Metamaterials—artificial media with tailored subwavelength structures—have now encompassed a broad range of novel properties that are unavailable in nature. This field of research has stretched across different wave platforms, leading to the discovery and demonstration of a wealth of exotic wave phenomena. Most recently, metamaterial concepts have been extended to the temporal domain, paving the way to completely new concepts for wave control, such as nonreciprocal propagation, time-reversal, new forms of optical gain and drag.
