Electromagnetic field dark spots

Abstract

Due to destructive interference electromagnetic fields contain minima, counterpart to bright maxima. But because light is a vector wave it is rare that all components of a field, for instance the electric field, are completely eliminated in a wave superposition. An entity satisfying E = 0 we call a dark spot and it can only be realised artificially by control of coherent monochromatic interfering fields, yet doing so embeds light with deeply rich topological structures beyond simple scalar dislocation lines. We study the imprints of vector dark spots in this thesis, including two-dimensional (2D) paraxial dark spots, three-dimensional (3D), point-like non-paraxial dark spots and propose a simple technique for their synthesis and position control. More specifically, paraxial dark spots are shown to always carry non-diverging polarisation structures infinitely into the far field (contrary to the universal phenomena of diffraction), point-like dark spots are reported to develop complex, possibly vortex-like flows of energy and momentum as well as one of six polarisation skeletons. Dark spots like these could be powerful experimental tools for topological control, atom traps and sub-wavelength optical microscopy. In the final chapter we present a decomposed representation of light's spin angular momentum density akin to the spin-orbit decomposition of the Poynting vector, a sum of two terms, the canonical spin and Poynting spin, whose physical meanings we lay out. The two terms relate to the difference in canonical and spin momenta carried by left- and right-handed photons and we apply the decomposition to a range of electromagnetic fields including a linearly polarised vortex beam. Unifying this thesis we predict that at the centre of the vortex beam, in its dark spot, a longitudinal chiral pressure force exists and presents a way for the beam's orbital angular momentum to couple to matter in a chiral interaction.

Date of Award	1 Jul 2024
Original language	English
Awarding Institution	King's College London
Supervisor	Francisco Rodriguez Fortuno (Supervisor) & Aliaksandra Rakovich (Supervisor)