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

TL;DR

This paper demonstrates experimentally that negative reflection of elastic waves in complex media can focus waves back to their source, mimicking phase conjugation passively, with potential applications across wave physics.

Contribution

It introduces the concept of negative reflection in elastic waves, linking it to phase conjugation and showing its super-focusing capabilities in complex media.

Findings

Negative reflection converts positive phase velocity waves into negative ones.

Negatively reflected waves focus back to the source, mimicking phase conjugation.

The phenomenon is demonstrated in elastic waves and applicable to various wave systems.

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.