The static and dynamic electrical behaviour, the electrical conductivity, the macroscopic viscosity and the optical transmission characteristics of side chain polysiloxane liquid crystal polymers (LCPs) have been studied. Particular emphasis has been placed on the study of the molecular dynamics using dielectric relaxation spectroscopy and the low field (AC) and high field (DC) conductivity, with the intention of relating structural variations to the behaviour of these materials, and of identifying the most appropriate LCPs for use in practical optical storage devices. The effect on the molecular dynamics of the polymer backbone and the core, spacer and end groups of the mesogenic units was investigated. The LCP responses were shown to be broadly similar to those of low molar mass liquid crystals, though with longer relaxation times. The dynamic responses of structurally different LCPs were shown to be related when these were observed at a temperature referenced to the glass transition temperature. Values of the static permittivity of several LCPs have also been measured and tabulated. Low field electrical conductivity measurements were used to show that the magnitude of the conductivity was comparable to that of commercial liquid crystals. A method was identified whereby the conductivity of different LCPs could be compared. The texture of the measuring electrode surface was found to play an important role in determining the impedance of electrical double layers formed near the surface of the sample, with smooth polymer based surfactants reducing the formation of space charge. High field DC conductivity measurements showed that Schottky-type charge emission occurred at the electrodes at elevated temperatures, except when the smooth polymer based surfactants were present. This was probably a result of the reduction in the local field near the electrode due to a better physical match at the LCP/surfactant interface. The bulk viscosity of mixtures of an LCP and commercial liquid crystal were also measured and the results extrapolated to the viscosity of the LCP. The relationship between electrical conductivity and bulk viscosity was examined. The optical absorption spectra of blue pleochroic dyes have been measured to identify appropriate dyes for use with LCPs in laser written optical memories. The stability of the alignment of selected LCPs with time and temperature was also observed optically.

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Ph.D. / Please refer to full text to view abstract

Dans le monde moderne de l'information numérique, les volumes de transfert de données augmentent constamment et créent une demande correspondante pour des vitesses de transfert plus élevées et un traitement des données plus rapide.De nos jours, le transfert d'information sur la puce au moyen de signaux électriques commence à atteindre ses limites en raison des restrictions physiques des effets quantiques, rendant le transfert de données au moyen de signaux optiques attrayant pour le transfert d'information rapide sur les puces informatiques. Il en résulte que la photonique au silicium, qui utilise du silicium (à motifs) en tant que milieu optique (par exemple des cristaux photoniques), a un grand potentiel pour remplacer les parties des interconnexions métalliques actuelles par celles optiques.Il existe déjà une gamme de composants photoniques créés de manière monolithique sur des plaquettes de silicium sur isolant, mais il manque encore des sources compactes de lumière pulsée sur puce. Dans cette thèse, nous travaillons sur la création de telles sources de lumière pulsée, tout en étudiant l'interaction des lasers à cristaux photoniques avec l'absorbant saturable de graphène. Bien que n'atteignant pas réellement le but ambitieux et à long terme associé à la réalisation d'un laser à verrouillage de mode intégré, cette thèse a fait quelques progrès dans cette direction. Nous avons franchi quelques étapes, telles que l'intégration du graphène et des structures actives des cristaux photoniques, l'étude de l'interaction du graphène avec les lasers compacts, le développement d'un modèle théorique permettant d'étudier cette interaction et enfin la conception de multimodes et de cavités compactes à base de cristaux photoniques pour la miniaturisation de lasers à verrouillage de mode. Ces étapes seront importantes pour le développement de dispositifs compacts capables de générer un train d'impulsions optiques sub-picosecondes dans des plates-formes à puce. / In the modern world of digital information, data transfer volumes are constantly increasing and create a corresponding demand for higher transfer speeds and faster data processing.Nowadays, the information transfer on-chip by means of electrical signals begin to reach its limitations due to physical restrictions of quantum effects, making data transfer by the means of optical signals to become an attractive prospect for high-speed information transfer on computer chips. As a result, silicon photonics, which uses (patterned) silicon as an optical medium (e.g. photonic crystals), has a great potential to replace parts of the current metallic interconnects by the optical ones.There is already a range of photonic components created monolithically on silicon-on-insulator wafers, however, compact sources of pulsed light are still missing on-chip. In this thesis, we are working on creation of such sources of pulsed light, while investigating the interaction of photonic crystal compact lasers with graphene saturable absorber. Although not actually achieving the long-term and ambitious goal associated with the realization of an integrated mode-locked laser, this thesis made some progress in that direction. We have achieved a few milestones, such as the integration of graphene and active photonic crystal structures, the study of the interaction of graphene with compact lasers, the development of a theoretical model providing a basis for studying this interaction and finally the design of multimode and compact photonic crystal based cavities for the miniaturization of mode-locked lasers. These steps will be of importance for the development of compact devices capable of generating a train of sub-picosecond optical pulses in chip-based platforms.

