Quantum Monte Carlo (QMC) methods are powerful, stochastic techniques for computing the properties of interacting electrons and nuclei with an accuracy comparable to the standard post-Hartree--Fock methods of quantum chemistry. Whilst the favourable scaling of QMC methods enables a quantum, many-body treatment of much larger systems, the lack of accurate and efficient total energy derivatives, required to compute atomic forces, has hindered their widespread adoption. The work contained within this thesis provides an efficient procedure for calculating exact derivatives of QMC results. This procedure uses the programming technique of algorithmic differentiation (AD), which allows access to the derivatives of a computed function by applying chain rule differentiation to the underlying source code. However, this thesis shows that a straightforward differentiation of a stochastic function fails to capture the important contribution to the derivative from probabilistic decisions. A general approach for calculating the derivatives of a stochastic function is presented, where a similar adaptation of AD applied to the diffusion Monte Carlo (DMC) algorithm yields exact DMC atomic forces. The approach is validated by performing the largest ever DMC force calculations, which demonstrate the feasibility of treating systems containing thousands of electrons. The efficiency of AD also enables molecular dynamics simulations driven entirely by DMC, adding new functionality to the QMC toolkit. Another focus of this thesis is using the phenomenon of stochastic coherence to correlate DMC simulations, allowing finite difference derivatives to be obtained with a small error. Whilst this method is far easier to implement than AD, preliminary results show an instability when treating larger systems. A different approach is obtained from extrapolating this method to a finite difference step size of zero, producing algebraic expressions for a direct differentiation of the DMC algorithm.

This thesis reports the experimental investigation of intermolecular charge transport in dye monolayers anchored to the surface of nanocrystalline oxides. I use electrochemistry and transient spectroscopy to measure diffusion of holes within dye monolayers and interpret my observations on the basis of the non-adiabatic Marcus theory of charge transfer. I observe thermally activated hole diffusion for dyes used in dye sensitized solar cells (DSSCs) anchored to TiO2 and immersed in an inert acetonitrile based electrolyte. The corresponding values of reorganization energy of charge transfer between the dyes range between 700 and 1500 meV. Assuming negligible contribution from energetic disorder, this shows agreement with previously reported calculations of reorganization energy. Low outer sphere and low inner sphere reorganization energies correlate with delocalization of the HOMO and with rigid molecular structures showing extended conjugation. I show that hole diffusion in the monolayer can be controlled both at the μm and at the nm scale by varying the fraction of TiO2 surface covered with dyes. I present the effect of decreasing the dye surface coverage and consequently stopping hole diffusion on photo-electrochemical device structures. First, I observe a slowdown of the photo-induced recombination reaction of holes in the dye monolayer to electrons in the TiO2 when decreasing the dye loading. This result is consistent with the hypothesis that hole diffusion in the dye monolayer contributes to faster recombination. Second, I show that hole transport in the dye monolayer is responsible for increased dye regeneration efficiency in solid state DSSCs. I quantify improved regeneration yield by between 50% and 5% depending on the degree of the pore filling by the hole transporting material spiro OMeTAD. Finally I demonstrate that effective photo-conversion can occur in solar cell structures where dye monolayers function as the only hole transport phase.

The problem of the elotron capture by fast charged particles passing through a gas is examined mathematically. The applications of theoretical techniques to simple capture processes are discussed, with emphasis on the scattering of protons by helium for which experimental results are more readily available.

From galaxies, to clusters, to the Cosmic Microwave Background, there is strong gravitational evidence that the matter content of the Universe is not restricted to the particles of the Standard Model. Specifically, observations indicate that there must also be a large relic population of non-luminous Dark Matter. However, the character of this Dark Matter remains unknown: in particular, to what extent does it interact with the particles of the Standard Model, and with itself, through non-gravitational means? We seek to answer this question in this thesis. We first present constraints on the interaction of Dark Matter with quarks, through an analysis of data from the XENON100 and CoGeNT Direct Detection experiments. In order to do so, we develop a Bayesian technique, which aims to maximise the amount of information we can extract from the data. After this, we discuss potential constraints on the charge of Dark Matter due to its interactions with galactic magnetic fields, and the potential for constraints on its self-annihilation cross section from Cosmic Ray data. We also consider Dark Photons, which partner Dark Matter in many models, and place bounds on their couplings to quarks using the quark-gluon plasma, produced in heavy-ion collisions. We place emphasis on a multi-scale approach and on the robust statistical treatment of Dark Matter data. Our main scientific result comes from the analysis of CoGeNT data, where we show that there is less than one sigma evidence for DM recoils, in contrast to previous claims. We show that the ‘region of interest’ derived in previous analyses, is the result of a bias in the analysis from a particular choice of functional fit for the energy-dependence of the fraction of bulk events. When we account for this bias the preference for Dark Matter vanishes.

