Physicshttp://hdl.handle.net/10523/982017-12-11T15:11:44Z2017-12-11T15:11:44ZCold Collisions of Ultracold AtomsThomas, Ryan Jameshttp://hdl.handle.net/10523/77762017-12-06T13:02:07Z2017-12-05T20:00:46ZCold Collisions of Ultracold Atoms
2017
Thomas, Ryan James
This thesis describes experiments investigating the collisions of alkali metal atoms at energies between 10 - 2000 uK, measured in units of the Boltzmann constant. The atoms are accelerated towards each other using a purpose-built collider comprised of a crossed-beam optical dipole trap, which enables us to collide dense ensembles of ultracold atoms in any internal state at relatively high energies. I present the results of two experiments centered around resonant enhancement of the collisions. The first is between homonuclear 40K atoms near a shape resonance where the fermionic nature of the atoms determines the character of multiple scattering effects. The second experiment involves heteronuclear 40K87Rb collisions near a magnetically-tunable Feshbach resonance where we measure parameters describing the resonance as a function of collision energy. Theoretical models are developed that let us describe the collisions using published empirical interaction potentials, and we find good agreement between these models and the experiment.
2017-12-05T20:00:46ZPhase ordering dynamics in a ferromagnetic spin-1 Bose-Einstein condensateWilliamson, Lewis Alexanderhttp://hdl.handle.net/10523/77182017-11-09T13:02:09Z2017-11-09T01:14:40ZPhase ordering dynamics in a ferromagnetic spin-1 Bose-Einstein condensate
2017
Williamson, Lewis Alexander
Spinor Bose-Einstein condensates exhibit both superfluid and magnetic order, and accommodate phases with rich symmetry properties and topological defects. Transitions between these phases can be induced by tuning external fields. In this thesis we explore the dynamics of order formation in a quasi-2D spin-1 ferromagnetic condensate following a quench from an unmagnetised phase to one of three ferromagnetic phases. The ferromagnetic phases exhibit distinct symmetry properties (easy-plane, easy-axis or isotropic) and support distinct topological defects. In each phase we observe scale invariant ordering and identify the relevant topological defect affecting the order parameter growth. We find that each phase is characterised by a distinct dynamic critical exponent. In the easy-plane phase we identify a persistent turbulent cascade that affects spin ordering long after all topological defects have annihilated. In addition to our exploration of phase ordering dynamics, we study a microscopic model of spin vortex dynamics in the easy-plane phase. Our work provides a comprehensive theoretical study of phase ordering in a conservative system, provides a thorough foundation for studies of phase ordering in antiferromagnetic and higher spin condensates, and offers prospects for further research into fundamental questions regarding the ordering properties of spin systems. Our work is pertinent to current experiments, which have explored the initial stages of phase ordering in both ferromagnetic and antiferromagnetic spin-1 condensates.
2017-11-09T01:14:40ZA Stand-Alone 'Self-Locking' LaserCowdell, Carolynhttp://hdl.handle.net/10523/75472017-09-17T23:20:08Z2017-09-17T23:18:17ZA Stand-Alone 'Self-Locking' Laser
2017
Cowdell, Carolyn
This dissertation presents a stand-alone ’self-locking’ laser. The aim of the project was to develop a prototype of a ’self -locking’ laser that requires no user input, other than to turn the laser on. The prototype was developed to be used as a repump laser for laser cooling experiments; however, its design is aimed at a market of individuals who do not have the background skills required to lock a laser. The project uses a frequency modulated spectroscopy setup to obtain the sub-Doppler atomic spectrum of Rubidium, which is demodulated to obtain zero-crossing linear slopes at the exact points of each atomic transition and crossover transition. The frequency modulation for the spectroscopy setup, the signal analysis, as well as the automatic locking and re-locking of the laser is all implemented digitally using an Arduino open source microcontroller. The distributed feedback laser used for the design is fully characterized and the lock of the ’self-locking’ laser is analyzed in detail. The finished prototype has been used in a laser cooling experiment as the repump beam to investigate how it performs under real experimental conditions.
