Breaking bandwidth limits in transformation optics with Brewster-enhanced metamaterials

Transformation optics (TO) enables unprecedented electromagnetic wave manipulation through the theory of coordinate transformations, yet its practical implementation has been fundamentally constrained by narrowband operation stemming from extreme material requirements. To break this spectral bottleneck, a dual-mode metamaterial architecture synergizing Brewster-angle broadband transmission with Fabry-Pérot multiband resonance is proposed. This approach leverages cascaded impedance-engineered slot cavities—constructed from conventional dielectrics and standard metallic patterning—to achieve transformation invariance while eliminating exotic material needs. This framework enables two spectral functionalities simultaneously in a single platform: 1) Omnidirectional multiband operation through discrete resonances; 2) Broadband unidirectional performance via angular-selective Brewster transmission. Experimental validation demonstrates a full-parameter invisibility cloak maintaining > 88.4% transmittance across X-band frequencies (7.5–12.5 GHz) with ± 70° angular tolerance, alongside a retroreflector achieving near-unity efficiency in X/K bands (12-24 GHz) within ± 60° illumination. By resolving the fundamental conflict between bandwidth and geometric complexity in TO designs, this work establishes a scalable paradigm for multifunctional wave-control devices spanning radar stealth to next-generation communications.