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

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.

A Review of Research on Seismic Metamaterials

“…As can be seen from the existing elastic metamaterials, some of the research is focused on low‐frequency regulation, some is focused on wide‐band range regulation, and some work considers implementing wide low‐frequency bandgaps simultaneously. Since there have been several good review articles on elastic metamaterials, we do not expect comprehensive coverage of elastic metamaterials with wide low‐frequency bandgaps but focus on introducing some novel wide low‐frequency bandgap realization techniques. These techniques have received little attention in SMs and some of them have not even been considered, so it is useful to review them.…”

Cloaking, trapping and superlensing of lamb waves with negative refraction

“…The underlying mechanism consists in a mode conversion between forward and backward propagating modes (or vice versa) either at a step-like thickness discontinuity 21 , 22 or at the interface between two different materials with an adequate acoustic impedance mismatch 23 . More recent studies investigated the negative reflection of Lamb waves at a free plate edge 24 – 26 and the conversion of propagating modes at a thickness step in order to optimize the negative refraction effect 27 .…”

May Rayleigh waves propagate with group- and phase-velocities of opposite sign in the valley of Mexico City?

“…So this short comment in some way serves the popularization and understanding of negative group velocities, well-known in many fields of physics and techniques [see e. g. Nedopasov et al (2017), Tamm et al. (2017), Gérardin et al. (2019)], in seismology as well [see Lysmer (1970), Forbriger (2017)] .…”