*2.2. Optical Approaches*

In photosensitive semiconductors (e.g., silicon, GaAs, and conducting oxide), conductivity can be controlled by pumping carriers from the valence band to the conduction band using an external laser beam with photon energy higher than that of the bandgap [84]. This dynamic photoconductivity provides temporal modulation of the metasurface. Figure 6a shows a hybrid circular SRR with an aluminum SRR placed on a circular silicon ring [84]. The top layer SRR was designed with eight patterns for 360◦ phase coverage, realizing a wideband (0.6–1 THz) cross-polarized wavefront deflection from 51◦ to 28◦ (Figure 6b). Stimulating the silicon with an external optical laser pump (800 nm) increased its conductivity and closed the SRR gap, eliminating the beam-splitting and deflection effect. A similar concept was applied to high-resistivity silicon resonators (Figure 6c) [85]. A supercell consisting of four resonators was designed, realizing a beam-deflection angle of 34.7◦ at 0.586 THz with 5 mW laser-pump power (Figure 6d). The rise time for the transient photocarrier was 14 ps. This symmetry-preserved Huygens' metasurface design achieved a high transmission efficiency of 90%.

**Figure 6.** Semiconductor materials for temporal modulation. (**a**,**b**) Optical active polarization switching and dynamic beam splitting. (**a**) An illustration of the hybrid circular split-ring resonator (h-SRR) pumped by near-infrared femtosecond pulses. (**b**) An active polarizing beam splitter. Reprinted from Ref. [84]. (**c**,**d**) Spatiotemporal dielectric metasurfaces for beam steering. (**c**) Ultrafast femtosecond laser pulses (@ 800 nm, 100 fs) pump high-resistivity silicon on a quartz substrate, providing transient photocarriers for temporal modulation. (**d**) Temporal beam steering of 34.7◦ at 0.586 THz. Reprinted from Ref. [85].
