Active Willis metamaterials for ultracompact nonreciprocal linear acoustic devices

Willis materials are complex media characterized by four macroscopic material parameters, the conventional mass density, the bulk modulus, and two additional Willis coupling terms, which have been shown to enable unsurpassed control over the propagation of mechanical waves. However, virtually all previous studies on Willis materials involved passive structures which have been shown to have limitations in terms of achievable Willis coupling terms. In this Rapid Communication, we show experimentally that linear active Willis metamaterials breaking these constraints enable highly nonreciprocal sound transport in very subwavelength structures. Furthermore, we present an experimental procedure to extract the effective material parameters expressed in terms of acoustic polarizabilities for media in which the Willis coupling terms are not constrained by the relationship holding in unbiased passive materials. The approach presented here will enable a generation of ultracompact Willis materials for enhanced sound control and improved acoustic imaging and signal processing.