# Potential of Liquid-Crystal Materials for Millimeter-Wave Application
## 1 Introduction  
Liquid crystals (LCs) are well known as excellent electro-optic materials, and their potential is sufficiently proven when we see the industrial success of the flat-panel display application. An LC device exhibits compactness, lightweight, low power consumption, and low driving voltage for all application fields in the optics and photonics [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. Furthermore, some optical control devices, such as variable focusing lenses, beam-steering prisms, and polarization control devices, gained attention in conjunction with conventional LC device applications, such as spatial light modulators and phase shifters [12; 13; 14; 15; 16; 17; 18]. Recently, it became evident that LC materials have a large birefringence in the wider frequency range of the electromagnetic wave spectra, such as microwave, millimeter wave (MMW), and THz regions [19; 20; 21; 22]. The novel frequency regions, which exist between the radio frequency and optics region, were forbidden for a long time because of the technological difficulties, i.e., even the very basic technologies, such as signal generation and detection, were insufficient for consumer applications. However, technological innovation occurred in the THz-sensing technology field upon using the femtosecond laser and photoconductive switch, and the unexplored frequency region including the MMW region gained attention from the high expectation of new application fields. Furthermore, the next-generation wireless communication standard (i.e., 5G) recently became a popular topic of information and communications technology (ICT). The 5G technology exhibitsan extremely short delay time and massive connectivity in conjunction with high-speed communication potential; thus, many people expect that many novel technologies leading to business opportunities will appear beyond smart phones. However, the frequency resources available for wireless communication were successively endangered after mobile phones came into existence, because the number of wireless terminals and the communication traffic increased steeply upon every generation change in the wireless communication standard. It is difficult to achieve the extremely high performance of 5G within the current frequency resources; thus, unexplored higher-frequency regions (i.e., MMW) enter the picture to realize 5G for many novel consumer services. MMW can propagate with narrow beams using a small antenna, bringing some advantages for energy saving and data security. However, some types of beam control device are mandatory for MMW usage, and low-cost and manageable devices for MMW propagation control are essential for the usage of MMW to become common in our daily life. We believe that LC materials are primary candidates to attain such devices if we reevaluate the technological success history of LC materials in optics. It is evident that LC materials have generally large birefringence in the MMW region as compared to that in the optics region, which is key for attaining variable LC devices working in a specific frequency region. However, molecules of all the commercially available LC materials are designed for display applications, and there is almost no information about their properties in the MMW region. It is essential to develop efficient measurement methods for LC materials to collect more information about excellent LC materials in the MMW region [23; 24; 25; 26; 27]. A uniform LC molecular alignment must be maintained in the measurement to evaluate the anisotropy of the LC materials; thus, we investigated a measurement method using a rectangular waveguide combined with a magnetic field [25; 26]. However, the amount of required LC material is still large, even though the waveguide size becomes smaller in the MMW region. We tried introducing the planar cell structure by using a coplanar waveguide (CPW) to significantly reduce the amount of test sample required [27].