Realization of tunable spectral shift and linewidth reduction of Fano resonances at a dielectric metasurface due to near-field interactions among multipole moments

Abstract: Fano resonance is a well-known phenomenon in fields like atomic physics and grating optics, and it has recently started to gain interest in artificially engineered dielectric, metallic or composite metasurfaces, and metamaterials, to name a few. This work elucidates the role of the induced multipole moments within a dielectric metasurface that leads to the interference, and the resulting scattering response in the far-field emerges into Fano resonance. Here, we report the details of the interference between fields generated by induced multipole moments and their effect on the scattered response when a maneuvered change in the excitation source is applied from the far-field plane wave excitation to a dipole emitter interacting in the near-field of the metasurface. This change in excitation leads to observed effects like resonance red-shift and linewidth narrowing for the out-coupled scattering. Starting from Maxwell's equations, we follow Feshbach formalism for electromagnetic scattering applied to the all-dielectric nanostructure used in this work, which leads us to satisfactory qualitative explanations along with the relevant results obtained from numerical calculations to support the observations we made experimentally. This work could help to 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.