Origin of the Fano interference and its tunability with near-field interactions in a guided mode-resonant metasurface

Asymmetric resonances emerging from the Fano interference are a well-known phenomenon in fields like atomic physics and grating optics, and they have recently started to gain interest in artificially engineered dielectric, metallic, or composite metasurfaces and metamaterials. The guided mode-resonant metasurface belongs to this class with grating-waveguide responses and shows asymmetric resonances. Here, we have theoretically studied the origin of the resonance, finding out the root of the Fano interference. We have followed the<i>ab initio</i>theory derived from the Feshbach formalism for the electromagnetic scattering. We have numerically simulated the metasurface to obtain different field parameters required for the<i>ab initio</i>theory; in this regard, we have used the multipole decomposition of the scattering fields for the induced moments. Motivated by our recent experiments, we have used a planewave and polarized dipole sources to excite the metasurface, and studied subsequent effects, like the resonance redshift and the resonance linewidth narrowing for the metasurface. These happened due to the change of the excitation, and we have numerically quantified them. The change of the excitation source bears the novelty of the work. Thus, it helps comprehend the experimental observations qualitatively. This work could help explain and explore possibilities to observe asymmetric resonances in metasurfaces for various excitation conditions, and the key results of resonance shift and linewidth narrowing could be significant for precision applications and fundamental optical studies.