2017-09-17T23:18:17ZCoherent Frequency Conversion from Microwave to Optical Fields in an Erbium Doped Y2SiO5 Crystal: Towards the Single Photon RegimeFernandez Gonzalvo, Xavierhttp://hdl.handle.net/10523/75462017-09-18T14:02:06Z2017-09-17T23:05:47ZCoherent Frequency Conversion from Microwave to Optical Fields in an Erbium Doped Y2SiO5 Crystal: Towards the Single Photon Regime
2017
Fernandez Gonzalvo, Xavier
In the context of quantum information technologies superconducting qubits (SQs) are very attractive devices for the manipulation of quantum states, and present themselves as one of our best candidates to build a quantum processor. They couple naturally to microwave photons, for which suitable quantum memories or a long distance propagation channel don’t exist. A way around these limitations is to turn these microwave photons into optical ones by building a quantum frequency converter: a device by which the frequency of a photon can be changed while preserving its non-classical correlations. Then, optical fibres could be used to link distant SQ-based devices together, facilitating the creation of a network of quantum computers. Moreover, SQs could then be coupled to quantum memories compatible with photons at optical frequencies, which are the most well developed kind of quantum memories at the present time.
This thesis explores the possibility to convert single microwave photons into optical photons using erbium doped in a yttrium orthosilicate crystal (Er3+:Y2SiO5). Er3+:Y2SiO5 is a good candidate because it has a naturally occurring optical transition near 1536 nm close to the point where silica optical fibres show their minimum loss. A microwave transition can be found in two different ways: one way is to use the 167 isotope of erbium, which is the only stable isotope that shows hyperfine splitting as it has non-zero nuclear spin. The hyperfine structure of the ground state of 167Er3+:Y2SiO5 spans over about 5 GHz. The other possibility is to use the other stable isotopes of erbium and Zeeman split their ground state using an external magnetic field. A microwave transition near 5 GHz can be achieved with moderate magnetic fields due to the high 𝑔-factors of Er3+:Y2SiO5.
The physical process of interest is a three wave mixing process involving two fields at optical frequencies and one field at microwave frequencies. In order to boost the efficiency of the frequency conversion process the Er3+:Y2SiO5 crystal is placed inside a microwave and an optical resonator. The problem is first explored from the theoretical point of view, where a nonlinear coefficient Λ(2) is derived (analogous to the 𝜒(2) often used in nonlinear optics), and the interaction between cavity modes and the nonlinear medium is studied. It is predicted that with a sample cooled down to millikelvin temperatures total frequency conversion between microwave and optical fields can be achieved.
A preliminary hole burning spectroscopy experiment is performed with the objective of reconstructing the hyperfine structure of the excited state of 167Er3+:Y2SiO5, but the complexity of the problem makes it too difficult to achieve this goal. Then a series of experiments are shown, aimed at determining whether or not frequency conversion at the single photon level is achievable using the even isotopes of erbium in a magnetic field. These experiments are based in the Raman heterodyne spectroscopy technique, which is used in combination with electron paramagnetic resonance and optical absorption spectroscopy. In all experiments the sample is cooled down to cryogenic temperatures near 4 K. A first experiment shows that the frequency conversion process exists in Er3+:Y2SiO5, in a setup where only a microwave resonator is used, but not an optical one. A second experiment is performed in a similar setup, this time presenting a quantitative study of the properties of the frequency conversion process, and its comparison with the theoretical model previously derived. A third experiment is performed, which incorporates an optical cavity to the system. The interaction between the erbium ions and the optical cavity introduces a whole new range of experimental complications, which are studied and discussed. Then, the frequency conversion signal is studied anew, showing an unexpected highly non-linear scaling behaviour with the input powers. A hypothesis explaining this unexpected behaviour is given, referring to stray optical absorption in the inhomogeneous line of Er3+:Y2SiO5 (and in particular 167Er3+:Y2SiO5), which can be bleached out under certain circumstances due to spectral hole burning effects. The overall maximum frequency conversion efficiency observed is of 3 × 10^−4 per Watt of pump laser power. While this value is still far from the target several ways of improvement are proposed, including cooling down the system to millikelvin temperatures, increasing the dopant concentration and modifying the geometry of the resonators.
