Medical research and clinical diagnostics require imaging of large sample areas with sub-cellular resolution. Conventional imaging techniques can provide either high-resolution or wide field-of-view (FoV) but not both. This compromise is conventionally defeated by using a high NA objective with a small FoV and then mechanically scan the sample in order to acquire separate images of its different regions. By stitching these images together, a larger effective FoV is then obtained. This procedure, however, requires precise and expensive scanning stages and prolongs the acquisition time, thus rendering the observation of fast processes/phenomena impossible. A novel imaging configuration termed Multi-Aperture Fourier Ptychographic Microscopy (MA-FPM) is proposed here based on Fourier ptychography (FP), a technique to achieve wide-FoV and high-resolution using time-sequential synthesis of a high-NA coherent illumination. MA-FPM configuration utilises an array of objective lenses coupled with detectors to increase the bandwidth of the object spatial-frequencies captured in a single snapshot. This provides high-speed data-acquisition with wide FoV, high-resolution, long working distance and extended depth-of-field. In this work, a new reconstruction method based on Fresnel diffraction forward model was developed to extend FP reconstruction to the proposed MA-FPM technique. MA-FPM was validated experimentally by synthesis of a 3x3 lens array system from a translating objective-detector system. Additionally, a calibration procedure was also developed to register dissimilar images from multiple cameras and successfully implemented on the experimental data. A nine-fold improvement in captured data-bandwidth was demonstrated. Another experimental configuration was proposed using the Scheimpflug condition to correct for the aberrations present in the off-axis imaging systems. An experimental setup was built for this new configuration using 3D printed parts to minimise the cost. The design of this setup is discussed along with robustness analysis of the low-cost detectors used in this setup. A reconstruction model for the Scheimpflug configuration FP was developed and applied to the experimental data. Preliminary experimental results were found to be in agreement with this reconstruction model. Some artefacts were observed in these results due to the calibration errors in the experiment. These can be corrected by using the self-calibration algorithm proposed in the literature, which is left as a future work. Extensions to this work can include implementing multiplexed illumination for further increasing the data acquisition speed and diffraction tomography for imaging thick samples.

Quantum state discrimination is a fundamental task in the field of quantum communication and quantum information theory. Unless the states to be discriminated are mutually orthogonal, there will be some error in any attempt to determine which state was sent. Several strategies to optimally discriminate between quantum states exist, each maximising some figure of merit. In this thesis we mainly investigate the minimum-error strategy, in which the probability of correctly guessing the signal state is maximised. We introduce a method for constructing the optimal Positive-Operator Valued Measure (POVM) for this figure of merit, which is applicable for arbitrary states and arbitrary prior probabilities. We then use this method to solve minimum-error state discrimination for the so-called trine states with arbitrary prior probabilities - the first such general solution for a set of quantum states since the two-state case was solved when the problem of state discrimination was first introduced. We also investigate the difference between local and global measurements for a bipartite ensemble of states, and find that in certain circumstances the local measurement is superior. We conclude by finding a bipartite analogue to the Helstrom conditions, which indicate when a POVM satisfies the minimum-error criteria.

