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Terahertz (THz) technology stands to solve a number of problems in everyday life, from next generation wireless communication to spectroscopic identification and imaging. However it is technically challenging to make a high power, compact source for terahertz radiation. The Quantum Cascade Laser (QCL), which produces gain at THz frequencies by exploiting inter-sub-band transitions in quantum wells, offers one solution to this problem. However controlling and detecting the emission from such sources remains a major challenge. This thesis investigates the theory and measurement of emission frequencies from aperiodic lattice THz QCLs. Crucially, realising both frequency control and detection provides a complete system for coherent THz characterisation of devices at precise, user defined frequencies. The author starts by studying the emission frequencies and threshold of discretely tuned aperiodic lattice lasers. This is achieved using a numerical transfer matrix method (TMM), which allows the calculation of the aperiodic lattice threshold spectrum for the first time. Calculations reveal that the low threshold modes of aperiodic lattice lasers form at peaks in the electromagnetic density of modes. This shows that lasing in aperiodic lattices arises from slow light propagation induced by multiple photonic band gaps, leading to both band edge and defect laser modes. Frequency selective lasing is maintained even under the influence of external facet feedback, albeit at the cost of precise knowledge of the mode frequency. Importantly this framework allows the understanding of essentially any aperiodic lattice laser system. Most significantly, the TMM is exploited in order to understand how graphene can be used to control a THz laser. Graphene interacts strongly with THz waves, and can be easily integrated with semiconductor structures such as lasers and waveguides. Here, numerical calculations reveal that graphene can be introduced into the waveguide of a THz QCL, generating electrically tunable THz surface plasmons. Such surface plasmons couple into an aperiodic lattice to change the scattering strength of each individual grating element. The TMM reveals that this change in scattering strength controls the modal selectivity of an aperiodic lattice THz QCL. This hypothesis successfully explains both earlier experiments and those performed by the author. Crucially, this model was central to a publication in the journal Science. Finally, this thesis demonstrates a novel coherent detection system for the characterisation of THz QCL emission. The technique exploits non-linear up-conversion of THz waves to a telecoms frequency side-band, a process shown to be sensitive to THz waveguide dispersion. By mixing the up-converted THz wave with a near infra-red local oscillator laser, coherent detection of QCL emission using all fibre coupled components is demonstrated for the first time. This measurement allows for the characterisation of laser emission with high frequency and temporal resolution. Specifically sub-microsecond pulses of THz emission and transients can be detected. When taken as a whole, the work of this thesis constitutes a major step towards realising cost effective THz characterisation and spectroscopy using QCLs.

By shining a precisely tuned laser through an atomic vapor, we can determine local mag- netic field strength in scalar form and in a way that is not affected by temperature changes. This technology has been used in space many times before on missions flown by NASA and ESA, such as SWARM, Øersted, and CHAMP to calibrate accompanying vector mag- netometers which are subject to offsets caused by temperature changes. The device we constructed is a small, low-cost application of this scientific principle and opens up new areas of scientific possibility for cubesats and the ability to define geomagnetic field struc- tures on a small (<10km) scale as part of the ANDESITE cubesat mission being developed at Boston University. Previously, magnetic sensors in orbit have been flown individually on a single spacecraft or in very small groups such as the International Sun-Earth Exporers (ISEE) and SWARM which each used three separate spacecraft. This method of analyzing the geomagnetic field cannot provide a spatial or time resolution smaller than that of the separation between magnetic field readings. This project has focused on producing a tabletop demonstra- tion of a compact sensor head which could enable measurements on unprecedented small scales. Toward this end we have accomplished the construction and preliminary testing of a compact sensor head which contains all necessary elements to function as a scalar atomic magnetometer.

