Vortex beams carrying orbital angular momentum (OAM) have attracted growing attention across fields, including optics and acoustics, for potential applications in particle manipulation and high-speed communication. Intracavity generation of OAM beams, such as OAM lasers, efficiently produces high-power, high-beam-quality vortices. This scheme, however, remains rarely explored in acoustics. Here, we propose and demonstrate an acoustic intracavity OAM generation mechanism with tunable topological charges via a single nonreciprocal nonlinear boundary in a compact resonator ring. In the linear regime, the boundary creates non-Hermitian complex effective magnetic fields piercing the ring, leading to a non-Hermitian Zeeman-like effect that splits clockwise and counterclockwise eigenmodes. Upon incorporation of nonlinearity to the boundary, all resonators are mutually locked, producing a single-mode self-oscillatory OAM radiation exhibiting hysteresis and bistability. Moreover, the topological charge is tunable by manipulating the boundary. Our work reveals intriguing physics related to nonlinear, non-Hermitian boundaries and offers potentials in the next generation of acoustic self-oscillatory OAM sources, switchers, and memory devices.

Controlling electromagnetic (EM) waves at will is fundamentally important for diverse applications, ranging from optical microcavities, super-resolution imaging, to quantum information processing. Decades ago, the forays into metamaterials and transformation optics have ignited unprecedented interest to create an invisibility cloak-a closed space with any object inside invisible. However, all features of the scattering waves become stochastic and uncontrollable when EM waves interact with an open and disordered environment, making an open invisible space almost impossible. Counterintuitively, here we for the first time present an open, cluttered, and dynamic but invisible space, wherein any freely-moving object maintains invisible. To adapt to the disordered environment, we randomly organize a swarm of reconfigurable metasurfaces, and master them by MetaSeeker, a population-based reinforcement learning (RL). MetaSeeker constructs a narcissistic internal world to mirror the stochastic physical world, capable of autonomous preferment, evolution, and adaptation. In the perception-decision-execution experiment, multiple RL agents automatically interact with the ever-changing environments and integrate a post-hoc explainability to visualize the decision-making process. The hidden objects, such as vehicle cluster and experimenter, can freely scale, race, and track in the invisible space, with the environmental similarity of 99.5%. Our results constitute a monumental stride to reshape the evolutionary landscape of metasurfaces from individual to swarm intelligence and usher in the remote management of entire EM space. © 2025. The Author(s).

Non-Hermitian physics has unveiled a realm of exotic phenomena absent in Hermitian systems, with the non-Hermitian skin effect (NHSE) showcasing boundary-localized eigenstates driven by non-reciprocal interactions. Here, we introduce a new class of non-Hermitian systems exhibiting pure decay modes- eigenstates with pure, smooth exponential decay, devoid of the oscillatory wave patterns typical of traditional NHSE. Modeled as directed graphs with nonreciprocal hopping, these systems reveal quantized decay charges, defined as the sum of decay constants along edges at each node, offering a novel topological invariant. We derive universal conditions for these modes, enabling versatile configurations from one-dimensional rings, directed graphs with complicated connectivity, to higher-dimensional lattices. Experimental validation using microwave resonant circuits confirms the predicted pure decay profiles. This discovery paves the way for potential applications in photonics, signal processing, and beyond, harnessing the unique topological properties of non-Hermitian networks.

Topological valley photonics, which exploits valley degrees of freedom to manipulate electromagnetic waves, offers a practical and effective pathway for various classical and quantum photonic applications across the entire spectrum. Current valley photonics, however, has been limited to two dimensions, which typically suffer from out-of-plane losses and can only manipulate the flow of light in planar geometries. Here, we have theoretically and experimentally developed a framework of three-dimensional (3D) topological valley photonics with a complete photonic band gap and vectorial valley contrasting physics. Unlike the two-dimensional counterparts with a pair of valleys characterized by scalar valley Chern numbers, the 3D valley systems exhibit triple pairs of valleys characterized by valley Chern vectors, enabling the creation of vectorial bulk valley vortices and canalized chiral valley surface states. Notably, the valley Chern vectors and the circulating propagation direction of the valley surface states are intrinsically governed by the right-hand-thumb rule. Our findings reveal the vectorial nature of the 3D valley states and highlight their potential applications in 3D waveguiding, directional radiation, and imaging.

