Shaojie Ma

Flat-band states in Hermitian systems exhibit compact localization in the bulk due to geometric or symmetry constraints, while non-Hermitian skin effects drive state accumulation at boundaries via asymmetric lattice hoppings. Despite extensive efforts devoted to studying these two effects separately, the interplay between these competing mechanisms remains largely unexplored. Here we reveal a tuneable interplay between them by introducing non-reciprocal hopping in low-dimensional lattice hosting isolated, symmetry-protected flat bands. We demonstrate that closure of the real or imaginary line gap between the flat band and dispersive bands induces exceptional skin effects (ESEs) - boundary localization of flat-band states despite their vanishing group velocity and intrinsic immunity to conventional skin effect. This phenomenon arises from exceptional points formed under periodic boundaries, where the flat band becomes enclosed by the spectral loop of dispersive states, enabling wavefunction mixing and skin accumulation. Supported by generalized Brillouin zone analysis, perturbation theory, and electrical circuit experiments, our findings establish band gaps as a primary tuning parameter for non-Hermitian localization, distinct from system size or reciprocity strength. These findings unveil a new class of gap-engineered non-Hermitian flat-band phenomena with implications for reconfigurable wave localization in photonic, acoustic, and electronic platforms.

Optical waveguides, typically composed of core and cladding layers, only utilize core layers to transmit signals via the total‐internal‐reflection mechanism, with cladding layers completely wasted. While waveguides consisting of photonic‐crystal (PhC) layers can enable signal transmission in both layers through equal‐frequency‐surface (EFS) engineering, such PhC‐based waveguides only support transverse‐magnetic modes, and the interfaces between adjacent layers are still not fully exploited. Here, we propose a novel metamaterial (MTM)‐based waveguide to allow transmission of waves with two independent polarizations simultaneously, not only in all bulk layers but also at the interfaces between two types of layers, thus boosting the waveguide capacity significantly. Via carefully tuning constitutional parameters, we design a hyperbolic MTM which, for both polarizations, possesses EFSs fully isolated from those of air and exhibits topologically protected boundary states at the interfaces. We experimentally realize a waveguide by combining such hyperbolic MTM layers and air layers and confirm the existence of three types of independent waveguide modes via microwave near‐field measurements. Our work pushes the capacity limits of cladding‐free waveguide architectures and lays the foundation for cladding‐free integrated photonic devices using natural hyperbolic materials.

On-chip photonic systems capable of efficiently generating pre-designed vectorial optical fields (VOFs) are highly desired in integrated photonics, but traditional devices are bulky and lack flexible control capabilities. Although ultra-compact metasurfaces (MSs) have exhibited powerful light-manipulation capabilities, they typically work under propagating-wave excitations and/or rely on only phase modulations to control light beams. Here, a general strategy is proposed to design MSs that, under surface wave (SW) excitations, can independently control the amplitudes and phases of locally scattered waves with two orthogonal polarizations, thus enabling efficient generation of pre-designed VOFs. As a benchmark test, a complex amplitude MS was constructed to experimentally demonstrate the generation of two directional beams exhibiting orthogonal polarizations and arbitrarily pre-designed intensities, under excitation of SW at 0.4 THz. Next, another MS was experimentally demonstrated to generate two focal points in the far field with distinct intensities, under the same THz SW excitation. Finally, based on a modified Gerchberg-Saxton (GS) algorithm incorporating both amplitude and phase modulations, a series of THz SW-excited complex amplitude MSs were designed and fabricated, and were experimentally demonstrated to respectively generate pre-designed scalar and vectorial holographic images in the far field, exhibiting much improved qualities and flexibility than those generated by their phase-only counterparts. The present work establishes a novel on-chip platform to generate complex vectorial fields, paving the way for many applications in integrated optics such as encrypted holography, and augmented reality.

Topolectrical circuits provide a versatile platform for exploring modern physical models, yet existing approaches lack full programmability and effective mechanisms for inverse state design. Here, we present a deep-learning-empowered programmable topolectrical circuit platform for physical modeling and analysis. Our system integrating fully independent, continuous tuning of on-site and off-site Hamiltonian terms, physics-graph-informed inverse state design, and immediate hardware verification, thereby bridges theoretical modeling and practical realization. Through flexible control and adiabatic path engineering, we experimentally observe boundary states in higher-order topological systems without global symmetry, the associated adiabatic phase transitions, and flat-band characteristics corresponding to Landau levels. Incorporating a physics-graph-informed generative model, we achieve arbitrary, position-controllable Anderson localization surpassing conventional random approaches. Leveraging this capability, we demonstrate physics-mechanism-driven probabilistic information encryption and product anti-counterfeiting. Our work establishes a paradigm where deep learning and programmable hardware synergistically enable on-demand inverse design, bridging physics and information technologies.

Converting surface waves to unidirectional far‐field beams is crucial for information readout in on‐chip photonic applications, but conventional grating‐like couplers suffer from low‐efficiency issue due to multi‐mode generation. Although recently proposed metasurfaces have addressed this challenge, meta‐couplers realized so far exhibit inevitable frequency dispersions, restricting their practical applications. Here, we propose a scheme to design on‐chip achromatic metasurfaces that can decouple SWs within a continuous frequency band to far‐field beams traveling along the same direction. Our metadevices, composed of composite meta‐atoms exhibiting pre‐designed reflection phases with tailored frequency dispersion dictated by both resonant and Pancharatnam–Berry phase mechanisms, can compensate the desired frequency‐dependent wavevectors for SWs at different frequencies, thereby generating unidirectional far‐field beams. As a proof of concept, we design and fabricate two on‐chip achromatic metasurfaces working in the microwave regime and experimentally demonstrate that they can achieve in‐plane and out‐of‐plane unidirectional far‐field decoupling of SWs, exhibiting relative frequency bandwidths of 22.22% and 37.04%, respectively. Both microwave experiments and full‐wave simulations successfully confirm our theoretical predictions. These findings pave the way for on‐chip photonic manipulations and related applications, including wireless communication, signal processing, high‐sensitivity sensing, and so on.

