Fig. 1. Schematic diagram of electromagnetic spectrum and location of THz in electromagnetic spectrum with blue underline ^[8]电磁波谱示意图及THz在电磁波谱中的位置^[8]

Fig. 2. Frequency-dependent birefringence Δn and refractive indices: (a) Real part n; (b) imaginary part κ of NJU-LDn-4 ^[23]THz波段大双折射液晶的双折射和折射率　(a) 实部；(b) 虚部随频率的变化^[23]

Fig. 3. Tunable THz waveplate: (a) The cell is composed of a front fused silica substrate covered with a subwavelength metal wire grid and a rear fused silica substrate covered with porous graphene, both substrates are spin coated with SD1 alignment layers, and 250- -thick Mylar is used to separate the two substrates, NJU-LDn-4 LCs are capillary filled into the cell; (b) UVO-treated and then SD1 spin-coated CVD-grown few-layer graphene films; (c) polarization evolution at 2.1 THz: linearly polarized at 0 V, elliptically polarized at 6 V, circularly polarized at 8.8 V, elliptically polarized at 20 V and linearly polarized at 50 V (orthogonal to the polarization at 0 V); (d) schematic illustration of the double-stacked cell ^[53]液晶THz波片　(a) 结构图；(b) 石墨烯传输特性；(c) 电调THz偏振态；(d) 双层液晶器件^[53]

Fig. 4. A reflective electrically controlled broadband tunable THz liquid crystal waveplate: (a) Schematic drawing; (b) polarization evolution (0−22 V) from linearly polarized to circularly polarized at 1.1 THz, to orthogonally linearly polarized at 2.2 THz^[60]一种反射式电控宽带可调THz液晶波片　(a) 示意图；(b) 不同电压下的THz偏振态^[60]

Fig. 5. (a) Theoretical optical axis distribution; (b) photo under crossed polarizers of the q-plate with q = 2, the scale bar is 1 mm; (c) the measured intensity, and (d) phase distributions of the transformed component at 1.0 THz with left circular incident polarization ^[62](a) 液晶光轴分布理论值；(b) q = 2的THz液晶q波片在正交偏振片下的照片, 标尺为1 mm；(c) 1 THz左旋圆偏振THz波经过该波片后所测强度和(d) 相位分布^[62]

Fig. 6. (a) Rendering of a single unit cell of the liquid crystal metamaterial absorber; (b) depiction of the random alignment of liquid crystal in the unbiased case (right) and for an applied ac bias (left); (c) frequency dependent absorption A(w) for 0 V (blue solid curve) and 4 V (red dashed curve) at f_mod = 1 kHz, dashed line is centered at A_max(V_bias = 0) = 2.62 THz ^[64](a) 液晶可调超材料吸收器单元；(b) 液晶在偏置电压下取向变化；(c) 吸收频率可调范围^[64]

Fig. 7. Liquid crystal tunable metamaterial/graphene absorber: (a) Schematic; (b) optical image of the metasurface (inset: a unit cell of the metasurface), P = 150 , l_x = 120 , l_y = 100 , w = 10 . Simulations of the static electric field and liquid crystal director distributions shown at a plane centered in the liquid crystal layer when the operating voltage is 10 V: (c) cross-shaped electrode; and (d) metamaterial/graphene electrode with the same metal ground. Tunability of the THz resonant frequencies and hot spots of the metamaterial absorber: (e) tunable absorption of TE and TM mode; (f) electric field of the corresponding points in (e) at a plane 1 above the cross-shaped metasurface. A, 0.864 THz, 0 V; B, 0.884 THz, 10 V; C, 0.742 THz, 10 V; D, 0.742 THz, 0 V. The orientation of liquid crystal is horizontal at 0 V while vertical at 10 V ^[74]一种石墨烯/超材料协同驱动的电控液晶可调THz波吸收器　(a) 结构示意图；(b) 十字超材料的显微图片；(c) 十字超材料电极驱动液晶指向矢分布；(d) 十字超材料和石墨烯复合电极驱动液晶指向矢分布；(e) 可调THz波吸收器的远场吸收特性和(f) 近场特性, A, 0.864 THz, 0 V; B, 0.884 THz, 10 V; C, 0.742 THz, 10 V; D, 0.742 THz, 0 V. 液晶方向在 0 V为平行, 在10 V为垂直^[74]

