Reconfigurable terahertz metamaterials: From fundamental principles to advanced 6G applications

Terahertz (THz) electromagnetic spectrum ranging from 0.1THz to 10THz has become critical for sixth generation (6G) applications, such as high-speed communication, fingerprint chemical sensing, non-destructive biosensing, and bioimaging. However, the limited response of naturally existing materials THz waves has induced a gap in the electromagnetic spectrum, where a lack of THz functional devices using natural materials has occurred in this gap. Metamaterials, artificially composed structures that can engineer the electromagnetic properties to manipulate the waves, have enabled the development of many THz devices, known as "metadevices". Besides, the tunability of THz metadevices can be achieved by tunable structures using microelectromechanical system (MEMS) technologies, as well as tunable materials including phase change materials (PCMs), electro-optical materials (EOMs), and thermo-optical materials (TOMs). Leveraging various tuning mechanisms together with metamaterials, tremendous research works have demonstrated reconfigurable functional THz devices, playing an important role to fill the THz gap toward the 6G applications. This review introduces reconfigurable metadevices from fundamental principles of metamaterial resonant system to the design mechanisms of functional THz metamaterial devices and their related applications. Moreover, we provide perspectives on the future development of THz photonic devices for state-of-the-art applications.