Researchers have experimentally demonstrated a multifunctional photonic crystal that combines high spatial degrees of freedom with high-quality-factor resonances, overcoming a longstanding trade-off in flat optics.
Study: Bridging the gap in flat optics: the dawn of partially nonlocal metasurfaces. Image Credit: narong sutinkham/Shutterstock.com
The findings were published in Light: Science & Applications by Lv et al. A later ‘News & Views’ piece in the same journal, written by Wei and Liu, places the work within a broader theoretical framework by introducing the concept of partially nonlocal metasurfaces and explaining how the design bridges local metasurfaces and nonlocal photonic crystals. Together, the studies suggest a new route toward multifunctional flat optical devices for wavefront shaping while preserving high-Q resonances.
Challenges in Flat Optics
Flat optics has revolutionized the manipulation of light at subwavelength scales, enabling precise control over phase, amplitude, and polarization using planar devices. Traditional metasurfaces achieve this through local modulation, where each structural unit independently imparts a specific optical response, resulting in vast spatial design flexibility.
However, such local metasurfaces generally suffer from pronounced radiative losses, leading to low quality (Q) factors limiting resonance sharpness. Conversely, photonic crystals (PhCs) that support nonlocal modes such as bound states in the continuum (BICs) exhibit ultra-high Q factors due to suppressed radiation leakage.
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These BIC modes depend on strict global periodicity, which restricts spatial modulation capability and encoding of complex wavefronts. Historically, these two regimes, local and nonlocal, have been treated as distinct paradigms, with a fundamental trade-off between spatial degrees of freedom (DoFs) for encoding light and the quality of resonance.
Meta-Notch Photonic Design
In the original study, Lv et al.’s team designed and fabricated a novel single-layer photonic crystal device using titanium dioxide (TiO2) nanopillars arranged in a planar array, integrating both local and nonlocal light manipulation mechanisms. A key innovation is the introduction of “meta-notches,” finely tunable local structural perturbations embedded within the photonic crystal pillars.
The physical principle enabling such coexistence is that the BIC mode displays field minima (nodes) precisely at the notch locations, meaning that local variations there impart negligible perturbation to the collective resonance. Topological phase control is employed, leveraging spectral singularities intrinsic to the nonlocal modes, to achieve a full 2π local phase coverage essential for arbitrary wavefront shaping.
The fabrication involved nano-patterning titanium dioxide arrays with spatially varying meta-notch geometries. To characterize the device, the researchers performed optical experiments including holographic imaging to verify real-space spatial modulation capabilities and angle-resolved band structure measurements to probe the momentum-space resonance behavior.
These measurements assess whether the quasi-BIC (qBIC) resonances persist despite breaking strict lattice periodicity due to local variations.
Wei and Liu’s subsequent analysis develops a conceptual framework based on information theory, defining “range of nonlocality” (RoN) as the spatial coherence length of dielectric profile coupling and mapping it against the DoF in spatial information encoding.
This framework contextualizes the partially nonlocal regime between the extremes of purely local metasurfaces (high DoF, low RoN) and perfectly periodic photonic crystals (low DoF, high RoN).
Partial Nonlocality and Encoding
Experimental results validate the successful integration of local wavefront shaping with high-Q resonances on a single planar platform. The fabricated TiO2 photonic crystals with embedded meta-notches exhibit wavelength-selective and high-contrast holographic projections, confirming efficient local phase control in the spatial domain.
For example, holograms of genus 1 and genus 2 patterns were demonstrated with high fidelity, signifying arbitrary wavefront encoding capability traditionally achievable only with local metasurfaces.
Simultaneously, angle-resolved reflectivity measurements reveal that the nonlocal BIC resonance near the Γ point in momentum space is preserved with an exceptionally narrow linewidth, demonstrating the retention of a near-perfect quasi-BIC despite induced spatial perturbations.
This indicates that breaking strict periodicity via locally varying structural units does not destroy the collective resonance. Wei and Liu interpret these results as evidence that the device supports a qBIC mode with a finite but large Q-factor, whose lateral coherence length remains significantly larger than the wavelength.
Wei and Liu’s framework suggests that the meta-notch perturbations fall within a tolerance window controlled by the spatial coherence of the qBIC mode. By placing meta-notches at node positions of the BIC field, local modulation can be introduced with minimal disturbance to the global mode.
From a fundamental physics perspective, Wei and Liu clarify that there is no perfect superposition of independent local and nonlocal mechanisms. Rather, the partially nonlocal regime arises from a relaxation of strict periodicity that balances spatial encoding and resonance quality.
The degree of nonlocality acts as a continuous parameter linking these regimes: qBICs naturally inhabit this intermediate state, where spatial coherence is strong but not infinite, allowing partial independence of unit cells.
Expanding the Flat Optics Paradigm
Building on Lv et al.’s experimental results, Wei and Liu argue that the work bridges the longstanding trade-off in flat optics between local spatial control and high-Q resonances by experimentally realizing partially nonlocal metasurfaces. Lv et al.’s innovative use of embedded meta-notches within photonic crystal nanopillars unlocks local tunability of optical phase without significantly perturbing nonlocal quasi-BIC resonances.
This integration expands the design’s DoF by relaxing strict periodicity, therefore recovering spatial encoding capacity previously locked by photonic crystal uniformity.
The demonstrated coexistence of high-contrast holographic imaging with stable high-Q resonances paves the way for sophisticated applications in advanced imaging, optical communications, and analog computing.
Wei and Liu argue that the initial experimental findings open a fertile frontier in flat optics, inviting further research into partially nonlocal metasurface design and their deployment in next-generation optical technologies.
Journal Reference
Lv et al., Local-nonlocal assisted multifunctional photonic crystals. Light: Science & Applications. 15(243). https://www.nature.com/articles/s41377-026-02308-3.
Wei J., Liu Y. (2026). Bridging the gap in flat optics: the dawn of partially nonlocal metasurfaces. Light: Science & Applications, News & Views. 15(314). https://www.nature.com/articles/s41377-026-02397-0.