The work presented in this thesis focuses on the influence of morphology and device structure on the performance of polyfluorene light-emitting diodes (LEDs). The influence of blend morphology has been investigated by considering how certain processing parameters give rise to different blend morphologies and the impact these morphologies have on device characteristics and efficiency. An understanding of how different blend morphologies arise has allowed the development of techniques to control blend morphology. In this way, vertically segregated polymer blend devices have been fabricated that increase device efficiency by up to 100%. The influence of multiple semi-conducting polymer layer structures on the efficiency of polymer LEDs has also been investigated and the results show that bilayer structures increase device efficiency relative to comparable blend devices by 20%. These bilayer structures have been adapted to incorporate one-dimensional periodic structures, which result in a further 20% increase in efficiency relative to unpatterned bilayer devices. The effect of these periodic structures on the electroluminescent radiation pattern has also been investigated and it has been found that they produce distinct non-Lambertian behaviour perpendicular to the pattern direction. Finally, one- and two-dimensional self-organised periodic structures have been induced into polymer blend LEDs by using surface energy modifications to control the blend morphology. It has been found that the 1- and 2-D patterned devices are 40% and 100% more efficient than comparable blend devices. As in the cased of the 1-D patterned bilayer devices, the radiation patterns of these periodically structured devices have been investigated and exhibit non-Lambertian behaviour perpendicular to the patterned directions. Both the 1-D patterned bilayer and 1- and 2-D self-organised blend device structures have been modelled to allow the observed optical behaviour to be accounted for. The results show that the increases in efficiency and the non-Lambertian radiation patterns are due to the periodic structures coupling light trapped within the organic-anode layers out of the devices in the forward direction without altering the device characteristics.

No description available.

This study is important in finding a suitable solvent-dispersant combination to stabilize dispersed single wall carbon nanotubes (SWNTs) at high (~0.16g/l) concentration. A method based on Photoluminescence excitation (PLE) spectroscopy is developed which can identify the presence of SWNT bundles regardless of the dielectric environment surrounding them. This method also enables correct interpretation of previously unassigned optical features in SWNT PLE maps. Enrichment of SWNTs by type and chiralities is investigated here as the next step towards the optimization of SWNT-polymer composites for optical applications. SWNT-polymer composites for sub-picosecond pulse generation at telecommunication <i>C</i> band (~1550nm) are prepared using SWNT-polycarbonate and SWNT-sodium carboxymethylcellulose composites. SWNTs of two different diameter ranges are used to demonstrate the importance of the correct band gap for wavelength specific operation. Passive mode-locking in a ring cavity using such saturable absorber SWNT-polymer composites exhibits stable operation. Up to ~170fs laser pulses at ~40MHz repetition rate for an output power of ~1mW are obtained at 1550nm. The cost, ease of fabrication and integration of these composites have huge commercial potential in low power ultrafast applications.

No description available.

This thesis examines the emission properties of dye-doped liquid crystal lasers, which are of interest because they have the potential to provide compact, low threshold, tunable laser sources for applications ranging from medicine to displays. The primary aim is to provide the first detailed study of the interactions between liquid crystals (LCs) and laser dyes and to examine their effect upon the emission properties of dye-doped liquid crystal lasers. Five different dyes are examined and, in all cases, the LC host (a nematic mixture, E49) is shown to have a significant effect upon the properties of the dye, through a combination of general and specific solvent-solute interactions. Lasing is demonstrated from all five dyes in the chiral nematic (N*) mesophase. Conventional xanthene laser dyes, rhodamine 6G chloride and rhodamine B chloride, are shown to be unsuitable for N*LC lasers because their ionic nature and isotropic molecular shapes limit their solubility in LC materials. Emission is shown to occur preferentially at the long wavelength reflection band edge for small degrees of alignment of the dye in the LC host. Three elongated laser dyes (DCM, PM597 and PM580) are shown to be more compatible with E49. N*LC laser samples were made using dye concentrations in the range 0.5 wt% to 2.0 wt%. For each dye, slope efficiency (η_s) values were used to calculate relative quantum efficiency values, which decreased linearly as the concentration increased. η_sis shown to peak at a characteristic value corresponding to the balance between increasing absorbance and decreasing quantum efficiency with increasing dye concentration. The largest η_s demonstrated is 32%, which corresponds to 1.0 wt% PM597. The E_th of both PM597 and PM580 is shown to increase linearly with increasing concentration, whereas the E_th of DCM shows the opposite tendency. It is proposed that the different E_th trends indicate different quantum efficiency quenching mechanisms. Lasing was demonstrated from novel bimesogenic mixtures that showed wide temperature range blue phase I* (BP I*) mesophases. The preparation and properties of the BP I* samples are described. the nature of the laser emission is shown to depend upon the input energy used. For low inputs energies (<~ 3μJ/pulse), multimode lasing is demonstrated from the BP I* texture, whereas for high input energies (> ~ 3 μJ/pulse) random lasing is demonstrated from optically induced focal conic (N*) textures in the BP I* samples.

