The Energy Focusing of Reflected Flexural Waves Via Two Adjacent Phase-Modulation-based Lenses

In this paper, two lenses attached on adjacent sides of a quadrangular plate are designed based on the phase modulation mechanism, with the aim of focusing flexural waves emitting from a point source. Two design schemes, i.e., changing the propagation distance or varying the phase velocity in the lens, are adopted. The lenses are finalized via theoretical analysis in the framework of Mindlin plate theory, with its working performance examined through FEM simulations and validated further through experimental measurements. Both the simulation results and experimental data demonstrate that the reflected wave has been successfully concentrated at the anticipated focal position. It is interesting that the wave focusing belongs to subwavelength focusing, with the focusing size broken the diffraction limit, i.e., smaller than half wavelength. Additionally, the subwavelength focusing is dependent on the interior angle of the plate, which indicates that the energy focusing can be artificially manipulated. The qualitative results and quantitative data are conducive for harvesting structure-borne noise.