Non-Hermitian Topological Acoustic Metamaterials for Robust Wave Control

The paradigm of topology has profoundly impacted condensed matter physics in recent years. This paradigm has been successfully extended to photonics and acoustics. A key outcome is the emergence of topologically protected edge, surface, and corner states, which exhibit remarkable robustness against defects and disorder.
This framework has been enabled by phononic crystals and acoustic metamaterials which are engineered structures offering unprecedented control over wave propagation. In acoustics, their macroscopic nature and experimental accessibility provide an ideal platform to explore novel wave phenomena while targeting concrete technological applications. Topological acoustic systems already enable robust waveguiding, delay lines, and offer strong potential for phononic circuits immune to fabrication imperfections. In elastodynamics, they support defect-immune guided modes relevant for optomechanics, on-chip acousto-optic modulation, and acoustic information processing. More broadly, these concepts open perspectives for high-performance sensing, vibration and noise control, energy localization and harvesting, as well as advanced biomedical ultrasound technologies where robustness and wave confinement are essential.
The topology of Hermitian systems is now well established. Introducing gain, loss, or non-reciprocity leads to non-Hermitian physics where topology is more delicate. However non-Hermitian systems exhibit phenomena that have no equivalent in Hermitian systems.

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