Antireflection Spatiotemporal Metamaterials

Abstract Reflection occurs when light impinges on an interface between two distinct media. Suppression of the reflection is of paramount significance to practical applications in information transfer and conversion. While technologies such as the Brewster effect and material coatings are widely adopted to eliminate the wave reflection from purely spatial or temporal interfaces, how to eliminate the reflection from a spatiotemporal interface remains elusive. Here, a new type of spatiotemporal metamaterial that functions as a global and generalized perspective of quarter‐wave impedance transformers is presented. With the proper geometric design, the proposed spatiotemporal metamaterial can fully suppress the reflection, applicable to arbitrary modulation velocities. Importantly, the tunability of modulation velocity in the spatiotemporal metamaterial allows flexible frequency conversions in high efficiency, indicating that the spatiotemporal metamaterials are much more powerful than the purely spatial or temporal counterparts. These findings not only enhance the controllability of electromagnetic waves but also can be adapted to other classes of physical dynamics, including water surface waves, acoustic, and elastic.