# Potential of Liquid-Crystal Materials for Millimeter-Wave Application
## 5 Conclusions  
In this study, we described some recent investigations intothe application of LC materials in the MMW frequency region. There is insufficient knowledge about specialized LC materials for MMW application; thus, the development of efficient measurement methods for LC materials is essential for exploring excellent materials in this frequency region. We successively investigated the issue using various types of waveguides, and a planar waveguide (CPW) was introduced to significantly decrease the size of the test sample. However, the metal plate inserted into the LC cell, which worked as a moving reflector, destroyed the alignment layer of the LC molecules in one measurement. To overcome this problem, an optical short was introduced using a semiconductor substrate for the microstrip line. An optical short, which can work as a reflection boundary of MMW, was confirmed by irradiating an intense laser pulse to create a high-density photo-carrier domain. The reflection boundary could easily be moved by changing the irradiation position of the laser light. The refractive index and absorption coefficient of the substrate material could be successfully evaluated from the periodicity and attenuation of the MMW peak intensity as a function of the laser light irradiation position. There are two approaches to develop LC MMW devices. The first involves scaling up an LC optical device according to the increase in the wavelength; this is called aquasi-optical device. A Fresnel lens is a typical quasi-optical device used in the MMW region, and it becomes tunable if we introduce LC materials into the lens. The device structure is generally simple and easy to design following almost the same method used with optical devices. In this study, we demonstrated the basic performance of a Fresnel lens fabricated using a 3D printer. The filament material of the printer had relatively low loss properties in the MMW region, and the fabricated Fresnel lenses exhibited excellent focusing properties, even if some roughness was caused by the laminating process. The LC material was combined with a concave Fresnel lens substrate fabricated by the 3D printer to develop an LC Fresnel lens. A magnetic field was applied to control the LC molecular orientation, and the complementary focusing MMW intensity change was observed upon changing the magnetic field direction. Another approach to develop LC MMW devices involves integrating LC materials with high-frequency electric circuits. If we adopt a planar wave guide, a thinner sandwich cell structure can be used for MMW LC devices. This is the biggest advantage in comparison with the quasi-optical approach. In this study, we demonstrated an LC phase shifter by combining LC materials with a microstrip line, which is a very common and excellent planar-type waveguide used to construct high-frequency integration circuits. We demonstrated the basic performance of the minimal twin antenna array system, which was attained upon combination with LC phase shifters. It was confirmed that the radiated MMW beam became narrower by the arrays, and the peak intensity tended to increase. A beam-steering angle of about \(\pm 12^{\circ}\) could be obtained by driving each LC phase shifter with a low voltage; however, the changing property was extremely nonlinear as a function of the applied voltage because of the LC molecular orientation phenomena in the homogeneously aligned LC cell. This approach is advantageous in terms of response speed, because the thinner LC layer is beneficial. However, some other components, such as a power divider, LC driver circuit, and antenna, must be integrated with the LC phase shifter to attain specific functionality. On the other hand, the device structure in the quasi-optical device is simple for achieving specific functionality. However, a thousand-fold increase of the LC layer is necessary to attain a similar operation to optical devices.