Fig. 8. The active multifunctional terahertz metadevice: (a) Schematic illustration; (b) decomposition diagram of the device, the yellow arrows indicate the alignment direction; (c) the micrographs of the metasurface; (d) the comb electrode, inset in (c) shows the unit dimension of the resonator; p, lattice periodicity, 50 ; l, CRR length, 40 ; r, SRR length, 20 ; w, structure width, 3 ; g, gap, 4 ; and x, asymmetry distance, 11 ; the inset in (d) shows the polarization selectivity of the subwavelength grating; (e) black line reveals the electro-optical response of the device at 45 V; the blue line depicts the 1 kHz square-wave voltage signals^[76]集成液晶的多功能THz超材料器件　(a) 示意图；(b) 分解图, 黄色箭头方向为液晶取向方向；(c) 超表面显微照片, 内插图为共振器的单位尺寸, p, 晶格周期, 50 ; l, CRR长度, 40 ; r, SRR长度, 20 ; w, 结构宽度, 3 ; g, 液晶层厚度, 4 ; x, 非对称距离, 11 ；(d) 梳状电极显微图和特性；(e) 器件响应时间实验测试^[76]

Fig. 9. (a) Schematic of the polarization-tunable THz emitter; a ferromagnetic heterostructure and a large birefringence liquid crystal are integrated in the emitter, the heterostructure acts as the THz source as well as the electrode on the front side, a few-layer porous graphene with a high transmittance is employed as the other electrode on the rear side; (b) the spin current J_s launched by the laser pulse excitation is converted into the in-plane charge current J_c due to the ISHE, the current J_c along the x-axis act as an electric dipole, emitting linearly polarized THz waves into free space, the polarity of the THz waveform is determined by the direction of the magnetic field H and reverses together with it ^[77](a) 偏振可调的THz发射器结构图；(b) 铁磁异质结THz源工作原理图, 由飞秒激光脉冲作用铁磁异质结产生的自旋电流J_s转化成面内电流J_c, 其沿x轴方向类似电偶极子, 发射出线偏振THz波, THz波偏振方向由磁场方向决定^[77]

Fig. 10. (a) Schematic of the liquid crystal-reflectarray, cells showing its different parts; (b) measured radiation patterns at 100 GHz of several scan angles^[79](a) 液晶电控反射阵天线结构单元示意图；(b) 增益与扫描角度关系^[79]

Fig. 11. (a) CLC picture taken by a digital camera; (b) hue image digitalized from (a) ^[88](a) 基于CLC的THz成像卡; (b) 数字化处理后的THz波束^[88]

Fig. 12. (a) Schematic and working principle of the capsulized CLC film, the inset shows a micrograph of the film, which is produced with a color 3D laser scanning microscope (VK-8710, KEYENCE, Osaka, Japan); (b) visible pictures are taken under different THz intensities by a smartphone camera with Bluetooth; (c) increase in the diameter of the color change with different THz powers in thermal equilibrium, similar to a dartboard shown in the inset; (d) increase in the diameters as a function of response time with 1.3 mW and 2.6 mW THz radiation, the inset shows image changes under different THz radiation times ^[91](a) 一种基于胶囊型CLC薄膜的可视化THz功率计结构示意图; (b) 在不同THz强度辐照下胶囊型CLC薄膜颜色变化情况; (c) 热平衡时THz功率与颜色变化区域直径的关系; (d) THz波辐照时间与颜色变化区域直径的关系^[91]

Table 1. Large birefringence liquid crystal materials in THz range