School of Physics - Theses
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ItemErbium for optical modulation and quantum computation( 2017)Erbium (Er) is a lanthanide element, mainly used in its trivalent ionic form (Er3+), as an active dopant in optical devices, for light amplification or generation. The luminescence of Er3+ lies within the conventional wavelength band, 1530-1565 nm, for fiber-optic communication. The low noise, linear response, and stability of the optical gain provided by the Er3+ luminescence are ideal for applications in photonic systems that operate in the fiber-optic frequency. While much research has been done to understand the Er3+ luminescence in various lasing media, few studies have been conducted to tap the potential of Er for applications other than amplifiers or lasers. This thesis delves into two new areas, namely optical modulation and quantum computation, where the Er3+ luminescence may be able to be applied in a novel way. By incorporating Er3+ into a switchable optical material, an optical modulator could potentially be made that is capable of not only switching but also amplifying signal transmission or sustaining the signal intensity from propagation losses. This integrated approach could reduce device footprint and latency for on-chip as well as synchronous applications. Successful integration of Er, however, has never been demonstrated in conventional optical modulators because their reliance on electro-optic effects conflicts with the carrier-sensitive mechanism of the Er3+ luminescence. The compatibility between Er and a recently advocated optical material, namely vanadium dioxide (VO2), is examined in the first part of this thesis. VO2 exhibits a hysteretic, bistable phase transition that is accompanied by a high-contrast optical switching in infrared, including the fiber-optic, wavelength band. The phase transition can be triggered thermally as well as optically. When triggered optically, it can occur in picosecond timescale, making VO2 a promising material for ultrafast optical switching applications. Experimental characterizations of the Er3+ luminescence and the optical switching were performed on selectively prepared thin-film samples of VO2. The Er3+ luminescence could be observed after the samples were implanted with Er and then annealed between 800*C and 1000*C. The optical switching could also be measured in the implanted and annealed samples as they were thermally heated up and then cooled down past the critical temperature of the phase transition. The Er-implanted samples, however, were found to have broader hysteresis and lower switching contrasts than the pure VO2 samples. It is concluded that although Er-implanted VO2 could probably work as a combined optical switch and amplifier, the poorer switching qualities do not guarantee that a device based on the material could provide better utility than a separated system of optical switches and Er amplifiers. The Er3+ luminescence could also be utilized for quantum frequency conversion, for implementation in interconnects that interface superconducting quantum computers to a fiber-optic quantum network. For two superconducting quantum computers to be able to communicate over a fiber-optic quantum network, the frequency of the signals transmitted from either computer needs to be converted into the fiber-optic frequency, and then back into the microwave frequency upon receipt at the other computer. Early proposals suggested that the interconnect be at least comprised of Er3+ ions, a microwave resonator, and an optical resonator. The realization of this system has been attempted recently in basic experiments, but the conversion efficiency was found to be too low. The weak couplings between the Er3+ ions and the two resonators were identified as one of the main reasons for the low conversion efficiency. One way to mitigate the weak coupling in the microwave part is to have a superconducting flux qubit bridge the interaction between the Er3+ ions and the microwave resonator. The second part of this thesis presents a theoretical and simulation study of the dynamics of a coupled system consisting of Er3+ ions, a superconducting microwave resonator, an optical resonator, and a superconducting flux qubit. It is shown that quantum information can be exchanged between the Er3+ ions and the microwave resonator with a high fidelity via the qubit coupling, and the exchange process is controllable by changing the frequency of the qubit. The frequency conversion between the microwave and the optical regime is shown to be infeasible to be realized at the limit where the number of optical excitations (n) is much less than the number of ions (N). A high-efficiency frequency down-conversion is demonstrated to be achievable in the case where there is no decoherence, and both n and N are small. However, the time it takes to complete the down-conversion is very long, leaving the efficiency prone to decoherence. It is argued that for the frequency conversion to be able to be accomplished in a typical decoherent environment, both n and N need to be large. The study of the dynamics, in this case, is left for future research.
