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Ultrathin structures that can bend, focus, or filter light, metasurfaces are reshaping how scientists think about optics. These engineered materials offer precise control over lights behavior, but many conventional designs are held back by inefficiencies. Typically, they rely on local resonances within individual nanostructures, which often leak energy or perform poorly at wide angles. These shortcomings limit their usefulness in areas like sensing, nonlinear optics, and quantum technologies.

A growing area of research looks instead to nonlocal metasurfaces, where interactions between many elements create collective optical effects. These collective behaviors can trap light more efficiently, producing sharper resonances and stronger interactions with matter. One of the most promising possibilities in this field is the development of photonic flatbands, where resonant behavior stays uniform across a wide range of viewing angles.

Another is creating chiral responses, which allow devices to distinguish between left- and right-handed circularly polarized light. Until now, however, achieving both flatband and chiral behavior with high efficiency on a single platform has remained a major challenge.

In new work, scientists from Shandong Normal University and the Australian National University have found a way forward. As in Advanced Photonics, they designed a class of metasurfaces combining principles from coupled-resonator optical waveguides (CROWs) with anisotropic planar structures.

In these designs, arrays of weakly linked optical waveguides—with their symmetry deliberately broken—produce photonic flatbands across wide angles while preserving ultrahigh quality factors. The waveguides' carefully tuned lateral coupling slows light to near zero group velocity, which boosts light–matter interactions and ensures consistent resonance across different incidence angles.

The researchers went a step further by manipulating the in-plane symmetry of their metasurfaces. Through this engineering, they demonstrated unidirectional and bidirectional flatbands that respond to linearly polarized light, as well as chiral flatbands that react only to one "handedness" of circular polarization. These achievements, verified through both simulations and experiments, mark the first demonstration of high-Q flatband and chiral effects coexisting in a single metasurface.

The approach offers a new framework for building multifunctional optical devices. By integrating CROW physics into metasurface design, the team shows how to expand the range of available tools for controlling light at extremely small scales. The work could open new directions in quantum optics, advanced sensing, communications, and compact flat-optics technologies.