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

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-ω 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.

15 pages, 4 figures

References in corpus (4)

• Topological Photonics
• Topological Acoustics
• Observation of topological valley transport of sound in sonic crystals
• Wood Anomalies and Surface-Wave Excitation with a Time-Grating

Cited by in corpus (10)

• Photonics of Time-Varying Media
• Observation of dynamical degeneracy splitting for the non-Hermitian skin effect
• Quantum Entanglement of Non-Hermitian Quasicrystals
• Exact analogue of the Hatano-Nelson model in 1D continuous nonreciprocal systems
• A multiple scattering formulation for elastic wave propagation in space-time modulated metamaterials
• Harnessing the natural resonances of time-varying dispersive interfaces
• Elastic wavepackets crossing a space-time interface
• Non-Bloch band theory for time-modulated discrete mechanical systems
• Symmetry-driven Phononic Metamaterials
• Noise-induced nonreciprocal topological dissipative solitons in directionally coupled chains and lattices