This thesis describes the magnetic properties of three-dimensional (3D) magnetic nanostructures fabricated using two-photon lithography (TPL) and measured by the magneto-optical Kerr effect (MOKE), with supporting micromagnetic simulations. Simulations were performed on individual cylindrical nanowires composed of permalloy (Py) with diameter 50nm and length 1.5μm. Hysteresis loops were taken where the external field was applied at angles of 0o, 35.25o, 54.75o, 70.5o and 90o with respect to the wire axis. From investigating the visualisations of the magnetisation it was observed that Bloch point domain walls were propagated down the wire during the switching process. The resulting hysteresis loops were compared with those generated from simulations performed on tetrapod structures in the principle directions [100], [110] and [111]. It was found that the [100] and [110] loops were reproducible from the individual wire loops where the field angles matched. However, the [111] loop could not be reproduced due to domain wall propagation through the vertex. Angled 3D nanowires and 3D magnetic structures in tetrapod geometry made of cobalt (Co) were fabricated using two-photon lithography and electrodeposition. From SEM imaging, the nanowires in both structures were found to have an elliptical profile with feature sizes of 750±40nm lateral width, 910±30nm length perpendicular to the wire axis and approximately 8μm length for the individual nanowires, and feature sizes of 610±55nm lateral width, 900±75nm length perpendicular to the wire axis and approximately 6.5μm length for the nanowires comprising the tetrapods. MOKE measurements were performed on 300μmx300μm arrays of both, in the polar and longitudinal geometries. It was found that the polar MOKE loops were comparable between the two structures, whereas the longitudinal MOKE loops for tetrapods showcased unexpected transition events at higher fields and could not be reproduced from the measurements of the single wires.

This thesis presents the development of the first SQUID-based ac-magnetometer built in a dry dilution fridge. The possibility of expanding the frequency respond from tens of kHz of a commercial magnetometer to the bandwidth of muon-spin relaxation μSR (MHz), an indirect magnetic probe, which measures magnetic dynamics from the depolarisation of fundamental particles, is demonstrated. It opens a new scenario to investigate classical and quantum magnetic fluctuations by a direct probe in a range of frequencies that have an important role in exotic magnetic phases. Geometrical spin ices, frustrated magnets where magnetic excitations can be deconfined forming monopoles are good candidates to investigate. The study of magnetic fluctuations will help to understand the magnetic dynamics of these elementary excitations. Quantum spin liquids, frustrated systems without long-range order but with spins highly correlated that still fluctuate down to zero Kelvin are also interesting systems. The temperature dependence of their quantum fluctuations investigated by μSR have shown a dynamical plateau that might be corroborated by susceptibility measurements. The fabrication of the magnetometer has been combined with μSR investigations of the ground state of NbFe2 and CeRhIn5. The magnetic ground state of the ferromagnetic quantum critical point induced by growing around 1% Nbrich Nb1−yFe2+y is claimed to be reached in this kind of clean itinerant systems by a long–range spin density wave (SDW), although several attempts to identify the ground state by neutron scattering were unsuccessful. The μSR measurements prove that the ground state of the stoichiometric compound is governed by static and short-range correlations or by an incommensurate and helical SDW. The heavy fermion CeRhIn5 was investigated at ambient pressure due to the coexistence of antiferromagnetic and superconducting order in an intermediate region of pressures. The filamentary or bulk nature of the superconducting phase is still until debate and the onset of superconductivity at ambient pressure reported in some works may shed some light into the nature of the ground state. μSR and resistivity measurements were carried out to investigate the antiferromagnetic and helical phase without any signature of a superconducting transition.

We have developed a method of site-specific attachment of proteins to pristine carbon nanotube (CNT) sidewalls using genetically encoded unnatural amino acids with functional moieties. Here, we incorporated an azido phenylalanine (AzPhe) group at different positions in the protein to assess the importance of different protein-CN configurations. This site-directed mutation of the protein structure has provided routes for direct covalent attachment to the sidewall of the CNT, as well as providing a functional group for modification with another molecule such as pyrene for non-covalent CNT attachment. We have employed a variety of techniques to study these nanoscale systems in order to gain an insight into the binding mechanism, the protein-CNT interaction dynamics and their electronic properties. Using atomic force microscopy (AFM), we found that the proteins bind regular patterns dictated by the position of the phenyl azide. By integrating CNTs into electrical transistor-like devices, we have performed electrical measurements across the CNTs to monitor the attachment of various proteins to the CNT sidewalls. Using these protein-CNT systems, we have also been able to study the interaction between proteins and CNTs using Raman spectroscopy and total internal reflection fluorescence microscopy (TIRFM). Here we present evidence of covalent attachment of proteins to CNTs using Raman spectroscopy to study the changes in vibrational modes present in the CNTs. TIRFM has provided evidence of post cross-linking activity of proteins on CNTs using super-folder green fluorescent protein (sfGFP) as a marker. By analysing the protein's fluorescent properties, we have produced evidence suggesting the importance of the orientation of the protein with respect to the CNT, which in turn determines the distance between the CNT interface and the active site of the protein. The approach developed here provides a versatile and convenient generic approach to interfacing proteins, in defined orientations, to CNTs that holds promise for exploitation in bioelectronics tools and biomolecular sensors.