Superscattering offers an enticing route to significantly enhance the scattering cross section of subwavelength scatterers, far exceeding the single-channel scattering limit. It is of paramount importance to many applications, such as optical sensing, antennas, and imaging. However, the superscattering phenomenon is generally accompanied with nonzero forward-scattered light. Here, we find the possibility of creating the exotic phenomenon of superscattering with zero forward-scattered light by exploiting the optical gain. The underlying mechanism is that the optical gain can facilitate the realization of not only the single-channel superscattering but also the second Kerker effect, which is intrinsically related to the zero forward scattering phenomenon. Because of the importance of the second Kerker effect in regulating the directionality of superscattering, this revealed phenomenon is termed as Kerker-superscattering. © 2025 Optica Publishing Group. All rights are reserved.

In topological physics, it is commonly understood that the existence of the boundary states of a topological system is inherently dictated by its bulk. A classic example is that the surface Fermi-arc states of a Weyl system are determined by the chiral charges of Weyl points within the bulk. Contrasting with this established perspective, here, we theoretically and experimentally discover a family of topological chiral bulk states extending over photonic Weyl metamaterial waveguides, solely induced by the waveguide boundaries, independently of the waveguide width. Notably, these chiral bulk states showcase discrete momenta and function as tunnels that connect Fermi-arc surface states living in different two-dimensional spaces via a third dimension. Our work offers an alternative mechanism for robust chiral bulk transport of waves and highlights the boundaries as a new degree of freedom to regulate bulk Weyl quasiparticles.

The emerging field of free-electron quantum optics enables electron-photon entanglement and holds the potential for generating nontrivial photon states for quantum information processing. Although recent experimental studies have entered the quantum regime, rapid theoretical developments predict that qualitatively unique phenomena only emerge beyond a certain interaction strength. It is thus pertinent to identify the maximal electron-photon interaction strength and the materials, geometries, and particle energies that enable one to approach it. We derive an upper limit to the quantum vacuum interaction strength between free electrons and single-mode photons, which illuminates the conditions for the strongest interaction. Crucially, we obtain an explicit energy selection recipe for electrons and photons to achieve maximal interaction at arbitrary separations and identify two optimal regimes favoring either fast or slow electrons over those with intermediate velocities. We validate the limit by analytical and numerical calculations on canonical geometries and provide near-optimal designs indicating the feasibility of strong quantum interactions. Our findings offer fundamental intuition for maximizing the quantum interaction between free electrons and photons and provide practical design rules for future experiments on electron-photon and electron-mediated photon-photon entanglement. They should also enable the evaluation of key metrics for applications such as the maximum power of free-electron radiation sources and the maximum acceleration gradient of dielectric laser accelerators. © 2025 American Physical Society.

Janus dipoles have exotic features in near-field coupling since they have two distinct faces, namely, the so-called coupling and noncoupling faces. When the out-coupler (e.g., a waveguide) is oriented toward the noncoupling (coupling) face of Janus dipoles, the guided waves can be inefficiently (efficiently) excited. Moreover, the noncoupling face of Janus dipoles can be engineered on demand to fully suppress the excitation of guided waves with either s polarization or p polarization, giving rise to the single-polarization perfect Janus dipole. However, whether it is possible to achieve a dual-polarization perfect Janus dipole in near-field coupling remains elusive. Here we theoretically reveal the possibility to realize a dual-polarization perfect Janus dipole, which requires the judicious design of both the constituent dipole moments of Janus dipoles and the dispersion relation of waveguides. When placing the dual-polarization perfect Janus dipole in the middle of two parallel identical waveguides, we theoretically find that the p-polarized guided waves are exclusively excited in one waveguide, while the s-polarized guided waves are excited solely in the other waveguide. Our findings might provide an enticing route for the development of polarization-multiplexed photonic devices, quantum routing, and on-chip signal processing. ©2025 American Physical Society

Motion underpins various exotic phenomena in nature, such as the Doppler effect, and enables numerous practical applications. In this study, we reveal a counterintuitive phenomenon that p-polarized surface plasmons cannot be excited for all directions and Doppler frequencies, when a complex dipolar source moves parallel and close to the planar interface that supports the propagation of surface plasmons under a newfound critical suppression condition. The underlying mechanism is that the motion provides an extra degree of freedom to reshape the near-field interference of dipolar sources themselves and could be exploited to fully suppress p-polarized waves carried by the judiciously designed dipolar source at all Doppler frequencies. To be specific, this critical polarization suppression arises when the dipolar source acts as a moving Huygens dipole, with its electric and magnetic dipole moments being in-phase and mutually perpendicular. Moreover, this Huygens-like dipole must move with a critical velocity, which is determined solely by the ratio of the source’s electric to magnetic dipole moment, but irrelevant to surface plasmons’ phase velocity. © 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.