# Liquid Crystal-Tuned Planar Optics in Terahertz Range
## 1 Introduction  
The explosive expansion of data traffic in wireless communication systems prompts the use of spectra with higher carrier frequencies to increase the bandwidth and provide larger capacity. The terahertz (THz) spectrum, between microwave and infrared waves (frequency range 0.1 to 10 THz), theoretically supports a communication capacity of terabits per second (Tbps) [1]. THz is considered the most promising technology for next-generation wireless communication, and is known as \"the last piece of the RF spectrum puzzle for communication systems\" [2]. THz waves can penetrate most nonmetallic and nonpolar materials, such as cloth, paper, and wood, making them very suitable for nondestructive detection and security checks [3, 4]. Currently, one of the biggest obstacles to THz communication is the absorption of water vapor in the atmosphere which makes it difficult to transmit high fidelity over long distances [5]. Therefore, it is necessary to develop highly efficient and compact passive and active devices, such as focusing [6], deflecting [7] and filtering [8] devices, to control the propagation of THz beams in free space. Compared with electrical or optical counterparts, THz devices are far from mature. THz exceeds the cutoff frequencies of many semiconductor amplifiers and mixers, making them incompatible with existing RF components [9]. High-speed amplitude and phase coding based on photonics provide an opportunity for high-efficiency THz modulations [10, 11]. Early refracted or diffracted THz devices are made of polymers or crystals, and their bulky sizes and fixed functions limit the miniaturization, integration, and versatility of THz apparatuses. Recently, metamaterials and metasurfaces based on subwavelength artificial electromagnetic microstructures have become excellent means for THz wavefront shaping [12, 13, 14]. Research on metamaterials began in 1968, when [PERSON] first proposed a medium with negative permeability [15]. In 1996, negative effective permeability wasexperimentally demonstrated by significantly enhanced magnetic resonance using artificial structures with periodic split ring resonators (SRRs) [16]. Early metamaterials were used as frequency selective filters [17]. With the development of micro/nano processing and electromagnetic simulations, research on artificial electromagnetic microstructures has been extended to the THz band [18]. By properly designing the unit structures, various filters [19], polarization converters [20], lenses [21] and other components have been achieved. The modulation of electromagnetic waves by metamaterials mainly depends on the surface plasmon resonance (SPR) of the metallic microstructure, which is usually narrowband and has high backscattering loss and ohmic loss, especially in the THz band [22]. To improve efficiency, all-dielectric metasurfaces [23] and multilayer metal metamaterials [24] have been adopted. Metamaterials are compact, flexible in function, and compatible with CMOS technology, but their functions are usually fixed once fabricated. Active tunable THz elements are realized by integration with various functional materials [25]. The dielectric properties of these functional materials change under the excitation of an external field, thus regulating the electromagnetic properties of metamaterials accordingly. Compared with other functional materials, liquid crystals (LCs) exhibit the superiorities of mature fabrication and low processing cost [26]. Driven by external stimuli such as electric fields, temperature and magnetic fields, the birefringence of LCs which covers a broadband range from ultraviolet to microwave, can be tuned [27]. Therefore, LC-based and LC-integrated THz devices have attracted intensive attention in recent years. With the aid of artificial intelligence algorithms, LC-integrated metadevices promote the dynamic control of THz beams and are diversely applied in THz radar detection and compressed sensing imaging.