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Title: Polarisation and angular momentum of light in guided modes : theory and implementations of dipolar sources coupling
Author: Picardi, Michela
ISNI:       0000 0005 0287 5262
Awarding Body: King's College London
Current Institution: King's College London (University of London)
Date of Award: 2020
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Light possesses many degrees of freedom, the most distinctive of which are polarisation, angular momenta, Poynting vector and helicity, which, in confined geometries, behave very differently than in free space. In said confined geometries, such as those required for the existence of guided modes, these degrees of freedom can interact with one-another giving rise to phenomena such as Spin-to-Orbital angular momentum transfer or Spin-momentum locking. In this thesis, we focus our attention on the exchange between the degrees of freedom of dipolar sources, superpositions of electric and magnetic dipoles in the most general case, and those of guided modes. This allows to achieve interesting excitations of guided modes, such as polarisation-dependent coupling (or absence thereof), or unidirectional excitation. The dipoles needed to achieve these phenomena are found analytically, with a very simple notation. Via numerical analysis both realistic sources, usually being nanoparticles under plane wave illumination, and waveguiding structures are designed and optimised. We extend the unidirectional excitation of guided modes, previously obtained with circularly polarised electric dipoles, to circularly polarised magnetic dipoles and superpositions of electric and magnetic dipoles, such as the Huygens dipole. This is done using the angular spectrum approach, and we prove it provides the same results that would be obtained using Fermi’s golden rule. We also reveal the existence of a source, the Janus dipole, whose coupling to guided modes is not directional but can be arbitrarily switched on-and-off simply inverting its polarisation, of which we show an experimental realisation. Moreover, we demonstrate that a superposition of electric and magnetic dipoles is sufficient to control amplitude, phase and direction of individually up to five different guided modes in the same waveguide. Finally, we present accurate calculations, both numerical and analytical of spin, angular momenta and helicity for the eigenmodes of nanofibers and nanowires, showing, among other properties, the quantization of the total angular momenta of cylindrical guided modes without the need for paraxial approximation.
Supervisor: Rodriguez Fortuno, Francisco Jose Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available