Revealing mode formation in quasi-bound states in the continuum metasurfaces via near-field optical microscopy

TL;DR

This study uses near-field optical microscopy to analyze how quasi-bound states in the continuum metasurfaces form at the individual resonator level, revealing size thresholds and the influence of defects and edge states on mode formation.

Contribution

It introduces a new image processing technique combined with s-SNOM to quantitatively investigate mode formation in quasi-BIC metasurfaces at the resonator level, linking near-field and far-field responses.

Findings

Quasi-BIC mode forms at a minimum size of 10x10 units, smaller than far-field predictions.

Coupling direction, defects, and edges significantly influence mode formation.

Insights enable optimization of metasurface active areas for applications like catalysis and biospectroscopy.

Abstract

Photonic metasurfaces offer exceptional control over light at the nanoscale, facilitating applications spanning from biosensing, and nonlinear optics to photocatalysis. Many metasurfaces, especially resonant ones, rely on periodicity for the collective mode to form, which makes them subject to the influences of finite size effects, defects, and edge effects, all of which have considerable negative impact at the application level. These aspects are especially important for quasi-bound state in the continuum (BIC) metasurfaces, for which the collective mode is highly sensitive to perturbations due to high quality factors and strong near-field enhancement. Here, we quantitatively investigate the mode formation in quasi-BIC metasurfaces on the individual resonator level using scattering scanning near-field optical microscopy (s-SNOM) in combination with a new image processing technique. We find that the quasi-BIC mode is formed at a minimum size of 10 x 10-unit cells much smaller than expected from far-field measurements. Furthermore, we show that the coupling direction of the resonators, defects and edge states have pronounced influence on the quasi-BIC mode. This study serves as a link between the far-field and near-field responses of metasurfaces, offering crucial insights for optimizing spatial footprint and active area, holding promise for augmenting applications such as catalysis and biospectroscopy.