Toward complete coupling of light to strongly confined plasmons

Efficient optical coupling between propagating light and confined surface polaritons is crucial for nanophotonic device design. However, coupling efficiency diminishes significantly with increased mode confinement due to wavelength mismatch. Despite various proposed mechanisms, achieving flexible and efficient light-to-polariton coupling remains challenging. Our experimental demonstration showcases efficient light-to-surface-plasmon polariton coupling with engineered dipolar scatterers placed optimally from the surface. Gold disks separated by silica spacers from a planar gold surface exhibit perfect coupling conditions, achieved by tuning spacer thickness for a given scatterer geometry resonating at a specific optical frequency. We measure maximum coupling cross section close to the square of light wavelength at optimal distance, facilitated by strong particle-surface interaction and minimal surface-driven particle-dipole quenching, favored at small separations. Our findings, supported by analytical theory and electromagnetic simulations, advocate distant engineered scatterers as a disruptive solution for in/out-coupling challenges in nanophotonics.