Non-reciprocal wave phenomena through pump-signal wave interaction in discrete systems with asymmetric subwavelength geometry

Abstract

Acoustic non-reciprocity has been shown to enable a plethora of effects analogous to phenomena seen in quantum physics and electromagnetics, such as immunity from back-scattering, unidirectional band gaps, and topologically protected states, which could lead to the design of direction-dependent acoustic devices. One class of material that holds promise as a means to achieve acoustic non-reciprocity is the “Willis medium,” which exhibits strain-momentum coupling owing to asymmetry and non-local effects in the material microstructure. This work considers a method for obtaining non-reciprocal elastic wave propagation in a system with sub-wavelength asymmetry inspired by asymmetric Willis microstructures. Non-reciprocity is achieved through application of a slowly-varying pump wave that acts as a spatio-temporal modulation of the material, which is modeled as a discrete system with geometric nonlinearity. The modulation generates momentum bias for a fast-varying signal wave with pump-signal interaction enabled by weak nonlinearity associated with the variation of subwavelength asymmetric geometry. It is shown that momentum bias may be generated with a pump wave acting transverse to the direction of propagation, which may facilitate experimental realizations. [This work was supported by the National Science Foundation.]