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Subwavelength imaging of elastic/acoustic waves using phononic crystals (PCs)is limited to a narrow frequency range via the two existing mechanisms thatutilize either the intense Bragg scattering in the first phonon band ornegative effective properties (left-handed material) in the second (or higher)phonon band. In the first phonon band, the imaging phenomenon can only exist atfrequencies closer to the first Bragg band gap where the equal frequencycontours (EFCs) are convex. Whereas, for the left-handed materials, thesubwavelength imaging is restricted to a narrow frequency region where wavevectors in PC and background material are close to each other, which isessential for single-point image formation. In this work, we propose a PC lensfor broadband subwavelength imaging of flexural waves in plates exploiting thesecond phonon band and the anisotropy of a PC lattice for the first time. Usinga square lattice design with square-shaped EFCs, we enable the group velocityvector to always be perpendicular to the lens interface irrespective of thefrequency and incidence angle; thus, resulting in a broadband imagingcapability. We numerically and experimentally demonstrate subwavelength imagingusing this concept over a significantly broadband frequency range.

Broadband Subwavelength Imaging of Flexural Elastic Waves in Flat Phononic Crystal Lenses