Physics
http://hdl.handle.net/10523/98
Thu, 21 Sep 2017 21:27:51 GMT2017-09-21T21:27:51ZA Stand-Alone 'Self-Locking' Laser
http://hdl.handle.net/10523/7547
A 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.
Sun, 17 Sep 2017 23:18:17 GMThttp://hdl.handle.net/10523/75472017-09-17T23:18:17ZCoherent Frequency Conversion from Microwave to Optical Fields in an Erbium Doped Y2SiO5 Crystal: Towards the Single Photon Regime
http://hdl.handle.net/10523/7546
Coherent 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.
Sun, 17 Sep 2017 23:05:47 GMThttp://hdl.handle.net/10523/75462017-09-17T23:05:47ZQuantum Analogues of Two-Dimensional Classical Turbulence
http://hdl.handle.net/10523/7528
Quantum 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.
Fri, 01 Sep 2017 02:26:53 GMThttp://hdl.handle.net/10523/75282017-09-01T02:26:53ZAn Investigation of Interference Lithography Applications using Evanescent Fields
http://hdl.handle.net/10523/7527
An 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.
Wed, 30 Aug 2017 20:42:42 GMThttp://hdl.handle.net/10523/75272017-08-30T20:42:42ZAn Exergy Analysis of the New Zealand Energy System
http://hdl.handle.net/10523/7481
An 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.
Wed, 26 Jul 2017 01:13:53 GMThttp://hdl.handle.net/10523/74812017-07-26T01:13:53ZDevelopment of a Hyperspectral sensor system for model-based remote water constituent inference
http://hdl.handle.net/10523/7478
Development 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.
Thu, 20 Jul 2017 04:18:18 GMThttp://hdl.handle.net/10523/74782017-07-20T04:18:18ZTight-binding methods for overdamped Brownian motion and their application to molecular motors
http://hdl.handle.net/10523/7449
Tight-binding methods for overdamped Brownian motion and their application to molecular motors
2017
Nguyen, Thi Thanh Phuong
Biomolecular motors are molecules that transform energy to perform physiological functions. The operation of molecular motors has a number of interesting physical properties that have attracted a great deal of investigation. One of the most compelling theories describes molecular motors as Brownian motion on a multidimensional free-energy potential landscape. Approximation methods are required to be able to compare this theory with experimental results.
In this thesis, we develop two tight-binding methods to systematically transform the continuous equation describing Brownian motion on a potential with deep wells to a much simpler discrete master equation that can be used to facilitate comparison with experiments. Both methods are based on an expansion in a basis of localized states. One of them, the generalized Wannier-state method, exploits the periodicity of the system to define localized states in terms of Bloch eigenfunctions. The other method, analogous to the atomic-orbital approach of quantum mechanics, defines localized states in terms of the eigenfunctions of local metastable potentials around individual wells.
For simplicity, we focus on the tight-coupling regime where the average diffusion dynamics on the multidimensional potential landscape can be approximated by the diffusion along a one-dimensional coupling channel.
We show that the tight-binding methods connect the master equation to properties of the potential landscape and allow the regime of validity of the master equation to be determined. Thus, in contrast to more phenomenological treatments, our work provides a link between discrete master equations and an underlying theory of molecular motor operation.
Finally, applying our method to particular model potentials, we find that the theory predicts qualitatively different discrete operation for molecular motors in different regimes that could be observed in current single-molecule experiments.
Tue, 11 Jul 2017 02:30:03 GMThttp://hdl.handle.net/10523/74492017-07-11T02:30:03ZSimulation of the Effect of Ice Shelf Melt around Antarctica in an Earth System Model
http://hdl.handle.net/10523/7425
Simulation of the Effect of Ice Shelf Melt around Antarctica in an Earth System Model
2017
Pauling, Andrew
The observed increase in Antarctic sea ice area over time is not reproduced by Earth System Models. One proposed reason for this discrepancy is that these models do not realistically represent ice shelves and the associated freshwater flux into the Southern Ocean due to basal melting. Previous work on the artificial addition of fresh water to the Southern Ocean has produced conflicting results depending on the model used. In this thesis results are presented from new experiments artificially enhancing the freshwater to the Southern Ocean in the Community Earth System Model version 1 (Community Atmosphere Model version 5) CESM1(CAM5) Earth System Model, building on previous experiments with the same model. Results were compared to the CESM1(CAM5) Large Ensemble (LENS), an available set of control runs of CESM1(CAM5).
Experiments have been conducted to test the response of the Southern Ocean and Antarctic sea ice to seasonally varying freshwater input, and to determine the residence time of the artificial freshwater signal after the forcing has been turned off. We have also tested the response to freshwater input that increases linearly over time, both with and without the effect of the latent heat required to melt the ice that is entering the ocean. The amount of freshwater input is much larger than present observations, in an effort to isolate the response from the variability of the system.
The seasonal freshwater enhancement experiments showed no significant difference in response from constant freshwater input at the same annual mean rate, due to the residence time of the freshwater signal being much longer than the period of the artificial freshwater input. Experiments with linearly increasing freshwater input over time without latent heat uptake resulted in a small positive trend in sea ice area in the austral summer, winter and spring, although the response was not significantly different from the LENS in autumn. The experiments with linearly increasing freshwater enhancement and latent heat uptake resulted in positive trends in sea ice area that were significantly higher than the LENS, and sea ice area magnitude up to 2.1 × 106 km2 greater than the LENS mean. This response is attributed to a combination of the indirect cooling effect of the stratification-induced reduction in vertical heat advection from depth and the direct cooling effect of latent heat uptake. The enhanced sea ice melt/freeze cycle in the experiments with latent heat uptake resulted in less freshening near the continent and greater freshening further north. This reduced ocean stratification meant that the direct cooling effect of the latent heat uptake from the ocean was the dominant mechanism in determining the sea ice response to freshwater input from ice shelves.
Thu, 06 Jul 2017 04:00:03 GMThttp://hdl.handle.net/10523/74252017-07-06T04:00:03Z