2017-09-17T23:05:47ZQuantum Analogues of Two-Dimensional Classical TurbulenceReeves, Matthewhttp://hdl.handle.net/10523/75282017-09-01T14:02:06Z2017-09-01T02:26:53ZQuantum Analogues of Two-Dimensional Classical Turbulence
2017
Reeves, Matthew
Turbulence, the irregular motion of fluids, is a challenging problem in physics. Yet some properties of turbulence appear to be universal, independent of the underlying host fluid supporting the motion. Recent studies have found that turbulence in superfluid helium, a quantum fluid, exhibits two of the most fundamental laws of classical fluid turbulence: the Kolmogorov −5/3 law, and the dissipation anomaly. These laws appear despite the fluid being highly constrained by quantum mechanical effects, and completely lacking kinematic viscosity. Such findings suggest further insight into the universal features of turbulent phenomena can be gained by studying analogies between classical and quantum turbulence.
Atomic Bose-Einstein condensates (BECs) offer a new platform for the study of quantum turbulence; the geometric control available in BEC experiments offers the possibility of studying quantum turbulence in effectively two-dimensional fluids. As two-dimensional turbulence exhibits dramatically different features from its 3D counterpart, BEC systems allow for further study of the analogies between classical and quantum turbulence. In this thesis we numerically and theoretically study 2D quantum turbulence in BECs within the framework of the Gross-Pitaevskii model. We focus on analogies with classical 2D turbulence, with the aim of identifying common or universal features.
First we investigate coherent vortex structures in negative temperature equilibria via an experimentally accessible flow-field measure. Coherent vortices are shown to produce a clear signal in this measure that is independent of the confinement geometry, and we demonstrate that it can be observed in dynamical simulations.
Second, studying a quantum analogue of the two-dimensional cylinder wake, we investigate the phenomenon of Strouhal oscillations. We find that the Strouhal number obeys a universal relation, similar to the classical form, upon introducing a modified superfluid Reynolds number that accounts for the critical velocity for vortex nucleation. Like the classical Reynolds number, the superfluid Reynolds number is found to govern the transition from laminar to turbulent behaviour in the quantum fluid.
Finally, simulating decaying 2D quantum turbulence for very large vortex numbers, we show that quantum fluids are capable of supporting the direct enstrophy cascade, a fundamental feature of two-dimensional turbulent flows. The quantum fluid manifests key features of the classical cascade, including Batchelor’s −3 law of the inertial range, scaling of the inertial range against the superfluid Reynolds number, and the value of the Kraichnan-Batchelor constant.
The findings from this work thus provide some new insight into the universality of fundamental turbulence concepts, and their applicability to quantum fluids.
2017-09-01T02:26:53ZAn Investigation of Interference Lithography Applications using Evanescent FieldsBourke, Levi Earlhttp://hdl.handle.net/10523/75272017-08-31T14:02:08Z2017-08-30T20:42:42ZAn Investigation of Interference Lithography Applications using Evanescent Fields
2017
Bourke, Levi Earl
Since the dawn of large scale integrated circuitry photolithography has been the primary means of pattern production. Over the following 60 years the size of these patterns has shrunk massively, along with the consequent increase in the complexity of the photolithography process. The demand for smaller, more powerful, and more energy efficient computational devices requires further shrinking of the patterns. The development of processes for further pattern size reduction however is not a simple one, thus a great deal of research and investments has been focused towards it. The research within this thesis is aimed at discovering new methods and techniques for photolithography pattern reduction by exploiting fields in an interference lithography setting.
Previously it was shown that dielectric resonant underlayers could be employed to enhance the depth of field for evanescent interference lithography. This however is limited by the availability of transparent high refractive index dielectric layers. To extend this to higher effective refractive indices an investigation into applying Herpin effective media within resonant underlayers was carried out. These underlayers were shown to be effective for combinations which have propagating fields within at least one layer; for combinations where all layers were evanescent however, the method broke down. Investigations into generic resonant underlayers also led to the development of a resonant overlayer method for increasing the evanescent field strength within a PR layer while allowing thicker and/or lower refractive index IMLs.
Further to this a new form of BARC for hyper-NA photolithography termed an evanescent-coupled ARC was developed. These ARCs rely on evanescently-coupled dielectric or surface state polariton resonators to produce destructive interference within the PR. The properties and design constraints for each of these systems was explored and two experimental designs developed. Experiment verification of evanescent-coupled ARCs was successfully demonstrated for a SiO2jHfO2 dielectric resonator based ARC. Demonstration of a MgF2jCr surface state polariton resonator based ARC was partially demonstrated with resonance within the underlayer and the consequent alteration of the PR standing wave pattern observed.