Charged particles in a magnetic field undergo circular motion and result in quantized energy levels known as Landau levels (LLs). LLs around band singularites are particularly interesting due to their nontrivial topological properties. To date, through the introduction of the artificial magnetic field, LLs have been observed in systems only with closed equifrequency contours (EFCs) around band singularities, i.e. type-I Dirac or Weyl systems. Here, we demonstrate the existence of LLs with open EFCs around band singularities through combined action of artificial electric (E) and magnetic (B) fields, whereas they cannot be generated by B only. We construct an artificial platform of inhomogeneous metasurfaces of the two-dimensional type-II Dirac system with open EFCs, create suitable artificial gauge fields of E&B, and observe the emergence of E&B-induced LLs experimentally. This work presents a new insight for realizing LLs, and provides a platform for controlling optical waves propagating using artificial gauge fields.

Weyl orbits are topologically protected cyclotron trajectories that connect Weyl nodes of opposite chiralities in momentum space, coupling bulk chiral Landau levels and surface Fermi arcs to produce quantum oscillations under static magnetic fields. While well-studied in condensed matter systems, their photonic analogs remain elusive due to the difficulty in generating global pseudomagnetic fields. Here, we overcome these limitations by engineering a pseudomagnetic field domain wall, enabling a novel photonic cyclic orbit (PCO) bound to a single Weyl point, unlike conventional Weyl orbits that link paired WPs. In the PCO, photons traversing the domain wall experience pseudomagnetic field reversal and flipping of momentum-space motion along Fermi arcs, forming a closed trajectory through a single Weyl point. Experimentally, we observe a photonic analog of quantum oscillations, manifested as discrete photonic resonances, and PCO states in the real space, comprising Fermi-arc surface states and chiral zero modes, providing definitive evidence of photonic Weyl orbits. Our Letter establishes a new paradigm for topological cyclic orbits and opens practical pathways for three-dimensional light manipulation.

Fermi‐arc metals, unconventional semi‐metals featuring cylindrical Fermi surfaces formed by Fermi arcs, have recently attracted extensive attention for realizing a novel metallic phase that retains chiral anomaly responses yet suppresses quantum oscillations. Although it was proposed that spatially twisting a superlattice of thin Weyl metals can form a Fermi‐arc metal, previous local‐approximation analyses are valid only for slowly varying systems and cannot capture all rich physics in such systems. Here, we report an optical realization of such a phase in a natural magnetized plasma subjected to a helically modulated magnetic field. Unlike previous studies on artificial heterostructure platforms, we admit a fully analytical, nonperturbative treatment that tracks a complete evolution of Weyl node. In the slowly varying regime, our platform faithfully realizes and manipulates the Fermi‐arc metal state in a real system. As the modulation rate increases, Fermi‐arcs inheriting opposite chiralities start to hybridize. Remarkably, in the deep nonperturbative limit, chirality vanishes, not through the conventional Weyl point annihilations, but via Fermi arc recombination, resulting in a chirality‐free uniaxial optical medium. These findings unveil global topological transitions in nonuniform Weyl systems and open routes toward photonic devices based on engineered Fermi‐arc dynamics.

Dirac points, arising from intrinsic space-group symmetry in hexagonal lattices, have been extensively explored in graphene and photonic crystals. However, their characteristic scales, confined either to atomic level (∼Å) or limited by the operational wavelength (∼λ), pose fundamental challenges for precisely engineering their topological properties and thus freely controlling the wave-transport behaviors in such systems. Here, beyond the conventional lattice-dependent paradigm, we demonstrate a type-I photonic Dirac point in the surface modes of an electromagnetic metasurface. This realization, governed by the intrinsic properties of constitutional units rather than the global crystalline symmetry, enables us to control Dirac physics in deep-subwavelength scales. Owing to the vanishing density of states at the type-I Dirac degeneracy, by strategically integrating inhomogeneous local symmetry breaking as an artificial gauge field, we experimentally observe supersymmetric Landau levels and chiral zero modes with deep-subwavelength precision in the microwave regime. Such a platform offers a branch-new approach to freely control surface waves through precisely engineering the spatial distribution of the gauge field imposed, with bending, focusing, and spreading of surface waves experimentally demonstrated. By harnessing the mode-tuning flexibility of metasurfaces, our Letter establishes a versatile platform for topology-driven optical manipulation.

Periodic non-reciprocal systems have attracted significant attention for their striking non-Hermitian skin effect (NHSE) and the development of a comprehensive non-Bloch theory. In contrast, aperiodic non-reciprocal systems have been rarely explored, owing to the intrinsic complexity induced by spatial non-uniformity and absence of predictable physical behavior. Here, we establish a theoretical framework to analyze aperiodic non-reciprocal systems and reveal that the spatially varying non-reciprocal coupling is a powerful and deterministic knob to control the wave-functions of eigenmodes. By tailoring the non-reciprocity distribution, we combine theory and proof-of-concept electric-circuit experiments to demonstrate that the eigenmodes of a non-Hermitian system can be reshaped into arbitrary spatial profiles without altering their spectral distribution, a phenomenon termed as the non-Hermitian reshaping effect (NHRE). In the special case of a uniform non-reciprocity distribution, NHRE reduces to the conventional NHSE. This concept can be generalized to arbitrary higher-dimensional systems and to high-frequency regimes, offering an efficient route to mode modulation and deepening our understanding of non-Hermitian physics.