Negative refraction and focusing of elastic Lamb waves at an interface

We study negative refraction and focusing of elastic waves in a simple mechanical system comprised of a freestanding plate with a step change in thickness. A point-focused and intensity-modulated laser source is used to excite backward-propagating Lamb waves on one side of the step, and the displacement field is probed using an optical interferometer. Conversion between forward- and backward-propagating modes at the interface leads to negative refraction, and we demonstrate for the first time the operation of a flat lens, similar to that predicted by Veselago in negative-index media, for guided elastic waves in isotropic media. We propose that guided elastic waves provide a convenient and powerful experimental test bed for the study of negative-index physics.