This thesis discusses an experiment, which has demonstrated transverse laser cooling of a pulsed supersonic beam of strontium monofluoride (SrF) molecules. Producing ultracold molecules is important because they could advance many fields including many-body physics, quantum chemistry and precision measurements to explore fundamental forces in nature. Direct laser cooling of molecules is a new and promising way to produce molecules with temperatures in the sub-millikelvin range. In the experiment, SrF molecules produced from a pulsed supersonic source were cooled in the transverse direction using light from just two lasers. The molecular beam brightness was increased by about 20%. I discuss the detailed experimental setup, laser system and data analysis. I also present several theoretical models, which give insight into the cooling experiment. Finally, I discuss improvements to this experiment, which should enable higher yields of ultracold molecules to be produced.

As laser amplifiers and oscillators continue to see widespread use in all branches of science and engineering, they continue to develop in terms of operating parameters to keep pace with their applications. Importantly, the temporal and spectral characteristics of laser systems must be carefully tailored to match application requirements. This thesis reports advances in the development of laser systems, based upon optical fibre technology, which demonstrate the flexibility of optical fibre and fibre integrated devices to cover a wide range of temporal and spectral characteristics. First, the principle of spectrally masked phase modulation for short pulse generation is explored. Here, a phase modulator is used to generate a time dependent optical frequency shift, which can be turned into an effective amplitude modulation by the introduction of an optical band pass filter. This method is combined with nonlinear compression techniques based on solitonic propagation in optical fibre to generate optical pulses with duration of a few hundreds of femtoseconds and repetition rates of tens of gigahertz. Increasing the range of wavelengths over with doped fibre amplifier systems will operate requires the development of laser/amplifier systems based on new active dopants. To this end amplifier systems based upon bismuth activated alumosilicate fibre were evaluated. The amplifier stages were then incorporated into a master oscillator power fibre amplifier (MOPFA) scheme, demonstrating the applicability of bismuth doped silica fibre to advanced laser configurations. Finally, the development of a novel laser source for use in fluorescent microscopy is detailed. The source was based on a gain switched diode which is amplified in a two stage Raman fibre amplifier system, subsequently frequency doubled in a periodically poled lithium tantalate crystal. Nonlinearity and optical filtering are exploited to re-shape the output pulse's temporal profile.

Two mechanisms through which oceanic jets and the atmospheric storm tracks interact in midlatitudes are considered. Firstly, the response of a two-layer ocean model to large- scale stochastic forcing, a simplified model of forcing by the North Atlantic Oscillation, is investigated. Long Rossby waves are excited at the eastern boundary of the square model basin and the waves are baroclinically unstable. A novel aspect is that the instability leads to the generation of zonal jets throughout the domain. Unlike other theories of jet generation, the jets are actually wave-like in nature, and result directly from the instability. The “jets” appear when averaging the zonal velocity field over fixed periods of time. The longer the averaging period, the weaker the jets as the latter are actually time-varying. The jets occur for a wide range of stochastic forcing strength and the presence or not of a time mean circulation. The mechanism described here thereby provides an explanation for the recent observations of alternating zonal jets. The response of the Pacific storm track to the variability of the Kuroshio Extension jet is then studied. An index of the Kuroshio Extension front strength is produced using sea surface temperature and sea surface height observations. The index reflects the strengthening and weakening of the SST gradient associated with the bimodal states of the Kuroshio, and composites of the atmospheric state are presented during its positive and negative phases. The anomalous response of the transient eddy heat transport resembles a zonal dipole structure. With a weaker (stronger) SST front, the eddy heat transport is increased in the eastern (western) Pacific region, consistent with reduced (enhanced) low- level baroclinicity. The response of the large-scale atmospheric circulation is a barotropic blocking-type pattern in the east Pacific, which is interpreted in terms of the barotropic “eddy-straining” mechanism and eddy-mean flow interaction.

