Long-range nonspreading propagation of sound beam through periodic layered structure

Linear spreading of a wave packet or a Gaussian beam is a fundamental effect known in evolution of quantum state and propagation of optical/acoustic beams. The rate of spreading is determined by the diffraction coefficient D which is proportional to the curvature of the isofrequency surface. Here, we analyzed dispersion of sound in a solid-fluid layered structure and found a flex point on the isofrequency curve where D vanishes for given direction of propagation and frequency. Nonspreading propagation is experimentally observed in a water steel lattice of 75 periods (~1 meter long) and occurs in the regime of anomalous dispersion and strong acoustic anisotropy when the effective mass along periodicity is close to zero. Under these conditions the incoming beam experiences negative refraction of phase velocity leading to backward wave propagation. The observed effect is explained using a complete set of dynamical equations and our effective medium theory. Recently dispersion-free propagation of spatiotemporally-modulated optical pulses over long distances has been demonstrated, but acoustic analogues are less well-explored. Here, anomalous dispersion and strong anisotropy is demonstrated for the long-distance non-spreading propagation of acoustic waves through an underwater metamaterial.