Metamaterials, as artificially structured materials composed of subwavelength arrays of resonant unit cells, can exhibit exotic properties beyond those accessible to natural materials. They were initially proposed for challenging fundamental laws and demonstrating negative refraction in the microwave regime, and subsequently exploited as a versatile platform to manipulate electromagnetic waves throughout the spectrum via their extreme scalability. Over the past decade, research into metamaterials has been extended to a search for real-world applications, leading to the concept of metadevices, defined as metamaterial-based devices that can operate in an active manner. Due to their subwavelength scale, metamaterials present intriguing strategies for active tuning and provide flat, high-efficiency alternatives to conventional optical systems based on bulky components. In this topical review, we summarize the development of active metamaterials and metadevices ranging from microwave to visible wavelengths, including milestones as well as the state of the art. We survey tuning strategies based on mechanical reconfiguration and incorporation with active materials such as varactor diodes, semiconductors, liquid crystals, phase change materials, superconductors, and two-dimensional materials under various external stimuli, and discuss their fascinating advantages and potential challenges to be confronted. Finally, future prospects together with several emerging tuning strategies and materials are presented at the end.

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A resonant metasurface with high quality factor can not only localize light at the nanoscale but also manipulate the far-field radiation. In this work, we experimentally demonstrate an active Fano-resonant metasurface that combines an asymmetric silicon nanorod array with embedded germanium quantum dots. The collective resonance of the nanorods results in strong near-field confinement, and the nanorods also lead to directional emission. This gives rise to 3 orders of magnitude enhancement of the photoluminescence intensity with respect to the unpatterned area. Besides, due to the symmetry-breaking property of the structure, the light emission is of specific polarization. Moreover, by varying the geometric parameters of the nanorods, different resonances are spectrally overlapped, which can be utilized to manipulate the far-field radiation pattern. The metasurface shows enormous potential in manipulating light emission and provides a route for high-directionality, high-efficiency LEDs and potentially functional dielectric metasurface lasers.