  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.
