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A new design principle uses symmetry and Brillouin-zone folding to control several optical resonances in one nanostructure. In experiments, researchers tuned measured Q factors from about 300 to 20,000 and resonance splitting from 0 to 70 nanometres.
Metasurfaces are flat optical structures built from elements smaller than the wavelength of the light they manipulate. By shaping and arranging these elements, researchers can control how light is reflected, transmitted or concentrated.
One important property is how long an optical resonance can retain energy before it leaks away. This is described by the Q factor: the higher the Q, the sharper and longer-lived the resonance. Such resonances can strengthen interactions between light and matter, making them interesting for sensors, filters and other photonic components.
In a peer-reviewed study in Nature Communications, researchers now present a framework for creating several such resonances in the same metasurface and controlling them through the structure's symmetry.
The researchers worked with tetramer metasurfaces, in which four structural elements form a repeating unit. The starting point is a set of guided light modes that normally sit below the so-called light cone and therefore do not couple directly to radiation outside the structure.
Using Brillouin-zone folding, these modes can be moved to other points in the structure's effective band structure. The researchers can then choose which symmetries are preserved and which are broken. In this way, the modes can become guided resonances or so-called bound states in the continuum, optical states that under idealized conditions can be protected from radiative loss.
The aim is not simply to create one sharp resonance. Symmetry provides a systematic handle on several properties at once, including how degenerate modes split, how their electromagnetic fields are distributed and how the Q factor changes as the structure becomes more asymmetric.
The researchers fabricated metasurfaces and experimentally measured two types of high-Q resonances. By changing structural asymmetry, they tuned the separation between resonances from 0 to 70 nanometres. At the same time, measured Q factors were adjusted from roughly 300 to 20,000.
The study also shows that a rectangular lattice geometry provides another way to tune both resonance splitting and Q factor. This offers more design parameters than treating every desired resonance as a separate optical element.
A Q factor of 20,000 is the upper experimentally reported value in these measurements. The study discusses ultrahigh-Q resonances and bound states in the continuum as part of the design framework, but the result should not be interpreted as demonstrating a lossless physical component or an arbitrarily high measured Q factor.
The work is primarily a new principle for organizing resonances. Instead of treating each desired optical response in isolation, the researchers use the symmetry of the overall superlattice to determine which modes become accessible and how they behave.
That could be useful in systems that need several narrow optical responses in the same small area, such as multichannel sensors or spectral components. High Q can also strengthen light–matter interactions, potentially making small environmental changes easier to detect.
But the study demonstrates a research system, not a finished product. Practical applications will depend on factors including fabrication tolerances, losses, stability and how well the desired resonances can be maintained when the design is integrated into larger photonic systems.
The result nevertheless shows that symmetry can serve as more than a geometric description. It can be used as an active design tool for placing and shaping multiple optical resonances within the same nanostructure.
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