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have found that while simple consideration of the equilibrium geometries is unable to reproduce the experimental results, we have been able to use molecular dynamics and vibrational coordinate analysis to rationalize the rapid electron transfer. Specifically, certain vibrations of MesAcr promote the electron transfer, but through an interesting mechanism. We then proceed to describe three more systems where nuclear motion is critical to explaining their photophysical properties. A C6o-Ru dyad is shown to undergo efficient electron exchange at room temperature, but with a pronounced activation energy. We use molecular dynamics to show that the CwRu dyad should not be considered as a rigid system, but that the "butterfly" motion of the molecular "dumbell" can introduce new mechanisms for electron exchange. We also present preliminary calculations on a bodipy-Co, dyad which appears to exhibit dual emrssion from both bodipy- and C60 centred triplet states. These results indicate that instead of an equilibrium existing between these two distinct states, the actual emmissive state has characteristics of both, being delocalised across the whole dyad. Finally, a bodipy-pyrene dyad which exhibits a large increase in the rate of non-radiative decay when subjected to a protic environment is investigated using theoretical techniques. We show that this effect is not due to a large distortion of the bodipy dye inherent in the excited state's properties, but is likely to be due to the increased n-stacking of the pyrene units as they orient to minimize solvent contact Constrained biphenyl and quaterphenyl molecules are potentially effective units for use in molecular electronic devices. We describe how computational chemistry can be applied to the study of their excited states, providing a sound rationalization for both the excitation and emission spectra. Specifically, the differing strap lengths used to constrain the internal torsion angle is only partially effective, due to the accessibility of many more conformations as the strap length increases. Thus, we see that the excited SI state geometries remain comparable, regardless of strap length, and that the presence of almost iso-energetic conformers leads to interesting behaviour in the ground state. Finally, we use theoretical calculations as a mainstay for the design of a molecular memory device. This work proposes a biphenyl unit constrained by a disulfide bridge as a central switching unit. Calculations are concerned with both the reversible opening/closing reactions, as induced by electrochemical methods, and the possibility of using photo-induced electron transfer as the stimulus for conformational exchange.

TiO2 films are widely used as high refractive index dielectric layers in multi-layer optical devices and functional films. Indeed, the refractive index of titania is amongst the highest of the commonly used optical coating materials. Typical applications include low emissivity and solar control films, anti-reflection coatings and narrow bandpass filters. There are, though, two critical issues which affect the use of titania films in commercial products. First, it has a low deposition rate in comparison with other hi high index materials, such as Nb205. And, secondly, the optical properties of titania films, and their stability, are critically dependent on the film structure, which in turn, is dependent on the deposition conditions used.

Calculations based on a rigorous analytical model arc carried out to optimise the width of the avalanche region, w, in high-speed direct-detection avalanche photodiode- . based optical receivers. The model includes the effects of intersymbol interference (ISI), tunnelling current, avalanche noise, as well as dead space. The sensitivity of InP, InA1As and InAs avalanche photodiodes (APDs) were investigated. The interplay among the factors controlling the optimum sensitivity is confirmed. Results show that for a given transmission speed, as the device width decreases below the optimum value, increased tunnelling current outweighs avalanche noise reduction due to dead space, resulting in poorer receiver sensitivity. As the device width increases above its optimum value, the receiver sensitivity worsens as bandwidth decreases, causing TST to dominate avalanche noise and tunnelling current. For a 10 Gb/s system and a bit-error rate of 10.12, an optimum w of 0.191lm is predicted, yielding an optimum sensitivity of -28.1 dBm at an M of 13 for InP APDs. InAIAs APDs were found to provide an improvement of 0.5 dBm over InP at an M of 15 and w of 0.15Ilm. InAs APDs have been calculated to yield an optimum sensitivity between -29.6 and -30.2 dBm at an M of 76 and w of 4.2Ilm. A type-II InGaAs/GaAsSb superlattice p-i-n diode and separate absorption and multiplication (SAM) APD using TnAlAs as the multiplication region (both lattice- matched to InP) is reported. Optical and electrical characterisations of the devices are performed. The devices exhibited a cut-off wavelength of 2.511 m. Unity-gain responsivities of 0.53 A/W and 0.47 A/W have been obtained for the p-i-n diode and APD, respectively, when illuminated with 2.004f.lm wavelength light. Detectivity at this wavelength was calculated to be 7.1 x 10Y cmHzY'l/W for the p-i-n diode and 5.9x I O~ cmHz'f,/W for the APD. Excess noise measurements conducted on the APD show that the characteristics follow that of an In AlAs p-i-n diode with the same multiplication layer thickness, giving an effective k of 0.2. Significant multiplication factors, above 50 at 290K and above 200 at 200K, have also been achieved by the APD.

