# Potential of Liquid-Crystal Materials for Millimeter-Wave Application
## 1 Introduction
In this case, another problem occurs; the inserted metal plate, which works as a moving electrical short, destroys the LC molecular alignment layer in one measurement. Firstly, a recent study to overcome this problem using a photo-induced electric short is reviewed. When a suitably strong light is irradiated on the intrinsic semiconductor wafer, the wafer becomes an excellent miller for THz waves due to the densely created photo carriers on the surface [28; 29]. We fabricated a CPW by using an intrinsic Si wafer as a dielectric material, and the diode laser light was irradiated on a part of the waveguide to create a dense photo-carrier domain that could work as a moving reflection boundary of the MMW without any mechanical contacts. There are two approaches to develop MMW control devices using LC materials. One involves scaling up the LC optical devices according to the increase in wavelength; these are called quasi-optical devices [30; 31; 32]. Although the device structure in this approach allows one to simply attain a specific functionality, an extremely thick LC layer is required because the wavelength of the MMW is at least one thousand times that of the optics region. A simple scale up of the LC layer causes fatal degradation of the response speed; thus, we tried introducing a polymer-dispersed LC (PDLC) structure, which consists of a large number of tiny integrated LC cells, by using porous polymethylmethacrylate (PMMA) materials [33; 34; 35]. We investigated the MMW Fresnel lens, which is a typical quasi-optical device and can be tunable by introducing LC materials. In this case, some kind of machinery process is essential for attaining the Fresnel structure, and direct machining of the bulky porous PMMA material was demonstrated [35]. If we use a Fresnel structural substrate, the spray deposition method of the porous PMMA is acceptable for attaining a bulky LC layer. In the next section, it is demonstrated that three-dimensional (3D) printers are precise enough to fabricate the Fresnel lens substrate, which becomes the base of the LC Fresnel lens working for MMW control. Another approach to obtainan MMW LC device is the integration of LC materials into high-frequency electric circuits. The thickness of the LC layer can be significantly reduced if we choose a planar-type waveguide, because we can adopt the conventional sandwich cell structure to attain the LC devices. However, some extra parts must be integrated at the same time as the LC material to complete specific functionality, which leads to a complicated device structure in comparison with the quasi-optical approach. Many types of LC phase shifters, which are one of the important parts for integration circuits, were investigated by integrating the LC material into a suitable waveguide, such as a metal hollow waveguide [20; 21; 22; 36; 37; 38], microstrip line [39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50], or coplanar waveguide [51; 52; 53]. In this study, we investigated a planar-type LC phase shifter using a microstrip line. A microstrip line is a very common planar waveguide, which has a large technological background based on its long history. Additionally, we also integrated an LC driving circuit, power divider, frequency elimination filter, and antennas in conjunction with the LC phase shifter to attain specific functional devices, such as a phased array antenna. However, it is normally useless to integrate the LC materials on the microstrip line, because the MMW propagates inside the dielectric material of the substrate. We investigated a novel conversion circuit that can switch the propagation path of the MMW between the dielectric substrate and the upper LC layer to allow the introduction of the LC material onto the surface [47; 48; 49; 50]. In the last section, we describe the basic performance of a minimal twin array of LC phase shifters connected with a planar antenna.