image: (a) Schematic of the dual-mode metamaterial design. The structure employs PEC slot cavities filled with conventional dielectrics, exhibiting dual operational modes: omnidirectional multiband performance at Fabry-Pérot resonance frequencies, and unidirectional broadband functionality via Brewster-effect transmission. The right panel illustrates the transformation invariance achieved by cascaded impedance-matched metallic slot cavities. (b) Experimental validation of quasi-broadband cloaking performance. (c) Experimental characterization of quasi-broadband retroreflection.
Credit: ©Science China Press
In 2006, Prof. Pendry from Imperial College London proposed transformation optics (TO), which describes the correspondence between the propagation path of light and the constitutive parameters of materials. The proposal of TO has long offered unparalleled control over electromagnetic waves through coordinate mapping, enabling remarkable functional devices including invisibility cloaks, optical illusion devices. However, its potential has been constrained by the need for extreme material parameters—spatial inhomogeneity and anisotropy—that inherently restrict operation to narrowband frequencies. Although previous studies have tried to mitigate the requirement for material inhomogeneity, existing implementations remain confined to single-frequency operation and geometrically restricted designs. Until now, the development of TO devices capable of broadband operation in arbitrarily shaped configurations has remained a persistent challenge.
To solve this dilemma, the research group proposed a dual-mode metamaterial. This metamaterial synergistically combines two distinct spectral functionalities:
1. Multiband omnidirectional operation: Achieved through discrete Fabry-Pérot resonances.
2. Ultrabroadband unidirectional functionality: Achieved via angular-selective Brewster effects.
This universal framework leverages cascaded impedance-matched slot cavities filled with homogeneous dielectrics, eliminating the need for extreme constitutive parameters while enabling dynamic frequency reconfiguration. The team experimentally validated this paradigm through two benchmark devices: a full-parameter cloak capable of concealing objects and a retroreflector designed to reflect the incident light along its original direction. The experimental results demonstrated that the invisibility cloak has a transmittance greater than 88.4% across the entire X-band (7.5–12.5 GHz) with a 70° angular tolerance, and the retroreflector achieved near-unity retroreflecting efficiency in both X- and K-bands (12–24 GHz) under wide-angle illumination. These devices exhibit more than 10 times extension of the operational bandwidth compared to conventional TO implementations while maintaining full geometric adaptability.
The implications extend beyond proof-of-concept demonstrations. This methodology utilizes metallic slots and commercial dielectrics, ensuring scalability from microwave to terahertz regimes where metallic conductivity remains favorable. This positions the technology for immediate applications in broadband radar cross-section reduction, adaptive beam-steering systems, and next-generation communication networks, aligning with emerging 6G/7G infrastructure demands.