Far-field imaging of acoustic waves by a two-dimensional sonic crystal

Negative refraction of elastic waves at the deep-subwavelength scale in a single-phase metamaterial

“…Yang et al 7 experimentally achieved NR and focusing of ultrasound beams by using the anisotropy of the equifrequency curves (EFCs) in a PC consisting of tungsten carbide beads surrounded by water. NR imaging for all angles of incidence was demonstrated later through the EFCs in the second band of a two-dimensional (2D) PC, where the direction of the wave vector and group velocity are antiparallel to each other [8][9][10] . Intense Bragg scattering at the Brillouin zone can also result in the extreme deformation of the second band EFCs in PCs, which leads to backward-wave NR 11 .…”

Beam paths of flexural Lamb waves at high frequency in the first band within phononic crystal-based acoustic lenses

“…Analogously to heterostructures designed to achieve negative refraction of electromagnetic waves, 1,2 PCs have been conceived in order to focus acoustic waves in a frequency range where the negative refraction is possible. [3][4][5][6][7][8] It has been proved both theoretically and experimentally that such PCs can realize the subwavelength focusing or imaging. [3][4][5][6][7][8] To a certain extent, the resolution of the focusing was beyond the diffraction limit with the evanescent waves being gathered at the focusing through the bound modes.…”

“…[3][4][5][6][7][8] It has been proved both theoretically and experimentally that such PCs can realize the subwavelength focusing or imaging. [3][4][5][6][7][8] To a certain extent, the resolution of the focusing was beyond the diffraction limit with the evanescent waves being gathered at the focusing through the bound modes. 9,10 These studies of acoustic waves in fluids have inspired a series of works dealing with other polarized waves both on the negative refraction phenomenon [11][12][13] and on the focusing of negative-refractive PCs.…”

Focusing and directional beaming effects of airborne sound through a planar lens with zigzag slits

“…By employing these artificial materials, numerous transmitted devices have been proposed, e.g. flat lenses with imaging effects [2][3][4][5][6][7]. The improvement of the transparency in such artificial materials can be mostly attributed to the existence of air channels for direct sound propagation.…”

“…A critical factor is the extreme impedance contrast between air and natural solids, which strongly suppresses the coupling efficiency of the acoustic devices. This problem is considerably relaxed by the recent development of the artificial structures [1][2][3][4][5][6][7][8][9][10][11], such as sonic crystals and acoustic metamaterials. By employing these artificial materials, numerous transmitted devices have been proposed, e.g.…”