  Field transformation, as an extension of the transformation optics, provides
a unique means for nonreciprocal wave manipulation, while the experimental
realization remains a significant challenge as it requires stringent material
parameters of the metamaterials, e.g., purely nonreciprocal bianisotropic
parameters. Here, we develop and demonstrate a nonreciprocal field
transformation in a 2D acoustic system, using an active metasurface that can
independently control all constitutive parameters and achieve purely
nonreciprocal Willis coupling. The field-transforming metasurface enables
tailor-made field distribution manipulation, achieving localized field
amplification by a predetermined ratio. Interestingly, the metasurface
demonstrates the self-adaptive capability to various excitation conditions and
can extend to other geometric shapes. The metasurface also achieves
nonreciprocal wave propagation for internal and external excitations,
demonstrating a one-way acoustic device. Such a field-transforming metasurface
not only extends the framework of the transformation theory for nonreciprocal
wave manipulation, but also holds significant potential in applications such as
ultra-sensitive sensors and nonreciprocal communication.

Applied Physics