Abstract

In this theoretical work, we investigate the mechanisms, and efficiency of orbital angular momentum (OAM) generation in electromagnetic fields, i.e. formation of optical vortices. We employ the real-space and real-time code Octopus to perform our numerical simulations [1]. We employ computer-aided design (CAD) software to design various gold nanoplasmonic structures, represent them with a Drude model and excite them with circularly polarized pulses in our simulations. Our designed geometries include springs, Archimedean spirals and carved slabs, which are then compared in terms of their OAM generation potential. Thanks to the multisystem treatment of the Octopus code, it is possible to capture the electronic currents formed by the structure as well as the shaping of the pulse upon interaction. We characterize the resultant orbital angular momentum content of the scattered waves, and verify our findings by emulating the current response of the medium by prescribing current sources in further simulations. Finally, we place classical charges into the simulation box and probe their interaction with the pulse that is shaped by the medium. We characterize this interaction in terms of orbital angular momentum transfer from incoming light to the charged particles. A further objective of this work is to replace the current Drude model treatment with matter simulations using time-dependent density-functional theory. This will pave the way for ab-initio spectroscopy simulations with optical vortices, which is expected to expand our scope beyond the ubiquitous dipole approximation.

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