Jie Zhu

In non-Hermitian physics, coherent perfect absorption at an exceptional point (CPA EP) emerges as a unique degenerate state that can manifest anomalously broadened absorption line shape implying strong robustness, which apparently contradicts the extreme sensitivity rooted in EPs. To investigate this unique state, we design an underwater acoustic metasurface comprising a beam splitter and a retroreflector on opposite sides. We leverage fluid-solid interaction (FSI) as a powerful mechanism to engineer the non-Hermitian coupling, circumventing the necessity for difficult-to-tune gain or loss for the degeneracy. FSI-induced strong nonlocal elastodynamic coupling concurrently morphs the previously independent spatial subspaces, creating contrasting non-Hermitian behaviors in the respective subspaces, with one encoding the projected CPA EP. We prove that the CPA EP inherits core attributes from both states, manifesting the coexistence of macroscopic operational robustness (from CPA) and microscopic topological hypersensitivity (from EP). We further experimentally demonstrate that the CPA EP, in turn, acts as a knob to modulate the FSI coupling channel, allowing its full activation (for perfect anomalous deflection) or, by leveraging the missing dimension, complete suppression (for total reflection). This synergy between FSI and non-Hermitian degeneracies shows potential for multifunctional hydroacoustic devices.

We demonstrate that stacking topologically trivial layers, under enforced symmetry restrictions, yields emergent topological phases with protected boundary states. Remarkably, the number of layers itself acts as a topological switch, enabling the system to host topological bound states in the continuum (BICs). Analytically, we reveal that an odd-layer configuration not only renders the spectrum gapless but also supports BICs protected by interlayer reflection symmetry. Combined with entanglement-spectrum calculations, this confirms that odd-layer systems indeed support topological BICs. We provide experimental confirmation of these topological states in stacked acoustic lattices. Our findings establish a previously overlooked pathway to topology and demonstrate a readily applicable strategy for realizing exotic states in a wide range of artificial material systems.

Exceptional points (EPs) offer new routes to control wave-matter interactions in non-Hermitian systems. Whereas conventional EPs arise in Hamiltonians and mark eigenstate coalescence, scattering EPs recast them in input-output responses for free-space wave control. Yet the rich topology of EPs is largely concealed in steady-state observables, exhibiting similar asymmetric patterns governed by identical Jordan-block forms. Here we exploit scattering as an evolving process in space and time to resolve spatiotemporal dynamics. We establish a cross-domain mapping from momentum-frequency to space-time, connecting static EPs to dynamic scattering singularities. By projecting a unitary scattering matrix onto lower-dimensional subspaces, EP topology is imprinted into a wave packet as a spatiotemporal vortex (STV), whose phase singularity is anchored to the EP and carries its quantized topological charge. In a liquid-surface-wave platform, we observe a charge-1 STV and a disintegrated charge-2 STV, generated by distinct EPs in a single device. Our work links fixed exceptional topology to dynamic wave topology, establishing a framework for spatiotemporal wave control.

We demonstrate that stacking topologically trivial layers, under enforced symmetry restrictions, yields emergent topological phases with protected boundary states. Remarkably, the number of layers itself acts as a topological switch, enabling the system to host topological bound states in the continuum (BICs). We analytically show that the spectrum becomes gapless for an odd number of layers; combined with entanglement-spectrum calculations, this confirms that odd-layer systems indeed support topological BICs. We provide experimental confirmation of these phenomena in stacked acoustic lattices. Our findings establish a previously overlooked pathway to topology and demonstrate a readily applicable strategy for realizing exotic states in a wide range of artificial material systems.