# Liquid Crystal-Tuned Planar Optics in Terahertz Range
## 3 LC-Integrated Plasmonic Metadevices  
Plasmonic metamaterials consist of artificial metallic micro/nanostructures. By tailoring the structures and shapes of metamaterial units, functions not available with natural materials, such as [56], invisible cloaks [57], and perfect absorption [58], can be achieved. To realize dynamic or switchable functions, metamaterials are usually combined with semiconductors [59], graphene [60], vanadium dioxide [61], and other functional materials. LCs have attracted special attention due to their mature and cost-efficient fabrication [62]. However, they still suffer from slow operating speed as well as limited modulation depth, and there is still a long way toward practical active metadevices. In recent years, a lot of THz absorbers, phase shifters and polarization converters based on LC-integrated metamaterials have been presented. Because of their applications in sensing and imaging fields, THz absorbers have been widely concerned [63; 64; 65]. In 2011, a tunable bandpass filter was theoretically designed, which uses a woodpile metallic photonic crystal as a resonator and electrode and LCs as a defect layer to fill the photonic crystal [66]. In 2013, [PERSON] et al. demonstrated LCs embedded into metamaterials to form metal-LC-metal absorbers, and 30% amplitude modulation of absorption at 2.62 THz and over 4% bandwidth adjustment were presented, as shown in Figure 3a [67]. By combining different geometries of plasmonic resonators, a multiband tunable metamaterial absorber can be realized [68]. In Figure 3b, a few layers of porous graphene are integrated into the metamaterial to apply a uniform electric field to LCs and achieved an amplitude modulation of \(\sim\)80% at a voltage of 10 V [69]. In addition to metamaterial absorbers, modulators are also used to modulate THz wave amplitudes. Different THz modulators based on LC-integrated metamaterials have been reported. However, most of these devices operate in reflection mode, limiting their applications in THz systems. As shown in Figure 3c, [PERSON] et al. reported a metadevice that worked in transmission, with high modulation depth and low insertion loss, by integrating two layers of metamaterials with LCs [70]. In 2018, an LC-integrated metadevice operating in transflective mode was reported, and its structure is shown in Figure 3d. The comb electrodes exhibited polarization selectivity and realized a dynamic electromagnetically induced transparency analog (EIT) in transmission mode, and a dynamic absorber in reflection mode [71]. In the above tunable terahertz metadevices, LCs act as the intermedium with a tunable refractive index. Through reorienting the LCs driven by an external field, dynamic modulation of the THz intensity and phase can be realized. In this way, the modulation speed reaches the millisecond scale, but the modulation depth is commonly low. By integrating LCs as tunable waveplates with polarization-dependent metamaterials, dynamic THz resonant switches with large modulation depths of THz amplitude and phase were achieved,  
Figure 3: (**a**) Schematic diagram of the metal–LC–metal absorber [64]. (**b**) Illustration of a few-layer porous graphene-assisted metamaterial absorber [66]. (**c**) Structure of a double layer plasmonic metamaterial THz modulator [67]. (**d**) Structure of a THz filter with dual operating modes [68]. as shown in Figure 4a [72]. Through thermally [73], photo [74], or magnetically [75] tuning the LC birefringence, the resonances of metamaterials were also varied. In Figure 4b, an optically tunable and thermally erasable THz modulator was realized by doping LC with the azo dye methyl red [76]. Based on the rewritable photo-patterning of SD1, a spatial terahertz modulator featured by photo-reconfigurable and electric-switchable is also designed [77]. In addition to traditional nematic LCs, many other kinds of LCs have also been studied.