All active galactic nuclei (AGN) are known to vary in the rest-frame UV/optical. Typical variations are on the order of 30% or so and are stochastic in nature. Therefore, the discovery of a number of extreme AGN transients, which are smoothly evolving on year-long timescales and by a factor of four or more, is surprising and necessitates further analysis. Are these objects simply at the extreme end of the variability distribution seen in normal quasars or is there another mechanism which can explain their atypical behaviour? The primary focus for this work is on the possibility that a number of these extreme AGN transients are actually rare, high-amplitude microlensing events, caused by intervening stellar mass object(s). Not only do the microlensing models provide an explanation for the observed variability but they also allow constraints to be placed on the morphology of the emitting regions of the AGN, namely the accretion disc and broad line region (BLR). These transients have been monitored both photometrically and spectroscopically, since their discovery. The majority of spectroscopic observations have been conducted using the William Herschel Telescope. At time of writing (Sept. 2017), there are now 64 confirmed AGN and 235 individual spectra. The spectral reduction pipeline, calibration and initial measurements are described in Chapter 2. This chapter also details the microlensing models and procedures used in interpreting both the light curve information and spectral measurements. This includes: a comprehensive treatment of the simple point-source/point-lens model; quantitative point-lens models which allow for the use of extended sources; and also an initial exploration into more complex lensing morphologies involving multiple lensing objects and/or an external shear. Chapter 3 details the results of the spectroscopic monitoring campaign for the entire transient sample. A general classification scheme is developed which allows for a comparison of the evolutionary trends seen in objects exhibiting similar behaviour. A subset of transient AGN, the most extreme objects in the sample, is also discussed in detail, with a particular focus on the evolution of the continuum, line fluxes and equivalent widths. Chapter 4 details the results of the analysis of four key targets, selected for their suitability in addressing the microlensing hypothesis. For two targets the point-source point-lens model performs very well. Lens parameters for these objects are presented and in one particular case, the data is sufficient to allow constraints to be placed on the size of various components comprising the broad line region. Chapter 5 expands the microlensing analysis to include the entire AGN transient sample. Approximately 10% of objects are well matched by a simple point-source, point-lens microlensing model. In other objects, evidence is seen which requires a more complex lensing scenario to adequately explain. In one class of objects there is also evidence that the accretion disc is being resolved by the lens. Chapter 6 revisits a notable are seen in an AGN which lies behind M31. The analysis reaffirms that this event is well described by a simple microlensing model and provides an independent estimate that the most probable location for the lens is within M31 itself.

This Master's thesis presents numerical and experimental results that benchmark a state-of-the-art terahertz time-domain spectrometer. We begin by describing the theory behind the nonlinear optical mechanisms through which we generate and detect short pulses of THz radiation. Based on a coherent electro-optic detection scheme, our measurements trace out the oscillating electric field of the THz pulses generated from the optical mixing process of optical rectification. A numerical simulation based on the theory presented in this work helps present the physical intuition behind our use of these nonlinear optical processes and is furthermore used to complement our measurements with theory. Utilizing the simultaneous amplitude and phase information provided by our detection scheme, we perform terahertz time-domain spectroscopy on sample materials. Our samples of interest are the III-V zinc-blende semiconductors GaP and ZnTe which are nonlinear media popular for their advantageous dispersive and absorptive properties in the terahertz range. Therefore, the thesis culminates in the demonstration of a material parameter extraction procedure which we use to obtain the complex refractive index of a GaP crystal and a ZnTe crystal.

No description available.

The present work describes a theoretical and experimental investigation of the stimulated Raman effect excited by the focused output of a slow, Q-switched ruby laser in the self-focusing liquids benzene, nitrobenzene, chlorobenzene, and carbon disulphide. Time relationships between the exciting pulse, the transmitted laser pulse, and the time resolved stimulated Raman spectra have been studied using a high speed streak camera in conjunction with fast photodiodes. The spontaneous Raman spectra of these liquids have been investigated using a photoelectric Raman spectrometer and a method is outlined for measuring the relative values of the stimulated Raman gain coefficients. It is experimentally shown that, on a nanosecond time scale, stimulated Raman lines are not generated simultaneously but in the sequence Stokes, Anti-Stokes, and second harmonic Stokes. Further, the time delay between the start of first and second harmonic Stokes is shown to be dependent on the rate of rise of the exciting pulse. It was found that during stimulated Raman generation the laser pulse transmitted through the liquid was heavily distorted and that each liquid produced its own characteristic pulse distortion. No distortion was found in the absence of stimulated Raman generation. Time correlations were found between the duration of features of the pulse distortion and the duration of first Stokes and second harmonic Stokes. These correlations show that forward stimulated Raman generation is controlled by the transmitted laser pulse not the exciting pulse. The threshold for the onset of pulse distortion is shown to be dependent on the rate of rise of the exciting pulse whilst, in all the liquids, apart from carbon disulphide, the 'cut-off' threshold is dependent on the peak power of the incident pulse. Investigation of the pulse distortion thresholds for the range of liquids produced relative values which did not agree with those predicted from either the optical Kerr coefficient or the calculated stimulated Raman gain coefficients. A theoretical model of the self-focusing of a focused beam in a medium for which both electrostriction and the optical Kerr effect are significant is presented and has been used to explain the anomalous threshold results. Within the experimental scatter of the results obtained, this model appears to explain the observed threshold effects. Since the forward stimulated Raman generation was weak in comparison to the transmitted laser pulse and followed this distorted pulse rather than the exciting pulse, it is concluded that stimulated Raman is not the dominant mechanism in the interaction. A brief review is presented of the theory and predictions of steady and non-steady state Brillouin scattering. Although the phonon lifetimes for the other liquids appear to be too short to be of significance, the results are similar in form to those of carbon disulphide. An explanation based on multiphoton absorption is suggested to explain the anomalous behaviour of these liquids. It is concluded that the observed effects in the forward stimulated Raman process depend upon the development in time of the non-linear field dependent self-focusing of the exciting beam and the strong backward scattering process.

