On the identification of novel bragg phenomena in static and dynamic media
File(s)
Author(s)
Koufidis, Stefanos
Type
Thesis
Abstract
This dissertation extends scalar coupled-wave theory through the application of Möbius transformations, thereby reducing the coupled-wave equations to a first-order non-linear differential equation of a single real variable. This reformulation facilitates both analytical and numerical approaches to complex refractive index modulation scenarios, establishing an innovative connection between coupled-wave theory and coupled oscillators, and offering novel insights into photonic bandgaps. The theory is applied to optically active structurally chiral media, demonstrating that giant chirality may induce back-scattering of both polarisations under conditions intrinsically aligned with negative refraction. A novel Bragg-like phenomenon is subsequently introduced, observable in uniform media, where tuning is accomplished by adjusting the medium parameters, rather than by wavelength-matching, thereby enabling broadband, polarisation-selective reflection. Additionally, non-axial wave propagation in axially bi-anisotropic media reveals a similar circular Bragg-like effect with more flexible tuning requirements, with the inclination angle exercising control over the resonance location and corresponding bandwidth.
The second part of this dissertation initiates by examining photonic time-crystals characterised by a periodically time-varying permittivity. Employing Möbius transformations, it explores the dispersion and response characteristics of a finite ‘time-slab' of the considered dynamic medium, uncovering the temporal analogue of Bragg gratings and elucidating time modulation as a platform for parametric amplification. The analysis also considers the influence of almost-periodicity on the first-order momentum gap formation in photonic time-crystals, demonstrating how material imperfections can paradoxically broaden modal coupling to augment amplification. Further examination of light propagation in time-periodic chiral media, characterised by time-varying permittivity, permeability, and chirality parameter, reveals distinctive synchronisation phenomena for contra-handed modes. The findings illustrate that extreme optical rotation triggers a temporal analogue of the chirality-induced negative refraction and highlight the potential for designing an all-optical modulator capable of simultaneously amplifying the signal whilst controlling its polarisation.
The second part of this dissertation initiates by examining photonic time-crystals characterised by a periodically time-varying permittivity. Employing Möbius transformations, it explores the dispersion and response characteristics of a finite ‘time-slab' of the considered dynamic medium, uncovering the temporal analogue of Bragg gratings and elucidating time modulation as a platform for parametric amplification. The analysis also considers the influence of almost-periodicity on the first-order momentum gap formation in photonic time-crystals, demonstrating how material imperfections can paradoxically broaden modal coupling to augment amplification. Further examination of light propagation in time-periodic chiral media, characterised by time-varying permittivity, permeability, and chirality parameter, reveals distinctive synchronisation phenomena for contra-handed modes. The findings illustrate that extreme optical rotation triggers a temporal analogue of the chirality-induced negative refraction and highlight the potential for designing an all-optical modulator capable of simultaneously amplifying the signal whilst controlling its polarisation.
Version
Open Access
Date Issued
2024-12-16
Date Awarded
01/04/2025
License URL
Advisor
McCall, Martin
Sponsor
Bodossaki Foundation
Publisher Department
Department of Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)