Three-dimensional acoustic sub-diffraction focusing by coiled metamaterials with strong absorption

The diffraction limit restricts the smallest diameter of a wave's focal spot in a homogeneous medium to no less than half of the operating wavelength. The diffraction limit originates from the interference between converging and diverging waves, however, in this paper, a focus spot beyond the diffraction limit is realized by employing a point-like sink at the focus point to change this interference, thereby eliminating or reducing the diverging waves, using subwavelength coiled acoustic metamaterial absorbers. This sub-diffraction focusing effect is intuitively demonstrated in a number of three-dimensional time-reversal acoustic experiments, for which a planar coiled structure and two types of hemispherical coiled structures were respectively designed. Here, these linear, planar and spherical sub-diffraction focusing results are detailed and the corresponding physical mechanisms are discussed.