Compound elastic concentrator for achromatic superfocusing and amplified energy harvesting over an ultra-broadband spectrum

Elastic metasurfaces offer a compact route to manipulate wavefronts, yet their focusing performance is intrinsically chromatic because the retarded phase varies strongly with operating frequency. This spectral dispersion induces chromatic aberration and deteriorates energy capture, which limits ultra-broadband elastic-wave concentration and reliable piezoelectric energy harvesting in practical plate-based systems. Here, we propose and demonstrate a compound elastic concentrator that integrates a thickness-programmed elastic metasurface with a conformally mapped gradient-index concentrator, enabling achromatic superfocusing over an ultra-broadband spectrum. A central advance is a unified, analytically tractable unit-cell platform grounded in Reissner–Mindlin plate theory, which allows both components to be designed and fabricated by modulating only the local plate thickness. In this two-stage architecture, the metasurface provides high numerical aperture steering and compact pre-focusing, while the gradient-index concentrator continuously recaptures and confines the diffracted waves, thereby suppressing the chromatic aberration inherent to elastic metasurfaces. Numerical and experimental validations verify diffraction-limited, frequency-stable superfocusing in the compound elastic concentrator over the 40–100 kHz operating band, together with substantial intensity enhancement compared with standalone configurations. When a piezoelectric energy harvester is positioned at the confocal hotspot, the concentrated elastic-wave energy converts efficiently into electrical output, yielding markedly enhanced output power across the operating band. These results establish a design- and fabrication-friendly, thickness-only pathway toward ultra-broadband elastic-wave concentration for self-powered sensing, structural health monitoring, and vibration-energy harvesting.