Active metamaterials and metadevices: a review

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.