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Entanglement is one of the strangest phenomena to arise from the theory of quantum mechanics. The counter-intuitive connection between the wavefunctions of two completely separate bodies is the basis for some of the most exciting applications of quantum theory that have been proposed so far, including teleportation, quantum computation and long-range unconditionally secure communication. An inexpensive solid-state device capable of producing controlled entanglement on demand would be a huge step towards the practical realisation of many of these schemes. In 2006 the world’s first triggered semiconductor source of polarisation-entanglement photon pairs was produced in a collaboration between Toshiba Research Europe Ltd and the University of Cambridge. The work described in this thesis begins immediately after that demonstration. The source is used for a proof-of-principle experiment to demonstrate its viability for quantum-enhanced imaging resolution. Ways of improving the source are examined through careful manipulation of the growth of the quantum dots, and various types of optical cavity designed to couple the emission out of the device. The quantum mechanics of the biexciton cascade are studied closely, using both original theory and experimental results, testing the robustness against dephasing effects and the stringency of the requirement for degenerate exciton states. Finally a new method is developed for significantly enhancing the fidelity of the source using a temporal filter on the emission.

This thesis investigates the performance of solution-processable solar cells made from blends of two conjugated polymers or a polymer and a semiconductor nanocrystal. Device characterisation, steady-state spectroscopy and ultrafast spectroscopy are used to illuminate the factors affecting charge-pair generation and separation and their impact upon device performance. An improved processing technique based on spin-coating polymer blends from a controlled solvent mixture is described. The resulting optimisation of device performance is shown to be the result of a change in the drying time of the film during spin coating. This affects the morphology of the films, changing both the geometry of the phase-separated domains and, where semicrystalline polymers are used, the ordering of the polymers within those domains. Parameterisation of the IV curves of solar cells processed in this way shows that the enhancement in performance is the result of an increase in charge-pair separation efficiency and occurs despite a decrease in charge-pair generation.

This thesis examines the emission properties of liquid crystal (LC) lasers. The aim is to investigate correlations between the emission properties of the laser, in particular the threshold energy and the slope efficiency, and the macroscopic physical properties of the liquid crystal host. Using the threshold gain term obtained for a distributed feedback laser in the context of the coupled mode theory, an expression for the threshold energy (<i>E</i>_th) is obtained, in the form <i>E_th</i> ∝ <i>d</i> + 1/Δn²d² where <i>d</i> is the cell thickness and Δ<i>n</i> is the birefringence. The slope efficiency is considered to be inversely proportional to the threshold energy and thus the laser emission properties are evaluated in the context of the host physical parameters. These relationships provide fits that are in good agreement with experimental data for the threshold energy and slope efficiency dependence on cell thickness. It is shown theoretically that a threshold-less laser can be achieved for large cell thicknesses if the absorption losses are neglected. For a given cell thickness, the emission properties from a range of monomesogens, nematogen mixtures and bimesogens provide evidence that LCs with high birefringence give rise to a low threshold energy. This is in accordance with the above expression. However, examining the emission properties of a high birefringence LC laser, suggest that a high birefringence does not necessarily give rise to high slope efficiency. The slope efficiency is shown to follow the relation, <i>η_s</i>= P₃/E_th where <i>P</i>₃ depends on parameters such as saturation intensity and addition loss mechanisms. One possible loss mechanism highlighted in this thesis is associated to the elastic moduli of the host LC. This parameter provides an indication of the structural integrity of the helical structure of the LC host. The bimesogen with the largest elastic moduli gives rise to a slope efficiency of 20%. In addition to the chiral nematic phase providing the host structure for the band edge laser, the emission properties from three addition LC lasers, using alternative LC phase is also shown; the one dimensional chiral smectic C phase, the three dimensional blue phase I and the random laser that utilises the high scattering texture of the smectic A phase.

This thesis investigates the performance of terahertz Quantum Cascade (QC) devices. The main emphasis is on the reproducibility and uniformity of Molecular Beam Epitaxy (MBE) growth and the optimization of semiconductor processing. By exploring performance with reference to fabrication, this work aims to assist the physicist in characterizing new Quantum Cascade Lasers (QCLs) by yielding knowledge of performance variations unrelated to design. Work on QCLs and their forerunners, Quantum Cascade Emitters (QCEs), is described. Preliminary research investigated the ohmic contacts used in the fabrication of QC structures. It was found that traditional recipes such as AuGe or AuGeNi are unsuitable because the ohmic material can diffuse excessively and penetrate into the laser active region. Instead, PdGe ohmic contacts were identified as the most appropriate. Different types and lateral configurations of top contact were tested on a QCL structure, in which it was observed that for a shallow ohmic, complete coverage of the laser surface is preferred for uniformity of current injection. For penetrating contacts such as AuGe, it was found that a stripped configuration along the edges of the laser stripe could be used. To investigate MBE growth reproducibility and uniformity, four nominally identical structures of the first terahertz QCL were grown, across two growth campaigns, at varying chamber ‘cleanliness’. MBE chamber conditions were assessed from lateral mobility measurements of high-electron-mobility-transistor test samples. Sets of lasers were characterized from the middles and edges of all the wafers, all processed at the same time for maximum comparability. Samples were subjected to electrical, optical, magnetic and structural characterization. Substantial differences in lasing properties were observed between wafers. Lasers fabricated from high-mobility wafers emitted up to five times more power than those from low-mobility wafers and the threshold current density in devices from poor quality material increased by up to 90%. These results are attributed to variations in the lifetime of the upper state of the lasing transition. It is argued that the mechanism of this change is the degree of non-radiative scattering, affected by the quality of the heterostructure interfaces.

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