No description available.

Many JTC systems are constructed with the use of ferroelectric liquid crystal (FLC) spatial light modulators (SLMs) as they provide fast two dimensional binary modulation of coherent light. Due to the binary nature of the FLC SLMs used in the JTC systems, any image addressed to the device need to have some form of thresholding. Carefully thresholding the grey scale input plane and the joint power spectrum (JPS) has significant effect on the quality of correlation peaks and zero order (DC) noise. A new thresholding technique to binarise the JPS has been developed and implemented optically. This algorithm selectivity enhances the desirable fringes in the JPS which provide correlation peaks of higher intensity. Zero order noise is further reduced when compared to existing thresholding techniques. Keeping in mind the architecture of the JTC and limitations of FLC SLMs, a new technique to design rotation and scale invariant binary phase only filters for the JTC architecture is presented. Filers design with this technique have limited dynamic range, higher discriminability among target and non-target objects, and convenience for implementation on FLC SLMs. Simulation and experiments shows excellent results of various rotation and scale invariant filters designed with this technique. A rotation invariant filter is needed for various machine vision applications of the JTC. By fixing the distance between the camera and the input object, the scale sensitivity of the correlator can be avoided. In contrast to the industrial machine vision applications scale factor is very important factor for the applications of the JTC systems in defence and security. A security system using a scale invariant JTC will be able to detect a target object well in advance and will provide more time to take a decision. The CCD camera used in the current JTC system has limited dynamic range. Since intensities are distributed over a huge range in the JPS, a large amount of information is lost at the point of capture, particularly in areas where CCD array is saturated. Due to the camera saturation it is difficult to make a direct comparison with the experimental results. Hence, replacing the current CCD camera with equivalent CMOS camera would minimise the amount of information lost and avoid the camera saturation.

Although terahertz quantum cascade lasers (QCLs) are promising devices for cheap, compact and coherent sources of terahertz radiation, the full potential of these lasers has not been realised. In this work we shall examine the active region design of THz QCLs and study the behaviour of heterogeneous active regions, where there are several quantum cascade designs combined in the same active region. We will present a study of the systematic design of THz QCLs and show that it is possible to systematically tune the frequency of these devices by simple modifications to the active region design. We then present some active region designs, together with results for some high-performance designs, operating around 2 THz. Two of these designs have shown high efficiency and high temperature performance at this frequency range. From this we move on to the subject of heterogeneous active regions, and present electrically switchable emission from heterogeneous THz QCLs; devices which emit at different, widely separated frequencies depending on the electrical conditions. This is the first reported electrically switchable emission from THz QCL devices without a magnetic field.

A system for the launch of hypervelocity miniature flyer plates has been developed, characterised and applied. A laser-induced plasma is used to drive flyers, typically 5 μm thick and less than 1 mm in diameter, at velocities of a few km/s. Chapter III discusses the implementation of a fibre optic system to manipulate the spatial profile of the laser spot and thus influence the shape of the resultant flyer. A technique and procedure for coupling high power laser pulses into optical fibre has been developed. The surface finish of the fibre was found to be a critical factor. An investigation into surface preparation techniques has been conducted and an optimum procedure developed. Quantitative comparisons of fibres have been made through laser-induced damage threshold measurements and atomic force microscopy. The spatial profile of the emerging light spot was characterised. Altering the length of the fibre was found to be the most effective way of influencing the emerging intensity distribution. This fibre optic system has been successfully used to launch flyer plates and chapter IV describes the technique. Measurements of the flyer performance were made by high-speed streak photography. The flyer was found to show near perfect planarity and achieves velocities approaching 2 km/s. A maximum velocity was achieved within the first 100 μm of flight and sustained up to at least 400 μm. For a comparison, flyers launched with the raw laser pulse have been studied. These flyers were impacted on energetic materials to induce detonation and this work is described in chapter V. The sensitivity of various explosive charges was compared through measuring the threshold laser energy required to launch a flyer of sufficient velocity to initiate detonation. The effect of charge density was found to have a large influence on the laser energy required and an investigation into this has been conducted.

No description available.