2.3.2. Chalcogenide Phase-Change Materials

Phase transitions of chalcogenide phase-change materials are mediated by nucleation dynamics, providing an analog response by continuously varying the crystallinity fraction. Moreover, these analog states are nonvolatile, requiring zero-hold power [86]. A chalcogenide phase-change material, germanium–antimony–tellurium (GST), a ternary compound made of germanium (Ge), antimony (Sb), and tellurium (Te), exhibits a reconfigurable phase-transition response between amorphous and crystalline states under external optical, electrical, or thermal stimulation. A multifunctional tunable metadevice was described in [98] using Ge2Sb2Te5. A dual-split asymmetric SRR was designed for Fano resonance (Figure 8a). Four meta-atoms designed for different THz frequency responses were configured in a 2 × 2 array. A spatially selective reconfiguration was achieved by Joule heating from the isolated biasing current (Figure 8b). Continuously increasing the material temperature led to increased terahertz conductivity and refractive index, providing multilevel resonance modulation states (reaching 100% at 850 mA) (Figure 8c).

Germanium telluride (GeTe) with a crystallization temperature of ∼ 200 °C and a resistivity reduction of six orders has also been extensively studied for the construction of tunable metasurfaces [99,100]. Optical stimulus provides faster material phase-change modulation [98,101]. Recently, a two-bit coding metasurface based on GeTe was experimentally demonstrated with multifunctionality, including beam tilting, directing, and splitting at 0.3 THz (Figure 8d) [101]. A laser pulse excited the meta-atom between crystalline (conductive) and amorphous (insulating) states (Figure 8e), achieving a reflected phase difference of 180◦ (Figure 8f). Five different beam controls were demonstrated with five coding masks, as shown in Figure 8g.

**Figure 8.** Chalcogenide phase-change materials enabled a reconfigurable metasurface. (**a**–**c**) Germanium– antimony–tellurium (GST) incorporated with Fano-resonance mate atoms for multicolor spatial light modulation. (**a**)A2 × 2 array for four-color spatial light modulation. (**b**) Schematic of current biasing. (**c**) Multilevel Fano-resonance modulation (FRM) results from different input currents (stimulus period ≈ 15 s). Reprinted from Ref. [98]. (**d**–**g**) Phase-change GeTe material applied for a multifunctional coding metasurface. (**d**) Illustration of the coding metasurface. (**e**) GeTe- and gold-integrated unit cell with amorphous (insulating) state of GeTe and crystalline (conductive) state. (**f**) Reflected phase of 180◦ at 0.3 THz for the coding element at two different states. (**g**) Multifunctionality (beam tilting, directing, and splitting) is realized through different coding masks. Reprinted from Ref. [101].

## 2.3.3. Liquid Crystals

Liquid crystals are attractive for their inherent birefringent properties, which depend on the orientation of liquid crystal molecules and can be effectively controlled by an external electric field or light [102–105]. Figure 9a shows a THz spatial light modulator based on liquid crystals combined with metamaterial absorbers [102]. The authors used an isothiocyanate-based liquid-crystal mixture to fill the space around the electric ring resonator (ERR). The spatial light modulator consisted of a 6 × 6 pixel array (Figure 9b), and each pixel was individually controlled by a 15 V peak-to-peak square waveform at 1 kHz. The applied electric field forced the liquid-crystal molecule to align with its direction, achieving 75% reflectivity modulation at 3.67 THz (Figure 9c).

**Figure 9.** Liquid-crystal-enabled reconfigurable metasurface. (**a**–**c**) A spatial light modulator based on liquid crystals. (**a**) Schematic of metamaterial absorbers covered with a layer of liquid crystals. (**b**) Spatial light modulator device and an enlargement for the meta-atom dimensions. (**c**) A 6 × 6 pixelated absorption map measured at 3.725 THz. Reprinted from Ref. [102]. (**d**–**g**) A spatial phase modulator operating at 0.8 THz. (**d**) Schematic of the metasurface. (**e**) An optical microscopic image of the fabricated metasurface. (**f**) Phase difference as a function of liquid-crystal tilt angle. (**g**) Calculated beam deflection. Reprinted from Ref. [104]. (**h**,**i**) Programmable metasurface for beam steering. (**h**) Schematic of the beam steering metasurface with the control element (top). The unit cell consists of a liquid-crystal layer embedded between two metallic layers. Schematic of the metasurface with the applied coding sequence of /01.../(bottom). (**i**) Reflected angles for five different coding sequences with an incident angle of 20◦ at 0.672 THz. Reprinted from Ref. [105]. (**j**,**k**) Liquid crystalbased multifunctional transmissive coding metasurface. (**j**) Schematic of the functional metasurface (top) and the asymmetric unit-cell design (bottom). (**k**) Measured transmitted pattern for different coding sequences. Reprinted from Ref. [106].

The reconfigurable effective refractive index of liquid crystal makes it suitable for both amplitude and phase modulation [103]. Figure 9d shows a spatial phase modulator of a nematic liquid-crystal layer sandwiched between two orthogonally placed metasurfaces [104]. Meandering wires enabled the electric potential of each pixel to be selectively and spatially addressed. The anisotropic metapixel (unit cell) consisted of two metallic split rings to

enhance liquid-crystal birefringence (Figure 9e,f). The maximum phase change (32◦) resulted from a 90◦ tilt angle. A biasing voltage of 20 V achieved a deflection angle of 5◦ at 0.8 THz (Figure 9g). Combining liquid crystals with a programmable metasurface enabled more advanced dynamic beam steering (Figure 9h) [105]. A 25 μm thick liquid-crystal layer was embedded inside a metal–insulator–metal (MIM) resonator with a top metal layer patterned in the Jerusalem cross structure. Bias voltages of 0 and 40 V were applied to have "0" and "1" states with 180◦ phase differences while maintaining the same reflection amplitude. By changing the phase gradient through coding pattern, different reflected angles were achieved at 0.672 THz, for an incident angle of 20◦, as shown in Figure 9i. The maximum acquired deflection angle was 32◦, with a reflection efficiency of 19.1%. By designing unit cells with more phase changes, this method can be further extended to two-bit- or three-bit-coding liquid-crystal metasurfaces, achieving a wider beam-deflection angle and higher reflection efficiency. Recently, an liquid crystal -based transmissive coding metasurface was demonstrated for multifunctional control (Figure 9j) [106]. An asymmetric metasurface pattern was designed for Fano resonance, realizing a transmission efficiency as high as ~50% at 0.426 THz. Figure 9k shows the measured beam-splitting patterns using different coding sequences.
