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