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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.