This thesis describes the development of the two-frequency Planar Doppler Velocimetry (2n-PDV) flow measurement technique. This is modification of the Planar Doppler Velocimetry (PDV) technique that allows the measurement of up to three components of the flow velocity across a plane defined by a laser light sheet. The 2n-PDV technique reduces the number of components required to a single CCD camera and iodine cell from the two CCDs in conventional PDV. This removes the error sources associated with the misalignment of the two camera images and polarisation effects due to the beam splitters used in conventional PDV. The construction of a single velocity component 2n-PDV system is described and measurements made on the velocity field of a rotating disc and an axisymmetric air jet. The system was then modified to make 3D velocity measurements using coherent imaging fibre bundles to port multiple views to a single detector head. A method of approximately doubling the sensitivity of the technique was demonstrated using the measurements made on the velocity field of the rotating disc and was shown to reduce the error level in the final orthogonal velocity components by ~40 to 50%. Error levels of between 1.5ms-1 and 3.1ms-1 depending upon observation direction are demonstrated for a velocity field of ±34ms-1. The factors that will influence the selection of a viewing configuration when making 3D PDV measurements are then investigated with the aid of a computer model. The influence of the observation direction, the magnitude of the flow velocity, and the transformation to orthogonal velocity components are discussed. A new method using additional data in this transformation is presented and experimental results calculated using four-measured velocity components are compared to those found conventionally, using only three components. The inclusion of additional data is shown to reduce the final error levels by up to 25%.

Neste trabalho foi investigado o controle do processo de fluorescência excitada por absorção de dois fótons no polímero conjugado MEH-PPV, utilizando um sistema de formatação espectral da fase dos pulsos ultracurtos. Para tal estudo, foi utilizado um oscilador laser de Ti:Safira (15 fs, ~ 800 nm e largura de banda de 60 nm). Através de dois distintos métodos de formatação, observa-se a influência destes no processo de fotodegradação do MEHPPV, inferido pela diminuição da intensidade do sinal de fluorescência. No primeiro método de formatação, é estudada a influência de diferentes chirps impostos aos pulsos no processo de fluorescência do MEH-PPV. Observa-se uma menor taxa de fotodegradação para pulsos com maiores chirps, independente do sinal, em comparação a pulsos no limite de transformada. Esse efeito foi relacionado ao acréscimo na duração temporal dos pulsos com chirp, com consequente diminuição da intensidade. Numa segunda etapa, através do uso de um espelho deformável, a fase espectral do pulso é formatada usando uma máscara de fase senoidal. Neste caso, a intensidade de fluorescência foi modulada em aproximadamente 25%, num claro processo de controle coerente, sem nenhuma diferença apreciável no processo de fotodegradação. Desta forma, técnicas de controle coerente com formatação espectral da fase poderiam ser utilizadas para modular a intensidade do sinal de fluorescência no MEH-PPV, sem detrimento ao processo fotodegradativo. / In this work we studied the control of two-photon excited fluorescence in the conjugated polymer MEH-PPV, using pulse-shaping techniques to manipulate the pulse spectral phase. The experiments were carried out with a Ti:sapphire laser oscillator (15 fs, ~ 800 nm and 60 nm of bandwidth). We investigated the influence of two distinct pulse-shaping methods in the MEH-PPV photodegradation, inferred by the decrease in the fluorescence intensity. In the first method, we studied the effect of different pulse chirps on the MEH-PPV fluorescence. A smaller photodegradation rate was observed for pulses with higher chirps, independently of its sing, in comparison with pulses close to the Fourier Transform limit. This effect was attributed to the increase in the pulse duration for chirped pulses, and consequent decrease in the pulse intensity. In a second stage, we modulate the pulse spectral phase by employing a senoidal phase mask though a deformable mirror. In this case, a 25% modulation in the fluorescence intensity was determined, whereas no considerable effect was observed in the photodegradation. In this way, coherent control techniques employing spectral phase pulse-shaping could be used to modulate MEH-PPV fluorescence, without any negative effect to its photodegradation.

PolarizaÃÃo quÃntica tem sido usada extensivamente no processo da informaÃÃo quÃntica e emaranhamento quÃntico à essencial para muitas Ãreas de pesquisas, incluindo computaÃÃo quÃntica. Nesse trabalho foi realizada uma investigaÃÃo do grau de polarizaÃÃo e do emaranhamento de uma famÃlia de estados quÃnticos conhecidos como estados coerentes de fÃtons adicionados. Tais estados podem ser Ãteis na distribuiÃÃo quÃntica de chaves e distribuiÃÃo de emaranhamento. Usamos os parÃmetro de Stokes e a funÃÃo Q para demonstrar que, de uma forma geral, a polarizaÃÃo de uma superposiÃÃo de dois estados coerentes bimodais de fÃtons adicionados aumenta significantemente com o nÃmero de fÃtons adicionados. TambÃm utilizamos a concorrÃncia para mostrar que a quantidade de emaranhamento nas superposiÃÃes citadas apresenta um comportamento que depende se o nÃmero de fÃtons adicionados em cada modo sÃo iguais ou diferentes. / Polarization has been used extensively in quantum information processing, and quantum entanglement is essential to many areas of research, including quantum computing. Here we investigate the degree of polarization and the entanglement of a family of quantum states known as photon-added entangled coherent states. Such states could serve as means of entanglement distribution and quantum key distribution. Using the quantum Stokes parameters and the Q function, we demonstrated that, in general, the polarization of a superposition of two two-mode photon-added coherent states increases significantly with the number of added photons. And using the concurrence, we showed that the amount of entanglement in this kind of superposition presents a behavior that is dependent on whether or not the number of added photons on each mode is the same.

