The optical properties of textured metallic surfaces are governed by the scattering of surface plasmon polaritons (SPPs) and of quasicylindrical waves (CWs), which are both excited by the nano-objects located on the surface. We study here a fundamental scattering process of these fields, namely, the cross conversion of a CW into a SPP. We first show that this inelastic process is inevitable in multi-nanoobject ensembles and then propose a procedure enabling a rigorous calculation of the cross conversion scattering coefficients. Additionally, by mapping this intricate process to a much simpler one, we derive general and simple expressions for the cross conversion efficiency. All predictions are carefully supported by fully vectorial computational results.

A fully vectorial Fourier-modal-method approach combined with the Lorentz reciprocity theorem is used to study the capability of isolated subwavelength objects to launch surface plasmon polaritons on a nearby surface under illumination by light. Various 2-D subwavelength geometries, like grooves or ridges, are considered and compared. The highest efficiencies are achieved for metallic ridges isolated from the surface by a dielectric post. We derive general trends with respect to the geometry, the incidence angle, and the frequency of the incident illumination.We additionally discuss particular effects that provide very high efficiencies or unidirectional launchings. The predictions may be useful for further understanding and engineering plasmonic systems formed by nanoparticle ensembles.

Using a scanning near-field optical microscope, we visualize, in three dimensions, the electromagnetic field distribution near an isolated slit aperture in a thin gold film. At the metal-air interface and for a TM incident polarization, we confirm some recently observed results and show that the slit generates two kinds of surface waves: a slowly decaying surface plasmon polariton and a quasi-cylindrical wave that decreases more rapidly when moving away from the slit. These waves are not generated for a TE incident polarization. In a noncontact mode, we also observe how the transmitted light diverges in free space. At a small distance from the slit 2 m, we find that the emerging light spreads in all directions for TM, forming an electromagnetic cloud, whereas it is concentrated above the slit for TE, forming a more directive light jet. The experimental images are in good agreement with the numerical simulations.