Mackinnon, Neel (2021) Aspects of chirality in light and matter. PhD thesis, University of Glasgow.
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Abstract
This thesis presents a series of complementary pieces of work, each related to chirality and handedness in lightmatter interactions. There is a particular focus on the electromagnetic helicity – especially when viewed as a pseudoscalar density constructed from the electromagnetic vector potentials which obeys a local continuity equation. A central theme is the ways in which matter (both chiral and achiral) can act as “sources” of helicity, and the circumstances under which helicity is locally conserved. This feature is connected by Noether’s theorem to the duality symmetry of the freespace Maxwell equations – the invariance of the equations under an interchange between electric and magnetic fields.
We begin by discussing free fields, and as the thesis progresses introduce various treatments of matter involving both microscopic and macroscopic electrodynamics. The five chapters focusing on original work (3, 5, 6, 7 and 9) are preceded by chapters introducing the necessary background material and concepts.
The first piece of work is an examination of the difference between electromagnetic helicity and electromagnetic chirality in polychromatic fields. The two quantities are proportional to one another in monochromatic fields, but not in general. We explicitly calculate the two quantities for some simple field configurations, and the origin and nature of the differences is discussed.
This is followed by a discussion of helicity and angular momentum radiated from elementary multipole sources. We examine radiation from the simplest chiral source that can be constructed from point multipoles, consisting of colocated oscillating electric and magnetic dipoles. This is contrasted with the radiation from a rotating dipole, which is an achiral source of circularly polarised light. The former is a net source of helicity, but not of angular momentum, and the latter a net source of angular momentum, but not helicity.
We then move on to macroscopic electrodynamics, and consider the generation of helicity at a dielectric interface. We provide and discuss expressions for the helicity fluxes when light is incident on chiral dielectric interfaces, obtained from the Fresnel coefficients for chiral media.
Following this, we propose an extension to the definition of the helicity density within chiral media. The standard definition of helicity within a dielectric is found to be unsatisfactory when the medium is chiral – it produces an incorrect helicityperphoton for left and righthanded circularly polarised light in the medium, and also is not locally conserved even when the medium is macroscopically dualsymmetric. We present a modification to the definition of the density which yields the correct helicity per circularly polarised photon within the medium, and find by inspection that the new density is indeed locally conserved in a dualsymmetric chiral medium. Our definition is then formally justified by using Noether’s theorem in order to derive the locally conserved quantity associated with macroscopic duality symmetry in chiral media.
Finally, we conclude with an extension of a standard semiclassical treatment of molecular light scattering to include higherorder multipole terms. These represent a higherorder correction to the standard results, which should become appreciable when the ratio of scatterer size to the wavelength of the incident light is around 1/10. Expressions for the scattered light from an arbitrary scatterer are presented in terms of its polarisability tensors, with appropriate orientational averaging to describe scattering from an isotropic fluid sample. The transformation of the results under a change in multipolar origin is also examined, and they are found to behave acceptably.
Item Type:  Thesis (PhD) 

Qualification Level:  Doctoral 
Keywords:  Chirality, electromagnetism, lightmatter interactions. 
Subjects:  Q Science > QC Physics 
Colleges/Schools:  College of Science and Engineering > School of Physics and Astronomy 
Supervisor's Name:  Barnett, Professor Stephen M. and Götte, Dr. Jörg B. 
Date of Award:  2021 
Depositing User:  Neel Mackinnon 
Unique ID:  glathesis:202182166 
Copyright:  Copyright of this thesis is held by the author. 
Date Deposited:  07 May 2021 10:55 
Last Modified:  07 May 2021 11:01 
Thesis DOI:  10.5525/gla.thesis.82166 
URI:  http://theses.gla.ac.uk/id/eprint/82166 
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