Negative refraction and focusing of backward propagating Lamb waves.

It is known that some Lamb modes referred to as backward‐waves have a negative phase velocity. In this work, the mode conversion between a forward and a backward Lamb wave is observed at the interface formed by a stepped thickness change and on a finite bandwidth around the frequency where the dispersion curves of the forward propagating mode S2 and the backward propagating mode S2b on opposite sides of the interface intersect. To this end, a linear array of transducers connected to a fully programmable electronic device is used to generate selectively the S2 Lamb mode in the thick part of the plate. The resulting surface displacements are detected using a heterodyne interferometer. We observe that the forward S2 mode propagating in the thick part is converted into the S2b mode in the thinner part. We demonstrate that such mode conversion leads to wave focusing across the interface as originally predicted by Veselago for negative index optical media.