Fig. 1. Active terahertz metamaterial modulators integrated with liquid crystals. (a) Active terahertz modulator with broadband and large modulation depth by switching the operation regime between Brewster angle and total reflection angle^[14]; (b)(c) liquid crystal integrated with perfect absorber metamaterials^[17-18]; (d) programmable metasurface integrated with liquid crystals^[19]; (e) liquid crystal metasurface working in transmission mode^[20]

Fig. 2. Active terahertz metamaterial modulators based on MEMS. (a) Active polarization modulator with comb drive actuators^[29]; (b) SEM image of a simple bimorph cantilever^[34]; (c) chiral switches actuated by electricity or pressure difference^[42-43]; (d) programmable binary chiral modulator with metamolecules^[34]; (e) coupling regime switchable terahertz cavity with electric actuation^[32]; (f) programmable terahertz spatial light modulator with electric actuation^[33]

Fig. 3. Active terahertz metamaterial modulators with semiconductors. (a) All-optical Si-metamaterial amplitude modulator^[49]; (b) ultrafast all-optical Si-metamaterial polarizing beam splitter^[53]; (c) all-optical modulation of all-dielectric metamaterial absorber^[63]; (d) high Q bound state in the continuum excited with all-dielectric metamaterials and all-optical modulation of the mode^[66]; (e) Huygens’ dielectric metasurface for active angle deflection^[68]

Fig. 4. Active terahertz metamaterial modulators based on graphene. (a) Broadband terahertz modulation by directly biasing graphene layer^[87]; (b) enhancing modulation depth by dual pumping of electrical and optical stimuli^[89]; (c) larger modulation depth by operating at Brewster angle^[90]; (d) modulating plasmonic resonance frequency of patterned graphene^[91]; (e) flexible graphene modulator with metamaterials and flexible dielectric^[96]; (f) improved modulation depth with metamaterials and application in single pixel imaging^[97]

Fig. 5. Active metamaterial devices based on phase change materials, superconductors, nonlinear effect, diodes, transistors, and chemical reactions. (a) Terahertz nonlinear effect realized by integrating metamaterial with VO₂^[102]; (b) laser writing patterns on GST film for rewritable optical devices^[100]; (c) active metamaterial with high quality factor fabricated with YBCO film^[109]; (d) ultrafast polarization modulation by second order nonlinearity of gold surface^[115]; (e) programmable modulation by integrating with diodes^[117]; (f) GHz super-fast programmable modulation by integrating with transistors^[118]; (g) dynamic optical modulation by reversible chemical reaction of Mg and hydrogen^[119]

Table 1. Performance of liquid crystal based metamaterials