Conformal transformation method has been extensively applied to control propagation of electromagnetic waves and acoustics waves due to the form-invariant property of Maxwell equations and acoustic equations. However, conformal transformation method’s application in elastic waves is rarely reported due to the governing equation of elastic waves don’t have form invariant property. In this paper, through igniting evanescent waves at the interface of conformally mapped Mikaelian lens with hyperbolic secant refractive index profile, conformal transformation method is successfully used to achieve highly efficient (above 75%) broadband (30kHz -80kHz) achromatic high-resolution flexural wave focusing in thin plate with full width at half maximum (FWHM) around 0.2λ. The proposed Mikaelian lens is designed by linking refractive index with the thickness in plates. Simulated results agree well with theoretical prediction. This high performance for flexural wave focusing could be used for energy harvesting and medical imaging.
A 3D-printed ultra-broadband planar Luneburg lens composed of periodic gradient structures is proposed in this paper. Compared to the previous studies, the proposed lens has a pretty large operating frequency range, spanning from 5 to 22 GHz. The measured results of the near-field and far-field illustrate the feasibility and validity of this lens antenna. High efficiency achromatic sub-diffraction focusing with a full-width at half-maximum (FWHM) of 0.3λ and a highly directive far-field radiation pattern with side lobes below -10 dB are achieved. Triple input and triple output function is also realized experimentally.
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