The use of prism coupling for interference lithography is limited by the maximum refractive index of the coupling prism; above this refractive index all fields are evanescent and no energy will coupled into the PR. To overcome this limit grating coupled evanescent near-field interference lithography methods are employed. The higher order diffraction orders from the grating can have NAs far greater than the refractive index of naturally occurring materials, thus patterning with these diffraction orders produces far smaller interference pitches than prism coupled systems are capable of. Grating coupled systems involve the use of evanescent fields, plasmonic resonances,as well as coupled resonators all within subwavelength scales, consequently simulation and optimization of these systems is very computationally intensive. To improve this a genetic algorithm process was applied to reduce the computational time for optimization, and to allow the use of an inverse design process. Application of this method produced an order of magnitude improvement in optimization time compared to a full parameter sweep. Models including resonant overlayers, overlayers and underlayers, as well as those employing extremely high NAs and/or higher |m| diffraction orders were produced. Simulations showed that extremely high NAs up to 20 may theoretically be used for patterning of structures with a pitch of lambda/40 equating to a full pitch of 10.1 nm with an exposing wavelength of 405 nm.
2017-08-30T20:42:42ZAn Exergy Analysis of the New Zealand Energy SystemTromop van Dalen, Caitlin J Rhttp://hdl.handle.net/10523/74812017-07-26T01:14:08Z2017-07-26T01:13:53ZAn Exergy Analysis of the New Zealand Energy System
2017
Tromop van Dalen, Caitlin J R
Exergy analysis has been shown to be a valuable method of assessing energy use on a national scale. Exergy analysis measures the work potential of mass and energy flows, and therefore more accurately identifies areas with potential for improvement. This thesis presents an exergy analysis of the New Zealand energy system. Exergy flows, from resource inputs through transformation to output end-use, are calculated and presented in Sankey diagrams. Exergy flows for the total energy system, each resource category and each sector of the economy are presented. Exergy efficiencies are calculated from these flows. Energy flows and energy efficiencies are also presented for comparison. The total exergy efficiency for NZ is 22% and energy efficiency is 38%. Results show the transportation, residential, and commercial end-use sectors have the largest exergy losses. In transportation, the losses arise from the inefficiency of the conversion of chemical fuels to motive force in internal combustion engines. In the commercial and residential sectors the losses arise from the use of high exergy sources for low temperature space and water heating. This second loss would not be captured in an energy analysis. Another significant finding is the large impact of geothermal energy resources on the analysis. Geothermal energy currently makes up 26% of NZ total primary energy resources. Energy analysis does not account for the work potential in geothermal resources used for electricity generation, and therefore over estimates the potential of the geothermal resource. Due to the large percentage of geothermal in New Zealand’s energy system, this has a significant impact on overall results. Policy implications of these finding are discussed including a proposal to evaluate New Zealand’s energy productivity, the ratio of economic outputs to energy inputs, based on exergy rather than energy inputs to more accurately account for geothermal resources.
2017-07-26T01:13:53ZDevelopment of a Hyperspectral sensor system for model-based remote water constituent inferenceWest, Matthew Thomashttp://hdl.handle.net/10523/74782017-07-20T14:02:07Z2017-07-20T04:18:18ZDevelopment of a Hyperspectral sensor system for model-based remote water constituent inference
2017
West, Matthew Thomas
This project undertook a model-based inversion of spectral data for remote sensing of water constituents, from satellite based hyperspectral sensors. As the atmosphere effects dominate the observed spectrum, a parameterised model is needed for both the atmosphere, the water/atmosphere interface and the water itself. In order to calibrate these models, a sensor was developed for recording hyperspectral data. This sensor is portable and lightweight so that is can be used to simultaneously gather downwelling irradiance, as well as upwelling irradiance from a boat or other mobile platform during a HICO satellite sensor observation. We present data from apparatus deploying this sensor and perform model based inference on this data, as well as data from the HICO system.
2017-07-20T04:18:18Z