Confining optical modes well below the size determined by the light wavelength at the same frequency is beneficial for the design of compact and efficient optical devices because light concentration impacts the electromagnetic energy density, the ability of light to interact with analytes in sensors, and the nonlinear response of materials in all-optical light modulators. However, such confinement comes at the prize that coupling of propagating light to those modes is made more inefficient because of the mismatch with the light wavelength. Light coupling in and out of confined modes is in fact a major pending problem that limits the practical applicability of the optical confinement strategy in nanophotonics.
Small scatterers are commonly employed to assist light coupling because they allow targeting designated spatial regions in space, matching the area occupied by the confined modes. However, when those scatterers are placed close to the materials supporting the optical modes, the coupling is reduced by losses introduced through the coupling itself, acting as a loss channel. In this work, we have proposed the use of small scatterers placed at a suitable distance from a surface supporting optical modes as a strategy to realize complete optical coupling into such optical modes. By employing lossless, resonant scatterers such as silica particles supporting dipolar Mie resonances, and illuminating the system with focused light, we demonstrate through rigorous theory the possibility of achieving complete optical coupling, provided the angular profile of the incident light is appropriately shaped, and the surface-scatterer distance is fixed to satisfy the so-called critical coupling conditions.
The solution here proposed to solve the in and out optical coupling problem has general applicability in nanophotonics and provides a viable route toward the design of compact optical devices in which suitably engineered optical scatterers are used to funnel light into the surface modes of a planar surface, where they can be used for optical sensing or signal processing, and eventually coupled out into propagating light following the same scheme.