Highly efficient singular surface plasmon generation by achiral apertures

We report a highly efficient generation of singular surface plasmon (SP) field by an achiral plasmonic structure consisting of $\Lambda$-shaped apertures. Our quantitative analysis based on leakage radiation microscopy (LRM) demonstrates that the induced spin-orbit coupling can be tuned by adjusting the apex angle of the $\Lambda$-shaped aperture. Specifically, the array of $\Lambda$-shaped apertures with the apex angle $60^\circ$ is shown to give rise to the directional coupling efficiency. The ring of $\Lambda$-shaped apertures with the apex angle $60^\circ$ realized to generate the maximum extinction ratio (ER=11) for the SP singularities between two different polarization states. This result provides a more efficient way for developing SP focusing and SP vortex in the field of nanophotonics such as optical tweezers.