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

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 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

Equireflectionality and customized unbalanced coherent perfect absorption in asymmetric waveguide networks

We explore the scattering of waves in designed asymmetric one-dimensional waveguide networks. We show that the reflection between two ports of an asymmetric network can be identical over a broad frequency range, as if the network was mirror-symmetric, under the condition of so-called latent symmetry between the ports. This broadband equireflectionality is validated numerically for acoustic waveguides and experimentally through measurements on microwave transmission-line networks. In addition, introducing a generalization of latent symmetry, we study the properties of an N-port scattering matrix S. When the powers of S fulfill certain relations, which we coin scaled cospectrality, the setup is guaranteed to possess at least one zero eigenvalue of S, so that the setup features coherent perfect absorption. More importantly, scaled cospectrality introduces a scaling factor which controls the asymmetry of the incoming wave to be absorbed. Our findings introduce a novel approach for designing tunable wave manipulation devices in asymmetric setups. As evidenced by our acoustic simulations and microwave experiments, the generality of our approach extends its potential applications to a wide range of physical systems