In this thesis the production of a Higgs boson in association with a hadronic jet at the Large Hadron Collider is studied using the effective interaction between the Higgs boson to gluons induced by a heavy quark. The Leading Order (LO), Next- to-Leading Order (NLO) and Next-to-Next-to-Leading Order (NNLO) perturbative QCD corrections are studied for all of the parton channels. The infrared (IR) diver- gent behaviour of the various contributions to the partonic cross section is regulated using the antenna subtraction formalism. This method has previously been used at NNLO in the calculation of three jets production in the e+e− annihilation and for the gluonic dijet production via proton collision. The research presented in this thesis extends the antenna formalism to include scattering processes in which the initial state parton changes its identity. All contributions to the pp → H+jet pro- cesses are calculated at LO, NLO and NNLO and numerically tested to demonstrate the convergence between the matrix elements and the antenna subtraction terms in the various unresolved limits. As an example of the phenomenological impact of this work, numerical results for the total and differential Higgs plus one jet cross sections are presented for the purely gluonic subprocesses.

T2K is a long baseline neutrino oscillation experiment which uses a beam of muon neutrinos to study muon neutrino disappearance and electron neutrino appearance. The measurement of v_e appearance is sensitive to CP violation, and reducing the systematic uncertainties is critical to enabling experimental determination of whether there is CP violation in the lepton sector. This thesis describes the first measurement of the v_e CC inclusive cross-section on carbon at energies relevant to T2K and other long baseline neutrino oscillation experiments. The T2K near detector, ND280, is used to select a sample of v_e events, and a Bayesian unfolding technique is used to extract differential cross-sections as a function of electron momentum, electron angle and Q^2. The total flux-averaged cross-section is also extracted, and is found to be 1.11 ± 0.20 x 10^{-38} cm^2/nucleon, which agrees well with both the NEUT neutrino interaction generator prediction of 1.23 x 10^{-38} cm^2/nucleon and the GENIE prediction of 1.08 x 10^{-38} cm^2/nucleon. A restricted phase-space analysis is also performed, in which only events where the ejected electron has momentum greater than 550 MeV/c and cos(θ_e) > 0.72 are selected. In this case, the flux-averaged cross-section is found to be 6.54 ± 1.22 x 10^{-39} cm^2/nucleon, again in good agreement with both the NEUT prediction of 7.38 x 10^{-39} cm^2/nucleon and the GENIE prediction of 6.41 x 10^{-39} cm^2/nucleon.

Light manipulation via nanoantennas, especially plasmonic nanoantennas, is an exciting new field, which aims to provide the same benefits at optical frequencies as those given by standard antennas in the radio and microwave frequency regimes. While at lower frequencies metals behave as perfect conductors, with negligible field penetration and absorption, at optical frequencies the electromagnetic field is able to excite plasmons which combine the electromagnetic wave with electronic excitations, giving raise to new properties such as high scattering and field confinement. This thesis focuses on understanding the physical principles of plasmonic nanoantennas and calculating their properties analytically, by deriving the solution for the scattering from cuboidal nanoantennas, and computationally, by presenting an improved discrete dipole approximation on cuboidal point lattices. A theory of scattering from anisotropic particles is derived, showing multiple plasmon resonance and different peaks shifts changing the background index, enabling the study of the magneto-optical effect on nanoparticles, with potential applications in nanospectroscopy, light manipulation and optical sensors. Plasmonic nanoparticles are studied numerically in order to enhance light absorption in thin film silicon solar cells and to enhance the anti-reflection coating properties of triple junction III-V quantum well high efficiency solar cells to increase scattering. Finally, plasmonic nanoantennas are also used experimentally to enhance the photoluminescence and decrease the lifetime of silicon quantum dots for light emitting device applications.