Generation of surface plasmons at single subwavelength slits: from slit to ridge plasmon

Understanding how surface plasmons can be launched by single subwavelength structures is crucial for the development of surface plasmon-based devices within highly integrated photonic circuits. In this paper, we study the coupling efficiency of light to surface plasmons for single slits milled in a thin metal film by analyzing the far-field optical images recorded in the Fourier plane of a leakage microscopy setup. A simple model based on a Fano-type interference allows us to give a clear physical interpretation of the plasmonic generation efficiency with an optimum value separating two regimes depending on the size of the system. This is an issue of practical interest in the context of plasmonic features with size close to or smaller than optical diffraction limit.