Nonreciprocal Willis Coupling in Zero-Index Moving Media.

Area of Science:

• Acoustics
• Metamaterials
• Wave Propagation

Background:

• Mechanical motion breaks sound propagation symmetry, but achieving significant nonreciprocity usually requires high speeds.
• Zero-index acoustic metamaterials offer unique wave manipulation possibilities.

Purpose of the Study:

• To achieve extreme acoustic nonreciprocity and induced bianisotropy at modest motion speeds.
• To explore the use of asymmetric Willis coupling in metamaterials for directional sound control.

Main Methods:

• Combining moving media with zero-index acoustic propagation.
• Designing a metamaterial with an array of waveguides loaded by Helmholtz resonators.
• Utilizing asymmetric Willis coupling to create opposite refractive indices for bidirectional propagation.

Main Results:

• Demonstrated extreme acoustic nonreciprocity and induced bianisotropy at modest speeds.
• Achieved nonreciprocal positive-to-negative sound refraction.
• Metamaterial exhibits opposite signs of the refractive index for opposite propagation directions.

Conclusions:

• Moving zero-index acoustic metamaterials offer a pathway to extreme nonreciprocity and bianisotropy.
• The asymmetric Willis coupling is key to achieving directional control of sound.
• Proposed a novel nonreciprocal metamaterial lens with one-sided focusing capabilities.