Covert Scattering Control in Metamaterials with Non‐Locally Encoded Hidden Symmetry

Abstract

Symmetries and tunability are of fundamental importance in wave scattering control, but symmetries are often obvious upon visual inspection, which constitutes a significant vulnerability of metamaterial wave devices to reverse‐engineering risks. Here, it is theoretically and experimentally shown that a symmetry in the reduced basis of the “primary meta‐atoms” that are directly connected to the outside world is sufficient; meanwhile, a suitable topology of non‐local interactions between them, mediated by the internal “secondary” meta‐atoms, can hide the symmetry from sight in the canonical basis. Covert symmetry‐based scattering control in a cable‐network metamaterial featuring a hidden parity () symmetry in combination with hidden‐‐symmetry‐preserving and hidden‐‐symmetry‐breaking tuning mechanisms is experimentally demonstrated. Physical‐layer security in wired communications is achieved using the domain‐wise hidden ‐symmetry as a shared secret between the sender and the legitimate receiver. Within the approximation of negligible absorption, the first tuning of a complex scattering metamaterial without mirror symmetry to feature exceptional points (EPs) of ‐symmetric reflectionless states, as well as quasi‐bound states in the continuum, is reported. These results are reproduced in metamaterials involving non‐reciprocal interactions between meta‐atoms, including the first observation of reflectionless EPs in a non‐reciprocal system.