Revealing Mode Formation in Quasi-Bound States in the Continuum Metasurfaces via Near-Field Optical Microscopy

Area of Science:

• Nanophotonics and Metasurface Engineering

Background:

• Photonic metasurfaces enable nanoscale light control for diverse applications.
• Resonant metasurfaces require periodicity, but are susceptible to size, defect, and edge effects.
• Quasi-bound states in the continuum (BIC) metasurfaces are highly sensitive to perturbations.

Purpose of the Study:

• To quantitatively investigate quasi-BIC mode formation at the individual resonator level.
• To understand the impact of finite size, defects, and edge effects on quasi-BIC modes.
• To bridge the understanding between far-field and near-field responses of metasurfaces.

Main Methods:

• Utilized scattering scanning near-field optical microscopy (s-SNOM).
• Employed a novel image processing technique for quantitative analysis.
• Investigated mode formation on individual resonator scales.

Main Results:

• Quasi-BIC mode formation was observed at a minimum size of 10x10 unit cells, smaller than predicted by far-field measurements.
• Resonator coupling direction, defects, and edge states significantly influence the quasi-BIC mode.
• Established a link between far-field and near-field optical responses.

Conclusions:

• Provides crucial insights for optimizing metasurface spatial footprint and active area.
• Offers a pathway to enhance applications like catalysis and biospectroscopy.
• Highlights the importance of near-field analysis for understanding quasi-BIC metasurfaces.