Experimental Observation of Efficient Nonreciprocal Mode Transitions via Spatiotemporally-Modulated Acoustic Metamaterials

TL;DR

This paper demonstrates a novel spatiotemporally-modulated acoustic metamaterial that achieves nonreciprocal sound steering and mode transitions without mechanical parts, enabling unidirectional wave control and potential applications in advanced acoustic systems.

Contribution

It introduces a compact, noiseless, and programmable acoustic metamaterial using electromagnetic coupling to achieve irreversible mode transitions and nonreciprocal wave manipulation.

Findings

Experimental validation of nonreciprocal wave steering

Demonstration of unidirectional evanescent wave conversion

Implementation of nonreciprocal blue-shift focusing

Abstract

In lossless acoustic systems, mode transitions are always time-reversible, consistent with Lorentz reciprocity, giving rise to symmetric sound manipulation in space-time. To overcome this fundamental limitation and break space-time symmetry, nonreciprocal sound steering is realized by designing and experimentally implementing spatiotemporally-modulated acoustic metamaterials. Relying on no slow mechanical parts, unstable and noisy airflow or complicated piezoelectric array, our mechanism uses the coupling between an ultrathin membrane and external electromagnetic field to realize programmable, dynamic control of acoustic impedance in a motionless and noiseless manner. The fast and flexible impedance modulation at the deeply subwavelength scale enabled by our compact metamaterials provides an effective unidirectional momentum in space-time to realize irreversible transition in k-{\omega} space between different diffraction modes. The nonreciprocal wave-steering functionality of the proposed metamaterial is elucidated by theoretically deriving the time-varying acoustic response and demonstrated both numerically and experimentally via two distinctive examples of unidirectional evanescent wave conversion and nonreciprocal blue-shift focusing. This work can be further extended into the paradigm of Bloch waves and impact other vibrant domains, such as non-Hermitian topological acoustics and parity-time-symmetric acoustics.