In this thesis, an original quantum dot material (InAsP) was introduced and characterised as a prospective laser material for applications in biophotonics and monolithic mode-locking. InAsP quantum dot material was grown in conditions that are appropriate for InP QD, which is the standard device in this study. The reasons for employing this material are to shift the emission to longer wavelengths than can be achieved with InP QD laser. In principle, by using both the InP and InAsP QDs in a single structure, a very wide gain spectrum can be produced that may be advantageous for passive mode-locking. The characteristics of the InAsP QD lasers were determined and compared with the standard device (InP QD laser) in this work, such as threshold current density, laser efficiency, lasing wavelength and temperature dependency of the threshold current density for different cavity lengths (1, 2, 3 and 4mm). The results show a shifting in the emission wavelengths by 55 nm toward longer wavelengths, while maintaining useable threshold current density and laser efficiency. For example, the 2mm long InP laser has a threshold current density of 170 A.cm-2 at room temperature, whereas for the same length, the InAsP QD laser has 260 A.cm-2. Moreover, both samples delivered optical powers of at least 250 mW. Edge-photo voltage spectroscopy (E-PVS measurements) confirmed deeper dot confinements for the InAsP materials by approximately 103 meV. The modal absorption spectra show a greater degree of inhomogeneous broadening for the InAsP QD materials, which was consistent with the dot size variation shown in TEM images for the InAsP wafer. This can support mode-locking in this material by broadening the optical gain spectra, which was also observed in this material. Gain-current measurements at different temperatures; specifically, 150, 200, 250, 300, 350, and 400 K, illustrate that InAsP QD material has a wider gain band-width at all studied temperatures. The carrier distribution study shows that the InAsP QD material tends to be non-thermally populated at 150 K. And also the recombination rate of this material is faster than the InP QD materials. Both of these points can be positive in relation to mode-locked performance.

In this thesis is presented a brief review of quantum computing, the DiVincenzo criteria, and the possibility of using a solid state system for building a quantum computing architecture. Donor electron systems in silicon are discussed, before chalcogen, \deep", double donors are suggested as a good candidate for fulfilment of the criteria; the optically driven Stoneham proposal, where the spin-spin interaction between two donor electron spin qbits is mediated by the optically controlled, excited, state of a third donor electron, forms the basis of this [1]. Coherence lifetimes are established as a vital requirement of a quantum bit, but radiative lifetimes must also be long. If the spin-spin interaction between qbits is decreased, or turned off, by the de-excitation of the mediating donor electron then the coherence of the qbit is rendered irrelevant; de-excitation will ruin quantum computations that depend upon an interaction that only happens when the mediating electron is in an excited state. Effective mass theory is used to estimate excited state donor, 2P, wavefunctions for selenium doped silicon, and recent Mott semiconductor to metal transition doping data [2] is used to scale the spatial extent of the 1S(A1) ground state wavefunction. Using these wavefunctions, the expected radiative lifetimes are then calculated, via Fermi's golden rule, to be between 9 ns and 17 ns for the 2P0 state, and 12 ns to 20 ns for the 2P_1 state. Fourier Transform InfraRed (FTIR) absorbance spectroscopy is used to determine the optical transitions for selenium donors in silicon, this has allowed agreement between literature, measured, and effective mass theory energy values for the particular samples measured. FTIR time resolved spectroscopy has then been used to measure the radiative emission spectrum of selenium doped silicon samples at 10-300K, following a 1220 nm laser pulse. Fitting to the exponentially decaying emission data, selenium radiative lifetimes as long as 80 ns are found; for the 2P0 to 1S(A1) transition in an atomic selenium donor complex at 10K. A factor of between 4 and 8 agreement is found between calculated and measured radiative lifetimes. This offers the possibility of nanosecond scale donor electron coherence times for chalcogen dopants in silicon.