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ItemFluorescent defects and optical structures in metal oxides( 2016)The emission properties from fluorescent defects and manipulation/control of this light were the main themes of this thesis. It begins with the exploration of the emission properties of a radiating dipole within a spherical nanoparticle (NP) and how it is affected by the surrounding refractive index environment. It is important to address this topic as it can give researchers preliminary insight to their system using relatively simple numerical method and modelling. The radiating dipole is treated classically and the finite-difference time-domain technique is used to solve the electrodynamics of systems with optically relevant environments. This method is not restricted to just the environments presented within, but can be generalised to any arbitrary system. The total radiated power from the dipole can be significantly affected by: the refractive index of the NP, the size of the NP and the surrounding refractive index environment of the NP. Experimentally relevant refractive index distributions were considered in this thesis. The mechanism of visible and infrared fluorescence from optical defects within metal oxides has been highly debated where materials scientists do not have a general consensus on the origin. One aspect that is agreed upon is that the visible emission is due to native point defects within the crystallographic structure. This thesis will focus on the optical characterisation of fluorescence defects in promising optical materials of zinc oxide (ZnO) and titanium dioxide (TiO2). The controversy begins with the abundance of fabrication methods for ZnO and TiO2 that inevitably introduces different defects based on the experimental conditions. The fluorescence characterisation by materials scientists involves measurements that sample many defects, due to a large spot size in commercial set-ups, and therefore individual defect signal is lost. The defects in ZnO and TiO2 explored here will be characterised using confocal microscopy, a high-resolution optical technique. This gives the ability to address individual defects with an appropriate resolution to isolate single defects. The fluorescence defects from a few morphologies of ZnO and TiO2 were shown to exhibit room-temperature single-photon emission and these were furthered characterised by investigating their: photoluminescence spectrum, photodynamics, power saturation and lifetimes. Finally, this thesis explores the control and manipulation of light through a structured environment which is central to the operation of an integrated optical circuit. An important integrated component known as an optical microcavity which confines and traps was explored in this thesis. A deterministic algorithm for an ultrahigh-Q nanobeam cavity was explored within TiO2 that operates at the visible wavelength of λ = 637 nm, the mean fluorescence wavelength of the TiO2 single-photon emitters also presented in this thesis. The electrodynamics of the system was calculated using the finite-difference time-domain method. Preliminary fabrication results were also presented. The trapped light with an optical cavity is routed and connected within an integrated optical circuit using waveguides. An optical waveguide design that exhibits coherent tunnelling adiabatic passage of light was explored. The solving and propagation of optical modes was obtained using a combination of _nite element method and beam propagation method techniques. The design showed robust light transfer despite significant perturbations to the optimised system The archetypal three-waveguide system can be extended to a five-waveguide system where this system acts as a power divider. The three-waveguide system was fabricated into tellurium dioxide, an emerging material for non-linear optical communication applications.
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ItemMagnetic fields and chaos in coupled atom-cavity systems( 2016)This thesis explores the manifestation of many-body quantum physics in coupled atom- cavity systems, through the lens of the Jaynes-Cummings-Hubbard model. This model captures the behaviour of a strongly interacting photon system, with the entailing complex many-body quantum states. In particular, this work explores two distinct, but related, topics in many-body quantum mechanics. The first part of the work is concerned with a number of phenomena arising from the introduction of (synthetic) magnetic fields to the Jaynes-Cummings-Hubbard model. In this context, the Mott-superfluid phase transition is suppressed by strong magnetic fields, and vortices are observed in the superfluids regime. Also studied is the regime of high synthetic magnetic field to particle density, where the fractional quantum Hall effect manifests. Laughlin-like states are indicated by a number of metrics. Pfaffian like states are also show to exist when a three-body interaction is induced by the inclusion of a three level atom in the photonic cavities. In the latter part of the thesis, the relationship between classical and quantum chaos is explored through the introduction of a periodically perturbing force to a pair of coupled cavities. Demonstrated in this system are such chaotic behaviours such as localization, partially chaotic phase space, dynamic localization and dynamic tunneling.