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

Yurii Zubov,Mathew Sofield,Arup Neogi,Arkadii Krokhin,Bahram Djafari-Rouhani,Yuqi Jin,Ezekiel Walker

doi:10.1038/s42005-020-00422-1

Abstract

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.

Highlights

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
In what follows we demonstrate that the dynamic mass density ρx may be very small in a periodic layered water–steel lattice that gives rise to long-range nonspreading propagation of an acoustic beam
Resonant interaction of the incoming sound with the surface acoustic wave excited at a corrugated surface reduces angular spreading of the beam, while numerical simulations[22] show presence of lateral diffraction maxima

Summary

Introduction

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. 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. It was demonstrated recently that initially spatiotemporaly modulated optical pulses may propagate in a linear medium without dispersion and diffraction[1] In these 3D light bullets, the effects of dispersion and diffraction are not manifested and evolution of the packets is reduced to translational displacement without form change. These and other drawbacks of acoustic devices may be improved by a robust and inexpensive method of collimation of acoustic beams

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