Dongyi Wang

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.

Holography, an optical technology to record and reconstruct a target light field, has attracted intensive attention recently for its numerous practical applications such as imaging, virtual reality, data encryption, and storage.^{1 – 3} Traditional approaches to implement holography require complex optics interference setups, and the generated holograms usually exhibit bulky sizes,^{4 – 6} all being unfavorable for optical integration. Such a limitation is partially addressed after the Gerchberg–Saxton (GS) algorithm was developed to retrieve the optical properties of a hologram by computer without doing realistic interference experiments. However, holographic images generated are typically scalar ones, which exhibit uniform polarization distributions.

Holography is a highly desired technology in modern photonics, yet setups for traditional generating methods suffer from complexity and bulky sizes. While metasurface‐based holography exhibits advantages of compactness and easy‐fabrication, most meta‐holograms realized so far exhibit only single functionality, with a few multifunctional ones suffering from imbalances of efficiency and device‐thickness. Here, we propose a generic approach to design ultra‐thin metasurfaces for realization of multiple holographic images with high efficiencies , and experimentally verify the concept in the telecom regime. We first design a series of high‐efficiency reflective meta‐atoms exhibiting incident‐spin‐delinked reflection phases governed by geometric and resonant mechanisms, and experimentally characterize their optical properties at wavelengths around 1,064 nm. We next experimentally demonstrate a single‐functional meta‐hologram as a benchmark test. Finally, we employ the designed meta‐atoms to construct a metasurface with the thickness ∼1/4 λ , and experimentally demonstrate its capability of generating two distinct holographic images under illuminations of circularly polarized light beams with different helicities, possessing generation efficiencies ∼48.08 %. Our work provides a highly‐efficient and ultra‐compact platform to generate multifunctional holographic images, which may inspire numerous applications in integration optics.

Propagating waves and surface waves are two distinct types of light-transporting modes, the free control of which are both highly desired in integration photonics. However, previously realized devices are bulky in sizes, inefficient, and/or can only achieve one type of light-manipulation functionality with a single device. Here, we propose a generic approach to design bi-channel meta-devices, constructed by carefully selected meta-atoms possessing reflection phases of both structural-resonance and geometric origins, which can exhibit two distinct light-manipulation functionalities in near-field (NF) and far-field (FF) channels, respectively. After characterizing the scattering properties of basic meta-atoms and briefly stating the theoretical strategy, we design/fabricate three different meta-devices and experimentally characterize their bi-channel wave-control functionalities in the telecom regime. Our experiments show that the first two devices can multiplex the generations of NF and FF optical vortices with different topological charges, while the third one exhibits anomalous surface plasmon polariton focusing in the NF and hologram formation in the FF simultaneously. Our results expand the wave-control functionalities of metasurfaces to all wave-transporting channels, which may inspire many exciting applications in integration optics.

Multiplexing multiple yet distinct functionalities in one single device is highly desired for modern integration optics, but conventional devices are usually of bulky sizes and/or low efficiencies. While recently proposed metasurfaces can be ultra-thin and highly efficient, functionalities multiplexed by metadevices so far are typically restricted to two, dictated by the number of independent polarization states of the incident light. Here, we propose a generic approach to design metadevices exhibiting wave-control functionalities far exceeding two, based on coherent wave interferences continuously tuned by varying the incident polarization. After designing a series of building-block metaatoms with optical properties experimentally characterized, we construct two metadevices based on the proposed strategy and experimentally demonstrate their polarization-tuned multifunctionalities at the wavelength of 1550 nm. Specifically, upon continuously modulating the incident polarization along different paths on the Poincare’s sphere, we show that the first device can generate two spatially non-overlapping vortex beams with strengths continuously tuned, while the second device can generate a vectorial vortex beam carrying continuously-tuned polarization distribution and/or orbital angular momentum. Our proposed strategy significantly expands the wave-control functionalities equipped with a single optical device, which may stimulate numerous applications in integration optics.