Esta dissertação descreve o movimento quântico de uma partícula numa barreira de potencial parabólica, condicionalmente este sistema é chamado de oscilador invertido. Determinamos estados quânticos semiclássicos como família de estados coerentes generalizados (ECG), sendo construídos através da adaptação do trabalho de Malkin e Man´ko, determinando-se um operador que seja integral do movimento, parametrizado por algumas constantes e reconhecido como operador de aniquilação, cujos auto-estados tem características dos estados comprimidos. Discutimos suas propriedades e observamos que é possível comparar ECG do oscilador invertido com ECG da partícula livre e oscilador harmônico obtidos na literatura. Também é feito um estudo do movimento puramente quântico (tunelamento) nas barreiras de potenciais do oscilador invertido e oscilador invertido truncado, por meio das soluções estacionárias exatas e aproximadas (JWKB), comparando os resultados obtidos em cada situação. / This dissertation describes the motion of a particle in a quantum potential barrier parabolic, this system is conditionally called inverted oscillator. Determined quantum states semi-classics like family generalized coherent states (GCS), being constructed by adapting work Malkin\'s and Man\'ko, determining an operator who is integral of the motion, parameterized for some constants and recognized as annihilation operator, whose eigenstates have characteristics of squeezed states. We discuss its properties and note that it is possible to compare the GCS of inverted oscillator with GCS the particle free and harmonic oscillator obtained in the literature. Also made a study of the movement purely quantum (tunneling) in the barriers potential inverted oscillator and truncated inverted oscillator by means of exact and approximate stationary solutions (JWKB), comparing the results obtained in each situation.

This thesis work will investigate and implement a method for reconstructing an unstructured mesh sequence with evolving topology. The goal of the method is to increase frame-to-frame coherency of the triangulation. The motivation of the method is that many of current state-of-the-art mesh compression and decimation algorithms for mesh sequences are based on static connectivity.

Yacht races are won by good sailors racing fast boats. A good skipper takes decisions at key moments of the race based on the anticipated wind behaviour and on his position on the racing area and with respect to the competitors. His aim is generally to complete the race before all his opponents, or, when this is not possible, to perform better than some of them. In the past two decades some methods have been proposed to compute optimal strategies for a yacht race. Those strategies are aimed at minimizing the expected time needed to complete the race and are based on the assumption that the faster a yacht, the higher the number of races that it will win (and opponents that it will defeat). In a match race, however, only two yachts are competing. A skipper’s aim is therefore to complete the race before his opponent rather than completing the race in the shortest possible time. This means that being on average faster may not necessarily mean winning the majority of races. This thesis sets out to investigate the possibility of computing a sailing strategy for a match race that can defeat an opponent who is following a fixed strategy that minimises the expected time of completion of the race. The proposed method includes two novel aspects in the strategy computation: A short-term wind forecast, based on an Artificial Neural Network (ANN) model, is performed in real time during the race using the wind measurements collected on board. Depending on the relative position with respect to the opponent, decisions with different levels of risk aversion are computed. The risk attitude is modeled using Coherent Risk Measures. The proposed algorithm is implemented in a computer program and is tested by simulating match races between identical boats following progressively refined strategies. Results presented in this thesis show how the intuitive idea of taking more risk when losing and having a conservative attitude when winning is confirmed in the risk model used. The performance of ANN for short-term wind forecasting is tested both on wind speed and wind direction. It is shown that for time steps of the order of seconds and adequate computational power ANN perform better than linear models (persistence models, ARMA) and other nonlinear models (Support Vector Machines). The outcome of the simulated races confirms that maximising the probability of winning a match race does not necessarily correspond to minimising the expected time needed to complete the race.