Detection of vortex beams with the orbital angular momentum by using plasmonic nanoparticles

Vortex beams with orbital angular momentum (OAM) have many advantages in optical communications, such as high-capacity communication, communication under extreme circumstances, high-security communication, and so on. So far, vortex beams have been measured by using the interference and diffraction properties of them or by converting them back to plane waves. In this paper, we suggest an alternative way to directly detect twisted lights by using optical responses of a gold nanosphere and a gold nanodisk, which have the simplest structures. We simulate and interpret plasmonic resonance modes excited by vortex beams carrying different spin angular momentums (SAMs) and OAMs in a gold nanodisk and a gold nanosphere. Unlike an on-center nanodisk, the excited plasmon modes in an on-center nanosphere do not exactly satisfy the selection rules when OAM increases. The plasmon modes excited by vortex beams are also very sensitive to the location of a nanoparticle in the light field. The information about the SAM and OAM can be obtained from the spectral information and radiation patterns of the plasmon modes excited in on-center nanoparticles and off-center nanoparticles. We can reversely find out the location of the nanoparticles in the vortex beam by measuring the optical spectrum and radiation patterns. Our finding provides a route to develop the optical communication using vortex beams in the future.