Fig. 1. Refractive indexes of three nematic liquid crystals in THz regime. (a) Principle diagram of experiment; (b) refractive index spectra of 5CB liquid crystal; (c) refractive index spectra of E7 liquid crystal; (d) refractive index spectra of BNHR liquid crystal[62]

Fig. 2. Experimentally measured THz birefringent performance of dual-frequency liquid crystal (DP002-016). (a) Time-domain signal of incident light polarization along x-axis; (b) time-domain signal along y-axis; (c) refractive index spectra along x-axis; (d) refractive index spectra along y-axis; (e) extinction coefficient along x-axis; (f) extinction coefficient along y-axis[63]

Fig. 3. THz birefringent and phase shift performances of dual-frequency liquid crystal (DP002-016) versus frequency of alternating electric field. (a) Schematic of liquid crystal molecular arrangement under alternating electric fields of 1 kHz and 100 kHz; (b) group refractive index versus alternating frequency; (c) phase shift spectra of birefringence[63]

Fig. 4. Transmission characteristics of THz metal photonic crystal tunable filters. (a) Transmission spectra of liquid crystal-filled PC device under different effective refractive indexes nLC of liquid crystal; (b) structural diagram of PC device; (c) steady-state field distributions at ON and OFF frequencies of PC device; (d) transmission spectra of liquid crystal-filled PCW device under different effective refractive indexes nLC of liquid crystal; (e) structural diagram of PCW device; (f) steady-stat

Fig. 5. Experimental results of transmission characteristics of tunable THz EIT and EIA metamaterial devices. (a) Schematic and (b) transmission spectra of liquid crystal metamaterial when external electric field is along y direction at θ=0°; (c) schematic and (d) transmission spectra of liquid crystal metamaterial when external electric field is along x direction at θ=0°; (e) schematic and (f) transmission spectra when external electric field is along y direction at θ=90°[87]

Fig. 6. Experimental results of THz liquid crystal phase shifter based on grid-lattice composite dielectric metasurface structure. (a) 3D structural diagram of liquid crystal filled dielectric metasurface; (b) effective refractive index spectra for different external electric fields; (c) phase shift of three electronically controlled liquid crystal phase shifters based on different dielectric substrates versus bias electric field at 0.7 THz [97]

Fig. 7. Simulation results of broadband tunable QWP based on electronically controlled liquid crystal-graphene grating. (a) Schematic of broadband tunable QWP based on PGGLC; (b) schematic of polarization conversion and comparison of operating curves of two QWPs[98]