Generation of Orbital Angular Momentum with Archimedean Spirals

Electromagnetic vortex fields that carry orbital angular momentum (OAM) have been a topic of interest in a vast range of applications [1]. In this work, we investigate the generation mechanism of OAM by studying the interaction of circularly-polarized plane waves with plasmonic Archimedean spirals, making use of numerical simulations in real space and real time. By having access to the full spatio-temporal information, we are able to observe the first stages of the birth of an optical vortex upon interaction with matter. As a plane wave pulse interacts with the spiral, the induced OAM density propagates in different spatial directions. We model spirals with two different material descriptions, namely as a dielectric medium and with a frequency-dependent Drude model. While the first description demonstrates purely geometrical effects of matter on light, the second allows us to account for back emission from matter and its effect on the incoming light. The calculated OAM density in real space can be contrasted with the quantized OAM. We introduce a measure for the efficiency of OAM generation and use it to compare multiple systems and conditions. We use the Maxwell solver in the Octopus code[2], which employs the Riemann-Silberstein representation to propagate Maxwell’s equations in real time, and allows for full coupling to the quantum dynamics of matter systems, described by TDDFT.

[1] Padgett, M., Bowman, R. Nature Photon 5 (2011).

[2] N. Tancogne-Dejean et al., J. Chem. Physics 152 (2020).