Nanocavities inscribed in a gold thin film are optimized and designed to form a metasurface. We demonstrate both numerically and experimentally the creation of surface plasmon (SP) vortex carrying orbital angular momentum in the metasurface under linearly polarized optical excitation that carries no optical angular momentum. Moreover, depending on the orientation of the exciting linearly polarized light, we show that the metasurface is capable of providing dynamic switching between SP vortex formation or SP subwavelength focusing. The resulting SP intensities are experimentally measured using a near-field scanning optical microscope and are found in excellent quantitative agreements as compared to the numerical results.

Surface plasmon (SP) vortices typically have annular intensity patterns. Here we show that fractional SP vortices with arbitrary, asymmetric intensity patterns can be synthesized via simple geometric arrangements. We provide the direct experimental observations over the existence of fractional SP vortices. Triangular SP vortex intensities are synthesized by circularly polarized plane waves. On the other hand, subwavelength focusing with user-desired patterns is also experimentally demonstrated. The synthesized SP vortex and focusing patterns are experimentally measured using a near-field scanning optical microscope with uncoated fiber probe and are found in good agreements as compared to numerical results.

Nanocavities inscribed in a gold thin film is optimized and arranged to form a metasurface. We demonstrate both numerically and experimentally that surface plasmon vortex carrying orbital angular momentum can be generated under linearly-polarized optical excitation.

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