Negative reflection of elastic guided waves in chaotic and random scattering media

Benoît Gérardin,Claire Prada,Jérôme Laurent,François Legrand,Alexandre Aubry

doi:10.1038/s41598-019-38480-3

Abstract

The propagation of waves in complex media can be harnessed either by taming the incident wave-field impinging on the medium or by forcing waves along desired paths through its careful design. These two alternative strategies have given rise to fascinating concepts such as time reversal or negative refraction. Here, we show how these two processes are intimately linked through the negative reflection phenomenon. A negative reflecting mirror converts a wave of positive phase velocity into its negative counterpart and vice versa. In this article, we experimentally demonstrate this phenomenon with elastic waves in a 2D billiard and in a disordered plate by means of laser interferometry. Despite the complexity of such configurations, the negatively reflected wave field focuses back towards the initial source location, thereby mimicking a phase conjugation operation while being a fully passive process. The super-focusing capability of negative reflection is also highlighted in a monochromatic regime. The negative reflection phenomenon is not restricted to guided elastic waves since it can occur in zero-gap systems such as photonic crystals, chiral metamaterials or graphene. Negative reflection can thus become a tool of choice for the control of waves in all fields of wave physics.

Highlights

Controlling the propagation of acoustic or electromagnetic waves, is of fundamental interest for many applications ranging from imaging the living and detecting hazardous components, to information processing and structural health monitoring
Let us consider a negative reflection (NR) mirror and an incident forward wave with a Poynting vector, Pi, and a wave vector, ki, pointing in the same direction [see Fig. 1(b,c)]
The NR mirror gives rise to a reflected backward wave, i.e. with a Poynting vector, Pr, and a wave vector, kr, of opposite direction [see Fig. 1(b,c)]

Summary

Introduction

Controlling the propagation of acoustic or electromagnetic waves, is of fundamental interest for many applications ranging from imaging the living and detecting hazardous components, to information processing and structural health monitoring. The wave fields can be tamed in order to take advantage of the complexity of propagation media, for instance, to focus waves or image various objects This is realized in the temporal domain using time reversal (TR) mirrors[1,2,3] or in the spatial domain using phase conjugation (PC)[4,5,6] and wave-front shaping techniques[7] developed in optics. The first situation corresponds to elastic guided waves propagating, for instance, in isotropic[21,22,23,24] or anisotropic[25] plates, pipes[22], rods[26], strips[27], bi-layer structures[28], or even in the Earth’s crust[29] Such systems can support an ensemble of modes, the so-called Lamb waves, which exhibit complex dispersion properties[30]. We will describe how it can lead to the super-focusing of elastic waves in a cavity or to wave trapping in a scattering medium

Objectives

Methods

Results

Conclusion