# Electrically Tunable Hydrogen-Bonded Liquid Crystal Phase Control Device
## 1 Introduction  
The terahertz region of the electromagnetic wave spectrum ranges from 0.1 to 10 THz. Over the last few decades, there has been considerable interest in terahertz applications, such as imaging, security checking, nondestructive testing, and in information and communication technologies [1]. In order to successfully achieve these terahertz applications, terahertz wave control devices, including a beam former, a lens, an antenna, and a polarization control device, are essential. It is beneficial if these devices are tunable and have low absorption loss. To realize tunability in terahertz wave control devices, variable-focus terahertz lens using pumping oil [2], electrostatically actuated diffraction gratings [3], magnetically tunable ferrofluid [4], electrically tunable metamaterial [5], electrically tunable graphene-based polarizer [6], and optically tunable terahertz filter [7] have been demonstrated. Another advantageous possibility is the use of high-performance liquid crystal (LC)-based devices in a terahertz application due to its controllability at low drive voltage and its low power consumption without any mechanical movement. [PERSON] and coworkers reported the optical properties of LCs in the terahertz frequency region using a far infrared (FIR) continuous wave (CW) laser in 1997 [8]. They revealed that LCs exhibited birefringence in the terahertz frequency region, and thus the LCs were shown to be a promising material for terahertz devices. [PERSON] and coworkers reported the optical properties of LCs in a terahertz frequency region by using terahertz time-domain spectroscopy (TDS) [9]. They reported the properties of many LCs in the terahertz region [10; 11; 12; 13; 14] and in terahertz wave control devices [15; 16; 17]. [PERSON] and coworkers also reported the terahertz properties of many LCs using terahertz TDS [18; 19; 20; 21; 22; 23] and many types of terahertz phase control devices using the birefringence of LCs in the terahertz frequency region [24; 25; 26; 27; 28]. Several authors have reported the improvement of the LC-based terahertz phase shifter. A transparent electrode is very important for an electrically tunableLC-based terahertz phase shifter. [PERSON] and coworkers have reported tunable LC terahertz phase shifters by using indium-tin-oxide nanow whiskers as transparent electrodes [28]. [PERSON] and coworkers also have reported a high transmittance tunable LC terahertz phase shifter using porous graphene electrodes [29]. Since the LC-based terahertz phase shifter needs a large cell gap (500 \(\upmu\)m or more), improvement in response speed is an important subject. To achieve high speed operation, the introduction of a reflection structure [30], polymer-stabilized LC (PSLC) [17], and electrospun nanofiber [31] have been reported. In addition, the development of LC material is also important for high-performance terahertz LC phase shifters. Recently, LCs with high birefringence in the terahertz region have been reported by [PERSON] [32] and [PERSON] [14]. These two reports indicate the promise of high-birefringence LCs for use in LC-based terahertz devices. However, all the reported nematic LCs exhibited dichroism in the terahertz region. In other words, the absorption in the ordinary direction \(\alpha_{o}\) was not equal to the extraordinary absorption \(\alpha_{e}\). Previously, we investigated phase-shifting interferometry by using an LC phase shifter and reported that the dichroism of the LC caused unwanted variation in the intensity of the terahertz phase shifter [33]. Therefore, LC materials with no dichroism are significant not only in phase-shifting interferometry, but also in future terahertz applications. In this study, we fabricated an electrically tunable phase control device by using a hydrogen-bonded LC material. We investigated the performance of the LC phase control device by using an FIR CW laser and estimated the birefringence and absorption properties of the hydrogen-bonded LC at 2.5 THz.