with strong longitudinal dipolar groups, or molecules with carbon-carbon triple bonds between the aromatic cores, so-called tolanes, are good candidates for increasing birefringence not only at visible wavelengths—e.g., 589nm—but also in the microwave region. Since LC materials are tunable based on dielectric and magnetic anisotropy, the orientation of the molecules in the medium can be changed by applying an electro- or magnetostatic field. In addition, a low-viscosity system enables short switching times at room temperature. An optimal way to reduce melting temperature and at the same time to broaden the LC temperature range is to prepare mixtures of individual molecules, leading to eutectic compositions (in which the mixture's melting point is lower than those of the pure substances). Among others, we prepared and optimized several mixtures (see Figure 1) using different tolanes and lateral substituted quaterphenyls (see Figure 2). Lateral substituents shift the existence range of LCs to lower temperatures, whereas nonlaterally substituted quaterphenyls show much higher melting points.

Figure 1. Birefringence Δn at 589nm and 38GHz for a tolane mixture and for three tolane-quaterphenyl mixtures. The quality factor, η, of the tolane-quaterphenyl mixtures is for 38GHz and room temperature.

Figure 2. Example of a quaterphenyl molecule, substituted in the longitudinal direction (parallel to the long axis) by the C₃H₇and F (fluorine) groups, and in the lateral direction (perpendicular to the long axis) by the CH₃and F groups.

We carried out microwave measurements using a shortened rectangular waveguide as a cavity resonator perturbed by a polytetrafluoroethylene tube filled with nematic LCs. The microwave tunability at a given frequency is τ=(ε_∥−ε_⊥)ε_∥, where ε_∥ and ε_⊥ are the dielectric constants parallel and perpendicular to the unique axis. The quality factor is defined by η=τ tan δ_max, where tan δ_max is the maximum value of dielectric losses measured at a particular frequency and temperature.

For example, the birefringence of one tolane compound at 589nm is Δn=0:34; and the dielectric losses are tan δ_∥=0:0035 and tan δ_⊥=0:0133 at room temperature and 38GHz. By adding even a small amount of selected substituted quaterphenyls to the basic tolane mixtures, we obtained tan δ_max∼0:01 and η up to 21 (see Figures 1 and 4). As mentioned earlier, the quality factor depends on the ratio of the dielectric constants at the frequency of interest. Significantly, here we observe no direct relationship between the static dielectric anisotropy (i.e., as measured at low frequencies) and η at microwave frequencies. Furthermore, absorption resulting from reorientation around the long molecular axis of the LCs occurs up to only about 1GHz, and the reorientation process around the short molecular axis occurs at even lower frequencies (see Figure 3). Indeed, we observe no significant dependence of η at 38GHz on the strength of the diffusive reorientation around the long molecule axis.

Figure 3. Absorption in the megahertz region due to molecular reorientation. The reorientation around the long axis of the molecule shows tiny prolongations (‘tails’) into the microwave region.

Figure 4 shows the performance of typical BST thick- and thin-film samples and highly birefringent LC mixtures prepared by our group. Obviously, LCs exhibit lower tunability but also lower microwave losses compared with BST, leading to a higher material quality factor. The lower microwave losses of LCs are of key importance to applications, although tuning times are longer compared with BST.

Figure 4. Tunability (τ) and losses for highly birefringent LC mixtures and barium strontium titanate (BST) thick- and thin-film samples, all measured at 38GHz.³ tanδ_max: Maximum value of dielectric losses.

In summary, we have described work aimed at reducing losses in the microwave region—i.e., increasing the material quality factor at microwave frequencies—by improving the birefringence of LCs. Losses could be further reduced by using longer conjugated systems, such as four-ring tolanes (the work described above used three-ring tolanes) or even specifically selected lateral substituted pentaphenyls. Moreover, mixtures could be optimized using lateral substituted quaterphenyls. It is also possible to move the absorption peak of the molecular reorientation modes to lower frequencies through specific design of LC mixtures and thus to exclude the influence of ‘tail’ losses in the microwave region (see Figure 3). We will focus our future efforts on these questions.