Research progress of terahertz liquid crystal materials and devices

Wang Lei; Xiao Rui-Wen; Ge Shi-Jun; Shen Zhi-Xiong; Lü Peng; Hu Wei; Lu Yan-Qing

doi:10.7498/aps.68.20182275

Abstract

Liquid crystal (LC) is an excellent tunable functional material which can be controlled by the external stimulus such as electric field, magnetic field and temperature. Terahertz (THz) radiation in a frequency range of 0.1−10.0 THz, has enormous advantages such as a low photon energy, sensitivity to crystal lattice vibration, magnetic spins, hydrogen bonds, intermolecular interaction, and water, and high transparency to non-conducting materials. The THz technology, therefore, has great potential in a diverse range of applications from spectroscopy, security screening to biomedical technology and high-speed wireless communication. But the development of high-performance LC based tunable THz functional devices is still in its infancy stage. The dispersion of LC refractive index induces a comparatively low birefringence in the THz regime. The lack of transparent electrodes makes the electric tuning of LCs difficult to achieve. To achieve certain modulations requires a very thick THz layer, leading to several disadvantages such as high operating voltage, slow response and poor pre-alignment. In this paper, we first present the research progress of large birefringence LCs in THz range. A room-temperature nematic LC NJU-LDn-4 with an average birefringence greater than 0.3 in a frequency range from 0.5 to 2.5 THz is shown in detail. This kind of LC can remarkably reduce the required cell gap, thus reducing the operating voltage and response time. Then we summarize varieties of conventional THz devices based on LC. Many electrodes are used for THz range. Graphene which can be used as a perfect transparent electrode material in THz band is proposed. Not only tunable transmissive but also reflective THz waveplates are introduced. The thickness of the LC layer of the reflective one can be reduced to ~10% of that needed for the same phase shift at a given frequency in a transmissive waveplate. The same tunability as that in the transmissive type just needs half the thickness. We also introduce that LC can generate THz vortex beam based on a photopatterned large birefringence LC. In the area of LC based versatile THz metamaterial devices, the adjacent units of a metasurface layer, such as a fishnet or grating, are usually connected to each other which may cause low-quality (Q) factor and polarization sensitivity, which is undesirable. We emphasize a graphene-assisted high-efficiency tunable THz metamaterial absorber. Few-layer porous graphene is integrated onto the surface of a metasurface layer to provide a uniform static electric field to efficiently control the LC, thereby enabling flexible metamaterial designs. The THz far-field and near-field with large modulation and fast response are realized. A magnetically and electrically polarization-tunable terahertz emitter that integrates a ferromagnetic heterostructure and the large-birefringence liquid crystals is also demonstrated to be able to generate broadband THz radiation and control the polarization of THz waves perfectly as well as LC based THz reflectarray. Last but not least, a temperature-supersensitive cholesteric LC used for THz detection is shown. It can not only measure the beam profiles but also detect the power values of THz waves generated from a nonlinear crystal pumped by a table-top laser. Quantitative visualization based on not only the thermochromic but also the thermal diffusion effect, can be used conveniently and effectively at room temperature. In this review, we summarize the latest progress of liquid crystal materials and components in THz and discuss the possible prospects of the combination of liquid crystal technology and THz technology. We envision that LCs will play a unique role in THz sources, THz functional devices and THz detectors.

Keywords:

Authors and contacts

1.
College of Electronic and Optical Engineering and College of Microelectronics, Nanjing University of Posts and Telecommunications, Nanjing 210023, China
2.
National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures and College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China

Corresponding author: Wang Lei, wangl@njupt.edu.cn

Funds: Project supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 61605088), the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20150845), and the Open Foundation Project of the National Laboratory of Solid State Microstructures, China (Grant No. M31039).

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Cited By

图 1 电磁波谱示意图及THz在电磁波谱中的位置^[8]

Figure 1. Schematic diagram of electromagnetic spectrum and location of THz in electromagnetic spectrum with blue underline ^[8]

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图 2 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]

Figure 2. Frequency-dependent birefringence Δn and refractive indices: (a) Real part n; (b) imaginary part κ of NJU-LDn-4 ^[23]

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图 3 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]

Figure 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-${\text{μ}}{\rm m}$-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]

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图 4 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]

Figure 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]

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图 5 (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]

Figure 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]

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图 6 (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]

Figure 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]

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图 7 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]

Figure 7. Liquid crystal tunable metamaterial/graphene absorber: (a) Schematic; (b) optical image of the metasurface (inset: a unit cell of the metasurface), P = 150 ${\text{μm}}$, l_x = 120 ${\text{μm}}$, l_y = 100 ${\text{μm}}$, w = 10 ${\text{μm}}$. 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 ${\text{μm}}$ 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]

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图 8 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 ${\text{μm}}$; l, CRR长度, 40 ${\text{μm}}$; r, SRR长度, 20 ${\text{μm}}$; w, 结构宽度, 3 ${\text{μm}}$; g, 液晶层厚度, 4 ${\text{μm}}$; x, 非对称距离, 11 ${\text{μm}}$；(d) 梳状电极显微图和特性；(e) 器件响应时间实验测试^[76]

Figure 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 ${\text{μm}}$; l, CRR length, 40 ${\text{μm}}$; r, SRR length, 20 ${\text{μm}}$; w, structure width, 3 ${\text{μm}}$; g, gap, 4 ${\text{μm}}$; and x, asymmetry distance, 11 ${\text{μm}}$; 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]

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图 9 (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]

Figure 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]

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图 10 (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]

Figure 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]

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图 11 (a) 基于CLC的THz成像卡; (b) 数字化处理后的THz波束^[88]

Figure 11. (a) CLC picture taken by a digital camera; (b) hue image digitalized from (a) ^[88]

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图 12 (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]

Figure 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]

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表 1 THz大双折射液晶材料

Table 1. Large birefringence liquid crystal materials in THz range

液晶种类	频率范围/THz	n_e	n_o	Δn (1 THz)
LCMS107	0.5—1.6	1.80—1.85	1.50—1.62	0.2—0.3
BL037	0.3—2.5	1.76—1.78	1.56—1.62	～0.2
MDA-00-3461	0.3—1.4	1.74	1.54	0.20
RDP97304	0.2—2.0	1.77—1.79	1.55—1.61	0.22
NJU-LDn-4	0.4—1.6	1.80—1.82	1.50—1.51	～0.31
GT3-23001	0.4—4.0	1.76 ± 0.01	1.54 ± 0.01	～0.22
LC1852	0.5—2.5	1.85—1.89	1.55—1.57	0.32
LC1825	0.2—2.5	1.91—1.95	1.54—1.57	0.38
MLC-2142	0.1—1.6	1.85—1.88	1.61—1.64	0.24
2020+nps3	0.3—3.0	1.90—1.92	1.55—1.60	0.36

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