2.3.1. Vanadium Dioxide (VO2)

Vanadium dioxide (VO2) has a unique atomic rearrangement of monoclinic phases, exhibiting semiconductor behavior at room temperature and rutile phases at high temperatures (~68◦), experiencing an abrupt transition from insulator to metal when subjected to thermal, electrical, optical, or mechanical stimulation [88–92]. This insulator-to-metal transition property provided a diversified application for switching a metasurface between the broadband-absorber and half-wave plate states (Figure 7a) [90]. The metasurface had an absorption efficiency >90% at room temperature but became a reflector as the temperature rose above the phase-change temperature [90]. Supercells consisting of eight resonators were designed to have a beam-steering effect in the metallic state (Figure 7b). The metasurface had a simulated wideband deflection of ~28◦ at 0.8 THz (Figure 7c). Embedded VO2 with resonant structures for a multifunctional coding metasurface was also presented numerically [91,93–95]. An experimental demonstration of VO2-based reconfigurable beam steering was presented in [89] (Figure 7d). The unit cell had a cross-shaped aperture with a resistive heater electrode placed beneath for independent Joule-heating actuation. A 100 nm thick SiO2 layer was used to insulate the resistive-heater electrodes from the top Au layer. The refractive index of the VO2 was varied through temperature changes. The metasurface achieved a 44◦ beam deflection in both horizontal and vertical directions at 0.1 THz (Figure 7e,f). Recently, an electrically triggered VO2-based reconfigurable metasurface for both phase (90◦) and amplitude (71% at 0.79 THz) modulation was experimentally demonstrated in [96]. And a multi-state 8 × 8 pixelated reflective modulator was demonstrated with a modulation speed of 1 kHz [97].

**Figure 7.** Phase-change materials of a vanadium-dioxide (VO2)-enabled reconfigurable metasurface. (**a**–**c**) A thermally controlled reconfigurable metasurface with broadband absorption-to-reflection conversion. Reprinted from Ref. [90]. (**a**) Schematic of the VO2 integrated metasurface. (**b**) Unit-cell structure (top) and a unit-cell array for 2π phase control (bottom). (**c**) Broadband reflection when VO2 is in its fully metallic state. (**d**–**f**) An electronically controlled beam-steering metasurface operates at 0.1 THz. Reprinted from Ref. [89]. (**d**) Top-view scanning electron microscopic image of the metasurface (top left), unit-cell structure (bottom left), and beam steering (right). (**e**) Horizontal beam steering at −22◦, −14◦, 0◦, 12◦, and 22◦. (**f**) Vertical beam steering at −14◦, 0◦, and 12◦.
