Significance Recently, terahertz technology has made rapid progress in the fields of label-free analysis, cellular level imaging, chemical and biological sensing, security screening, and wireless communications. Furthermore, major advances have been made in terahertz sources, detectors, and modulators. Developing efficient modulators with natural materials is challenging due to the relatively weak interactions between terahertz waves and natural materials. Although functional modulators operating in the visible and infrared bands are mature for commercial applications, efficient, functional, and high-speed modulators are severely lacking in the technologically important terahertz band. Therefore, researchers are trying to obtain solutions that can improve light-matter interactions for terahertz applications. Metamaterials have extraordinary electromagnetic properties that show great potential to enhance local field strength significantly and improve light-matter interactions in practical terahertz modulators. The integration of metamaterials and certain active materials or techniques leads to the revolution of conventional modulators, which are named “metadevices” in this review. Metamaterials enable abundant functionalities of these devices, and integrated materials offer active responses to external stimuli. These types of hybrid metadevices lead to lower energy consumption, larger modulation depth, faster modulation speed, and more abundant functionalities due to the substantial local electric field enhancement of metamaterials. In this review, the current progress of active metadevices for terahertz applications is summarized with different approaches. In addition, the working mechanism, typical device configurations, major performance, and drawbacks are discussed.

Progress This review summarizes several typical configurations of active terahertz metadevices integrated with liquid crystals (Section 2.1), micro-electromechanical systems (MEMS, Section 2.2), semiconductors (Section 2.3), graphene (Section 2.4), phase change materials (Section 2.5), superconductors (Section 2.5), nonlinear materials (Section 2.5), and chemical reactions (Section 2.5). Electrically triggered liquid crystals integrated with metamaterials exhibit excellent terahertz modulation performance, operating in both transmission and reflection modes. Due to the great flexibility of electrical actuation, this type of metadevice can realize programmable control and operate in a complex configuration for wave deflection ( Fig. 1). The modulation speed of liquid crystal metadevices is limited by the intrinsic on-off speed of liquid crystals and can be optimized to 1 kHz. Moreover, the pixel density of spatial light modulators in this type of configuration is still low, severely decreasing imaging resolution. MEMS metadevices have similar electrical actuation modes and can realize programmable control for terahertz polarization control, wavefront deflection, and dynamic hologram. MEMS cantilevers are reconfigured by an electrostatic force, which is different from the refraction index change of liquid crystals with an external stimulus, and spatial deformation of cantilevers leads to modulation of terahertz resonance frequencies. This review mainly focuses on the polarization effects induced by MEMS metadevices for strong optical activity and artificial chirality ( Fig. 2). In addition to conventional electrical actuation, MEMS cantilevers can be controlled via external forces. The first order vibration frequency of the mechanical vibration of cantilevers determines the modulation speed of MEMS metadevices that depends on the geometric parameters of cantilevers and is in the order of kHz for terahertz metadevices. In addition, scalability, reliability, and uniformity of large-area MEMS metadevice arrays need to be improved for applications in the terahertz band and shorter wavelengths.

The major problem of limited modulation speed can be addressed by excluding electrical stimuli and applying an optical pump. All-optical metadevices have no theoretical limitations of modulation speed determined by the relaxation dynamics of the active materials. By integrating semiconductors (e.g., Si, Ge, GaAs, and WSe₂) with metamaterials, all-optical hybrid metadevices demonstrate excellent performance for ultrafast terahertz modulation (Fig. 3). Recently reported dielectric metadevices also reveal active, low-loss, and functional operations without integrating extra active media (Fig. 3) by directly pumping the resonators. Graphene can be controlled by electrical and optical stimuli and is a promising material for terahertz applications. By directly depositing a graphene layer on metamaterials, the hybrid metadevices enable fast and efficient terahertz modulation with easy fabrication and high efficiency. Phase change materials provide nonvolatile modulation and memory effects. Superconductors can induce high-quality factor resonance modes and provide low-threshold and ultrafast terahertz modulation but must operate in cryogenic temperatures. The nonlinear effects are theoretically instantaneous and can enable femtosecond or shorter switching time. Metadevices integrated with diodes or photodiodes are also very interesting; materials whose properties change under different chemical environments are also a possible solution for active metadevices (Fig. 4).

Conclusions and Prospects Different techniques have been discussed for hybrid metadevices with stimuli of electricity (e.g., liquid crystals, semiconductors, graphene, MEMS, and diodes/transistors), optics (e.g., semiconductors, graphene, phase change materials, and superconductors), heat (e.g., phase change materials and superconductors), forces (e.g., MEMS), and chemical reaction (e.g., Mg). Although there are certain limitations for different combinations, metadevices have made major progress toward realizing efficient terahertz modulators. With the maturity of the semiconductor industry, active metadevices with semiconductors are very attractive for programmable, fast, and efficient terahertz applications. Metadevices integrated with graphene are also attractive with easy fabrication and high efficiency that can be actuated by electrical or optical stimuli. All-optical metadevices are the solution to access faster modulation speed, and an appropriate combination of nonlinear materials and metamaterials would push modulation speeds to the GHz or even THz regime. All the approaches have pros and cons and should be utilized where most applicable.