At the onset of this study, the work presented in Chapter 3 of this thesis was the primary focus. The work was motivated by JF Prins where he observed the formation of diamond layers on copper followed by C+ implantation into copper. This initial result suggested that it may be possible to generate single crystal diamond layers on single crystal copper. Subsequent efforts to reproduce this result failed. A unique end station was developed where a number of parameters could be altered during the implantation process. A series of carbon ion implantations were carried out on copper and copper-nickel (FCC) single crystals in this end station. The layers were characterised using initially Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED) and later Raman Spectroscopy. During the early period of this study, the surface science equipment at the then Wits-Schonland Research Institute for Nuclear Sciences, was constantly giving problems. The time constraints on waiting for funds to be made available to repair the equipment, urged me to pursue alternative research endeavours and the results of this research is presented in chapter 4 and 5. The initial work will be investigated further in the future. Details of the end station are presented and the initial results of carbon layers generated in this end station are presented. In chapter 4, a study of C+ implantation into a type IIa (FCC single crystal) diamond using the cold implantation rapid annealing (CIRA) technique is reported. The Raman spectrum was recorded as a function of annealing temperature and C+ ion dose. De- fect peaks at 1450, 1498 and 1638 cm−1 appear in the Raman spectra, which have been previously considered to be unique to MeV implantation. The maximum energy of implantation used in this study was 170 keV. The peaks were monitored as a function of annealing temperature and ion dose. The annealing behaviour of the peaks were similar to those observed in the MeV implantation experiments. It is thus concluded that the defects that give rise to these peaks are related to the point-defect interac- tions that occur within the implantation regime and not to the implantation energy. 1 Understanding the nature of the defects that arise during the implantation annealing process, allows one to manipulate the implantation-annealing cycle, so as to generate defect structures that are useful in the fabrication of an active device in a diamond substrate. This is shown in chapter 5. A p-type (type IIb, FCC crystal) diamond was implanted with either carbon or phos- phorus ions using the cold implantation rapid annealing (CIRA) process. In each case, the energies and doses were chosen such that upon annealing, the implanted layer would act as an n-type electrode. The electroluminescence (EL) emitted from these carbon and phosphorus junctions, when biased in the forward direction, was compared as functions of annealing and diode temperatures. Typical luminescence bands such as those observed in cathodoluminescence (CL), in particular blue band A (2.90 eV) and green band (2.40 eV) were observed. Two bands centred around 2.06 and 4.0 eV were also observed for both the carbon and phosphorus junctions, while a band at 4.45 eV appeared only in the phosphorus implanted junction. This was the first time that the 4.45 eV band was observed in an electroluminescent junction.

The principal aim of this Thesis is the synthesis and characterisation of a range of novel liquid crystals designed to exhibit the twist-bend nematic phase (NTB), in order to enhance our understanding of the relationships between molecular structure and the observation of NTB behaviour. Moreover, the inclusion of chiral fragments allowed the effects of molecular chirality on the structure of the NTB phase, specifically the chiral twist-bend nematic phase, N*TB, to be studied. In Chapter 3, a series of non-symmetric odd-membered liquid crystal dimers are prepared and the terminal chain length m is varied. A change in the local molecular structure from intercalated to bilayer is seen on increasing m, but this has no apparent effect on the stability of both the nematic and twist-bend nematic phases, which show a regular dependence on m. A novel twist-bend smectic phase is reported. Chapter 4 investigates the effects of branching this terminal chain on phase behaviour. It is evident that in dimers with a shorter spacer, branching destabilises the N and NTB phases while stabilising smectic behaviour, but in longer homologues smectic behaviour is also destabilised. Chapter 5 explores the effect of molecular bulk on phase behaviour, specifically the stability of the NTB phase, by the incorporation of a pyrene moiety. This group supresses crystallisation such that stable, low temperature NTB phases are formed despite the bulky group. Chapters 6 and 7 study the inclusion of chiral moieties in bent-shaped, odd-membered dimers: specifically, 2-methylbutyl, 1-methylheptyl (6) and lactic esters (7). New examples of the rarely observed chiral twist-bend nematic phase are seen. Phase behaviour is investigated and compared to achiral and racemic analogues. Chapter 8 describes the mesogenic behaviour of molecular complexes assembled by hydrogen bonding between both achiral and chiral stilbazole-based and benzoic acid-based fragments. A selection of the complexes exhibit the N(*)TB phase despite only one or neither component being mesogenic.