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No description available.

Artificial spin ice (ASI) is a class of magnetic patterned arrays consisting of interacting ferromagnetic nanomagnets. The nano-scale size and the elongated shape of each nanomagnet ensure the formation of a single domain, which behaves as a ‘macrospin’ and results in only two possible magnetisation directions along the long axis of the nanomagnet. ASI system has the potential ability not only as a magnonic crystal because of the microwave properties being associated with its intrinsically intricate magnetisation topologies and inter-element interaction, but as a tool to model the microstructure of atomic scale allowing its fundamental physics to be studied. This thesis addresses the field-induced properties of the static and dynamic magnetisation in square and pinwheel ASI. Firstly, the magnetic properties of the square ASI specimens were characterised using alternating gradient force magnetometry, Brillouin light scattering and ferromagnetic resonance. Micromagnetic simulations were employed to assist in understanding the experimental results. Secondly, the field-induced evolution of the magnetisation configuration in a finite-size pinwheel ASI array was imaged using Lorentz transmission electron microscopy. The square structure of the square ASI lattices allows a comparison of the response of the spin-wave modes in the two groups of magnetic elements which are orthogonally aligned to one another. The frequency of the spin-wave mode is dependent on the direction of the applied field, either along the easy or hard axes of the nanomagnet. It has been found that more spin-wave modes are found when the magnetic field lies along the hard axis of the nanomagnet compared to when the field is aligned with the easy axis of the island. This attributes to the formation of more edge modes of standing spin waves in the former case. The experimental behaviour of the static and dynamic magnetisation can be well described via the micromagnetic simulations where only an individual island is considered with an assumption that the inter-island interaction is negligible. Additionally, the field direction with respect to the square ASI lattices is also responsible for the changes in spin-wave frequencies. The results imply that the square ASI could act as a reconfigurable microwave resonator due to its spin-wave frequency being dependent on the changes in magnetisation configuration that were controlled by the applied field. The dependence of the nanomagnet thickness on the static and the dynamic properties of the square ASI was studied. The nanomagnet thickness is found to be responsible for the coercivity and the number of observed spin-wave modes of the square ASI array. The thicker ASI array has a larger coercive field and produces more spin-wave modes. Micromagnetic simulations suggest that the inter-island coupling contributes weakly to the coercivity and the spin-wave frequency of the thicker array whereas it is negligible for a thinner array. Furthermore, fitting to ferromagnetic resonance data allows for access to information on ferromagnetic parameters, such as gyromagnetic ratio and saturation magnetisation. Finally, static and dynamic magnetisation topologies in a pinwheel ASI is explored as a function of magnetic field. The pinwheel ASI is a square ASI modified by rotating each nanomagnet 45° around its central axis in the same direction. The energy spread between the pinwheel vertices significantly decreased as the geometrical structure transforms from the square vertices to the pinwheel vertices. The ferromagnetic magnetisation process shows the domain growth mediated via the propagation of domain walls. Intriguingly, some of the observed mesoscopic domain-wall topologies resemble the Néel and the crosstie walls seen in natural ferromagnetic films, while others mimic the configurations of the charged walls found in the ferroelectric materials. In addition, a rotational-field demagnetisation was carried out in order to anneal the pinwheel ASI to the ground state. The results show that the net moment of the entire array decreases and the short-range ground state is attained through the presence of the vortices (Type III) and antivortices (Type IV) vertices, rather than the global Landau-like flux closure structure predicted by Monte Carlo simulations.