This work investigates the avalanche characteristics of ln0.53Ga0.47As, In0.52Al0.48As and type-II heterojunction material systems for applications in telecommunication and next generation mid-wave infrared avalanche photodiodes (APDs). This work carries out the first ever reported excess noise measured on a series of In0.53Ga0.47As p+-in+& diodes Electron initiated multiplication exhibit very low excess noise due to the large difference between the electron and hole ionisation coefficients in In0.53Ga0.47As. The behaviour is unlike Ge, which has a similar spectral response.

A systematic study of the avalanche multiplication behaviour in InP has been performed on a series of diodes with avalanche region widths, w, ranging from 2.50 to 0.08 μm, covering a wide electric field range from 180 to 850kV/cm. The local model for impact ionisation is found to increasingly overestimate the multiplication at low electric fields as w decreases due to the presence of dead space. The dead space is also found to decrease the excess noise. An excess noise factor of F = 3.5 at multiplication factor M= 10 was measured, the lowest value reported so far in InP.

Photonic crystal fibres (PCF) are a new generation of optical fibre that guide light via a periodic air-silica, photonic crystal structure instead of the more traditional step change in refractive index associated with traditional fibre. Careful design of the photonic structure causes the fibres to behave in interesting new ways and one of main aims of this thesis is to begin the investigation of the uses of PCF's in astronomy. Step index and large mode area (LMA) PCF's are introduced in Chapters 2 and 3, respectively. Chapter 4 then deals with the instrumental simplifications associated with the use of LMA PCF'ร in fibre stellar interferometry showing that up to four step index fibres and associated optics can be replaced with a single LMA fibre. One of the key features of LMA fibres, for astronomy, is that, unlike the step index fibre, the mode field size is independent of wavelength and the fibre can therefore be fed with a pupil image via a field lens. Chapter 5 investigates this important new parameter space showing that contiguous sampling using single mode fibres is now possible for the first time. Further, unlike the direct feed to the LMA fibre, maximised coupling over very large wavebands is now possible using just a single fibre. Chapter 6 deals with another new fibre technology in astronomy: Multi-mode fibre (MMF) to single-mode array (SMA) transitions. These fibre systems break out the modes of the multi-mode fibre into an array of single-mode fibres upon which Bragg gratings can be etched. The SMA is then refused into an output MMF resulting in a multimode device but with single-mode line suppression. The number of modes transmitted is numerically equal to the number of fibres in the SMA and the performance of these devices is investigated on a model telescope showing that only a few tens of modes is required to efficiently transmit either the J or н bands. Finally, Chapter 7 details the experimental investigation of fibre modal noise in high dispersion spectroscopy. This is a photometric error on a resolution element due to fibre modes interfering with each other at very high spectral dispersion. Worryingly, the results show that no current theory exists that can predict the performance of a fibre based high R spectrometer.

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No description available.

This thesis describes the continued development of square pore microchannel plate (MCP) X-ray optics with a view to their application on terrestrial and space-based instrumentation. The development of a lab-based Imaging X-ray Fluorescence Spectrometer (IXRFS) is described, incorporating an MCP optic to relay fluorescent flux, excited from uniformly illuminated target samples onto a Charge Coupled Device (CCD). Applications in mineralogy and scientific instrument calibration are investigated, with elemental maps spatially correlated to observed sample features in each case. The soft X-ray focusing properties of one planar and two slumped MCP optics are evaluated with a view to their application on the future Lobster-ISS all-sky monitor. Best FWHM foci of 9.4' x 7.5' (planar) and 9.1' x 9.1' (slumped) are reported. Observed temporal variation in the angular resolution of a slumped MCP optic is attributed to the mechanical deformation of the optic during focusing. Multifibre misalignment is determined to be the major source of error in the focusing properties of the planar MCP. The metallisation of the reflecting surfaces of MCP optics improves their high energy and wide angle response; metal films of iridium, ruthenium and nickel, deposited onto MCP optics using the Atomic Layer Deposition (ALD) and Electroless Nickel (EN) methods are characterised using the techniques of Auger Electron Spectroscopy (AES) and X-ray Reflectivity (XRR). Ruthenium and nickel-coated MCPs exhibit a shortfall in low-grazing angle reflectivity, not observed in the case of iridium, which is modelled with an RMS surface roughness of 26 Å. The Lobster-ISS all-sky monitor is modelled to a range of specifications using a Monte-Carlo raytrace code to optimise performance. Iridium-coated MCP optics are found to provide the best modelled sensitivity for a variety of astrophysical objects. In this configuration, Lobster-ISS is modelled to have a 1 day limiting sensitivity of 0.32 mCrab.