Odd Willis coupling induced by broken time-reversal symmetry

Area of Science:

• Acoustics and Metamaterials
• Wave Phenomena and Scattering

Background:

• Willis coupling, the interaction between pressure and particle velocity in sound waves, typically occurs in asymmetric structures.
• Existing Willis coupling is often perturbative and reciprocal, imposing symmetry constraints on acoustic scattering.
• Enhanced Willis coupling can lead to exotic acoustic phenomena when combined with tailored asymmetries and resonances.

Purpose of the Study:

• To introduce and experimentally observe a novel, dual form of acoustic Willis coupling.
• To investigate Willis coupling in geometrically symmetric structures by breaking time-reversal symmetry.
• To explore the potential of odd Willis coupling for advanced sound manipulation.

Main Methods:

• Theoretical derivation of conditions to maximize the dual Willis coupling effect.
• Experimental verification using a symmetric subwavelength scatterer.
• Application of angular momentum bias to break time-reversal symmetry.

Main Results:

• Observation of a purely odd-symmetric Willis coupling in symmetric structures.
• Demonstration of enhanced sound scattering phenomena enabled by odd Willis coupling.
• Experimental validation of the theoretical predictions.

Conclusions:

• The study introduces and confirms a new form of acoustic Willis coupling with odd symmetry.
• This discovery opens new avenues for acoustic metamaterials, enabling advanced sound control, non-reciprocal scattering, and wavefront shaping.
• The findings have broad implications for acoustics, nano-optics, photonics, and mechanics.