*2.3. Phase-Change Materials*

Phase-change materials (PCMs) transform from the amorphous to the crystalline state or between different crystalline phases upon external stimulation [86]. Their reversible phase transition, thermal stability, and fast switching speed make PCMs extremely popular [87]. Various phase-change materials, such as vanadium dioxide (VO2), chalcogenide phase-change materials, and liquid crystals (LCs), have been explored for reconfigurable metasurfaces.
