# Fast-Tunable Terahertz Metamaterial Absorber Based on Polymer Network Liquid Crystal
## 2 Structure Design and Absorption Mechanisms  
The design of the MA begins with the frequency selective surface (FSS), which is a two-dimensional periodic metal patch array on the dielectric surface and exhibits obvious band-pass or band-stop filtering characteristics when interacting with electromagnetic waves. The common basic units of FSS are dipoles, crosses, square rings, etc. In this paper, the circular patch array is adopted and fabricated by lithography and etching. Figure 1 shows the fabricated patch array.  
The PNLC (also known as anisotropic LC gel) has been extensively studied in the field of electrically tuned scattering devices [20]. In this paper, the mass ratio of 95% LC host SLC103014-200 (S200) (Shi jiazhuang Chenzhi Yonghua Display Material Co., Ltd., Shi jiazhuang, China) is first mixed with a mass ratio of 5% photoreactive difunctional monomer RM257 (Jiangsu Hecheng Advanced Materials Co., Ltd., Nanjing, China) and a small amount of photoinitiator Benzoin Methyl Ether (TOKYO CHEMICAL INDUSTRY CO., LTD., Tokyo, Japan). Then, the photopolymerization precursor is filled into the LC cell. After this, a UV curing process is performed to induce the crosslinking of the polymer network [21]. Figure 2 sketches a schematic diagram of the PNLC manufacturing process.  
Figure 1: Fabricated patch array.  
Figure 3 presents the structure of the PNLC-based MA. This device is composed of a patterned copper layer (FSS) and an ITO layer spaced apart by PNLC. The top copper layer is geometrically patterned in order to strongly couple to a uniform incident electric field and serves as a top electrode. By pairing the top layer with an ITO plane, it serves not only as a bottom electrode but also as a metal ground. Besides, the inner surfaces of the copper layer and the ITO layer are coated by the polyimide (PI) alignment layers to leave the orientation of the LC molecules parallel to the surface of the electrode without the bias voltage. When the driving voltage is applied, the alignment of the liquid crystal molecules is parallel to the excitation electric field.  
The resonant structure may be considered as a circular patch, with high aspect ratios. According to the antenna theory, the resonant mode within the structure is a cavity mode [22]. The orientation of the LC molecules may be redirected by an external electrical field excited by a bias voltage between the top metal pattern layer and the ITO ground. As a result, the permittivity of the LC layer continuously varies with the change of the bias voltage. Specially, when the LC molecules turn to the direction perpendicular to the surface of the electrode, permittivity reaches its peak value with the saturation state. Therefore, the LC permittivity may be electronically controlled between these two states. The resonant frequency of a MA depends on the dielectric permittivity of the LC layer so that it may be shifted by changing the bias voltage.  
Figure 3: Schematic diagram of the polymer network liquid crystal (PNLC)-based metamaterial absorber (MA).  
Figure 2: (**a**) Mixture of the liquid crystal (LC) host and photopolymer sable monomer filled into a LC cell. (**b**) The form of the polymer network after the UV exposure.