Infectious diseases remain a serious threat to human life and health as well as having important economical factor. One way of successful combating diseases is designing the most appropriate treatment plan following the correct diagnosis. Therefore, there is a need for a method combining reproducibility, precision and speed. The aim of this work was to evaluate the potential of micro-Raman spectroscopy for identifying bacteria at different taxonomic levels, strains revealing different antibiotic resistance profiles, and for phylogenetic investigation. The project was based on a selection of bacteria: Staphylococcus aureus (6571, Cowanl), Staphylococcus epidermidis (1457, 9142), Escherichia coli including wild- types (strain B, K12, Top 10), transformants expressing ampicillin and kanamycin resistance (Top10Amp, Top10Kan) and clinical isolates expressing extended-spectrum beta- lactamases (ESBL). Following a precise and detailed protocol, Raman spectra were recorded from bacterial colonies grown overnight on a Colombia Blood Agar. In order to remove background fluorescence, rolling-circle filter procedure was applied. The most critical peaks for differentiation between organisms as well as for characterising each microorganism were determined. The spectral data were analyzed using principal component and cluster analysis techniques. As expected, the degree of separation decreased in the order genus→species→strain. It was determined that DNA/RNA, proteins and amino-acids are responsible for the differentiation between strains on a lower level of similarity with more influence of the constituents of the bacterial envelope between more closely related organisms. Raman spectroscopy was capable of differentiating between susceptible and resistant strains as well as monitoring whether the organism has been grown under antibiotic pressure. Based on triplex PCR, clinical isolates of ESBL strains were assigned to one of the phylogenetic group characterising Esherichia genus and it was revealed that within CTX- M TEM-1 there were two distinct clusters of D and B2 groups. Overall we have demonstrated that the combination of micro-Raman spectroscopy, microbiology and bioinformatics has the potential for the successful discrimination of bacteria species and strains, for the determination of antibiotic resistance profiles and investigating phylogenetic grouping in a clinical environment.

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Photoelectron spectroscopy, with LEED, Auger electron spectroscopy (AES) and secondary electron emission crystal current (SEECC) measurements, has been used to investigate the electronic structure of well-characterized overlayers of Tl, Cr, Mn and Fe on Ag(100) and Fe on Cu(100). The Stranski-Krastanov growth mode has been found for Tl on Ag(100). In the chain structure observed at 70 K, there is no evidence for a Peierls distortion, in contrast to Tl chains on Cu(100). Band-mapping of the Tl 6p band has indicated the absence of an energy gap at the Fermi level, confirming the LEED result that the Peierls distortion has been suppressed. This is believed to be due to commensurability with the substrate. A study of Cr on Ag(100) has revealed epitaxial growth up to two monolayers, with a transition to bilayers before the completion of the first layer. Photoemission measurements of the Cr 3s multiplet splitting indicate an enhanced magnetic moment, which approaches the bulk value with coverage. This enhancement is thought to be a consequence of d-band narrowing, resulting from the reduced dimensionality of the overlayer. A p(1 x 1) surface alloy is initially observed when Mn is deposited on Ag(100), but with a magnetic moment smaller than the bulk Mn value; angle-resolved photoemission measurements indicate significant overlayer- substrate d-d interaction, in contrast to Cr on Ag(100). With further coverage, the magnetic moment increases and when simultaneous multilayer growth occurs on the AgMn surface, the bulk value is attained. Pseudomorphic 1ayer-by-layer growth has been identified for Fe deposited on Cu(100) and the band structure has been determined for Fe coverages of one and seven monolayers. From a comparison with calculated band structures, it is proposed that the monolayer is ferromagnetic, while the thicker fee film may be antiferromagnetic. An Fe monolayer on Ag(100) has also been shown to have ferromagnetic order.

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