# Fast-Tunable Terahertz Metamaterial Absorber Based on Polymer Network Liquid Crystal
## 1 Introduction  
Over the past few years, realization of telecommunication, imaging, sensing, and biomedical instruments and devices in the THz regime of the electromagnetic spectrum have attracted a great deal of research attention. This is due to the prominent potential of the THz frequency regime in the realization of highly efficient and ultra-broadband wireless communication systems [1; 2; 3]. A variety of THz devices have been recently emerged, in which the THz absorbers exert an important role in the THz systems, including the sensors, detectors and imaging instruments [4; 5].  
Metamaterials are widely known due to their unique physical properties not readily observed in natural materials [6]. They are artificially composited to affect the energy of the electromagnetic radiation to achieve abnormal properties such as negative permittivity and negative permeability [7; 8]. Ever since [PERSON] et al. presented the design for an absorbing metamaterial with near unity absorbance in 2008 [9], the metamaterial absorbers (MAs) have attracted an extensive research attention. Afterwards, various MAs were designed to fulfill different demands. [PERSON] et al. demonstrated an ultra-broadband, polarization-insensitive, and wide-angle MA for THz regime [10]. [PERSON] et al. fabricated a MA that had three distinctive absorption peaks at 0.5, 1.03, and 1.71 THz with absorption rates of 96.4%, 96.3%, and 96.7%, respectively [11]. However, most of them could only operate at a single frequency or in a limited range of frequencies. To overcome this issue, the tunable absorbers were presented by researchers [12; 13]. [PERSON] et al. put forward the electronically-tunable MA by incorporation of LC into strategic locations within the metamaterial unit cell [14]. Then, various tunableMAs based on LC, such as single-band MA [6], triple-band MA [15], and polarization-dependent MA [16], have been reported.  
LC, as a tunable material, is generally known that the rotation of the nematic LC embedded in the absorber can be guided by the bias voltage, resulting in the shift of the resonant frequency [17]. However, the response time of the LC-based devices is quite slow, which hinders their potential applications. The response time of the nematic LC depends on several factors, such as the cell gap, viscoelastic coefficient, and applied voltage [18]. The rotation time can be greatly cut down by applying a large bias voltage. However, the recovery time after removing the bias voltage may be diminished by the larger restoring elastic torque. Then, the free recovery time may be reduced by introducing the polymer network liquid crystal (PNLC) [19].  
In this paper, we present a tunable MA based on the PNLC in terahertz band. Compared to the previously reported LC-based Mas, the metal ground plane is replaced by a transparent indium-tin-oxide (ITO) film so that the UV light can be transmitted through the ground electrode plate to polymerize the internal network. The experimental results show that the PNLC-based MA not only provides wideband tunability but also greatly accelerates the response time.