This thesis details the design, construction and characterisation of an ultracold atoms system, developed in conjunction with a flexible optical trapping scheme which utilises a Liquid Crystal Spatial Light Modulator (LC SLM). The ultracold atoms system uses a hybrid trap formed of a quadrupole magnetic field and a focused far-detuned laser beam to form a Bose-Einstein Condensate of 2×10⁵ ⁸⁷Rb atoms. Cold atoms confined in several arbitrary optical trapping geometries are created by overlaying the LC SLM trap on to the hybrid trap, where a simple feedback process using the atomic distribution as a metric is shown to be capable of compensating for optical aberrations. Two novel methods for creating flexible optical traps with the LC SLM are also detailed, the first of which is a multi-wavelength technique which allows several wavelengths of light to be smoothly shaped and applied to the atoms. The second method uses a computationally-efficient minimisation algorithm to create light patterns which are constrained in both amplitude and phase, where the extra phase constraint was shown to be crucial for controlling propagation effects of the LC SLM trapping beam.
This thesis details the design, construction and characterisation of an ultracold atoms system, developed in conjunction with a flexible optical trapping scheme which utilises a Liquid Crystal Spatial Light Modulator (LC SLM). The ultracold atoms system uses a hybrid trap formed of a quadrupole magnetic field and a focused far-detuned laser beam to form a Bose-Einstein Condensate of 2×10⁵ ⁸⁷Rb atoms. Cold atoms confined in several arbitrary optical trapping geometries are created by overlaying the LC SLM trap on to the hybrid trap, where a simple feedback process using the atomic distribution as a metric is shown to be capable of compensating for optical aberrations. Two novel methods for creating flexible optical traps with the LC SLM are also detailed, the first of which is a multi-wavelength technique which allows several wavelengths of light to be smoothly shaped and applied to the atoms. The second method uses a computationally-efficient minimisation algorithm to create light patterns which are constrained in both amplitude and phase, where the extra phase constraint was shown to be crucial for controlling propagation effects of the LC SLM trapping beam.
This thesis describes the progress achieved towards two goals. One is the construction of a compact magneto-optical trap (MOT) apparatus, used both as a standalone undergraduate laboratory experiment and as part of a public engagement demonstration of cold atom physics. The setup, which utilises a dichroic atomic vapour laser lock (DAVLL) system plus homemade sideband generation electronics, is capable of trapping and cooling up to 8x10⁷ ⁸⁵Rb atoms in a standard six-beam laser cooling scheme. The second goal is the loading of an ultracold ensemble of ⁸⁷Rb atoms into novel optical trap geometries using a phase-only liquid crystal spatial light modulator (LC SLM) for applications to atomtronics and quantum simulation. Details of a double vacuum chamber apparatus, designed to produce the first Bose-Einstein condensates (BECs) at St Andrews, are provided. The setup incorporates a hybrid trap evaporative cooling scheme towards quantum degeneracy and reliably produces BECs of 1.9 10⁵ ⁸⁷Rb atoms. Two computational techniques, developed during the course of this project, are presented with the aim of creating flexible, smooth holographic optical traps. The first method illuminates a single SLM with overlapped, co-propagating light beams of different wavelengths to create a composite optical pattern for atom manipulation. The second uses conjugate gradient minimisation to create exceptionally high fidelity light patterns in both amplitude and phase for light in the output plane. These techniques, combined with the implementation of a light sheet into the experimental setup, are used to trap atoms in both a ring profile and in a set of narrow waveguides connected with a tunnelling barrier.
The development of new laser beam shaping methods is important in a variety of fields within optics, atomic physics and biophotonics. Spatial light modulators offer a highly versatile method of time-dependent beam shaping, based on imprinting a phase profile onto an incident laser beam that determines the intensity in the trapping plane laser field. The calculation of the required phase is a well-known inverse problem, which can be tackled with different approaches. Our method based on conjugate gradient minimisation [1] not only allows the calculation of smooth and accurate intensity profiles suitable for trapping cold atoms, but can also be used to generate multi-wavelength traps [2] and for simultaneous control over both the intensity and the phase of the light [3], with exceptionally high reconstruction fidelity. References: [1] T Harte, et al., "Conjugate gradient minimisation approach to generating holographic traps for ultracold atoms" Opt. Express 22, 26548 (2014) [2] D Bowman, et al., "Multi-wavelength holography with a single spatial light modulator for ultracold atom experiments" Opt. Express 23, 8365 (2015) [3] D Bowman, et al., "High-fidelity phase and amplitude control of phase-only computer generated holograms using conjugate gradient minimisation" Opt. Express 25, 11692 (2017)
This Thesis contains details of the construction and characterisation of a compact apparatus for the cooling of ultracold atoms to quantum degeneracy, and their manipulation in flexible holographic optical traps. We have designed and built two iterations of this apparatus. The first version consists of a stainless steel single-cell vacuum chamber, in which we confine ⁸⁷Rb and ⁶Li or ⁷Li in a Magneto-Optical Trap. We characterise the alternative methods of pulsed atomic dispenser and Light Induced Atomic Desorption (LIAD) to rapidly vary the background pressure in the vacuum chamber with the view to enabling efficient evaporative cooling in the single chamber, loading MOTs of up to 10⁸ atoms using pulsed dispensers. The LIAD is found to be ineffective in loading large MOTs in this setup, while the pulsed dispensers method gradually increases the background pressure in the chamber over time. \nBased on the results of this first iteration, we designed and built a second single-chamber apparatus for cooling of ⁸⁷Rb to quantum degeneracy. The LIAD technique was used to successfully load MOTs containing 8x10⁷ atoms in this single pyrex cell with a rapidly-varying background pressure. The lifetime of an atomic cloud loaded from the MOT into a magnetic trap increased by a factor of 6 when LIAD was used. \nThe holographic optical traps for cold atoms are generated using a Spatial Light Modulator, and we present our novel method for improving the quality of holographic light patterns to the point where they are suitable for trapping ultracold atoms using a feedback algorithm. As demonstrations of this new capability, we show power-law optical traps which provide an efficient, reversible route to Bose-Einstein Condensation and a dynamic ring trap for the investigation of superfluidity in cold atoms.
This thesis describes the design, construction and optimisation of two compact \nsetups to produce ⁸⁷Rb Bose-Einstein condensates and dual ⁷Li-⁸⁷Rb Magneto-Optical Traps (MOTs). The motivation for compact systems is to have simplified \nsystems to cool the atoms. The first experimental setup is based on a single pyrex \nglass cell without the need for atom chips. Fast evaporation will be achieved in a hybrid trap comprising of a magnetic quadrupole trap and an optical dipole trap \ncreated by a Nd:YVO4 laser and with future plans of using a Spatial Light Modulator \n(SLM). To enhance an efficient and rapid evaporation, we have investigated Light-Induced Atomic Desorption (LIAD) to modulate the Rb partial pressure during the cooling and trapping stage. With this technique, a ⁸⁷Rb MOT of 7 x 10⁷ atoms was loaded by shining violet light from a LED source into the glass cell, whose walls are coated with rubidium atoms. The atoms were then cooled by optical molasses and then loaded into a magnetic trap where lifetime measurements demonstrated that LIAD improves on magnetically-trapped atoms loaded from constant background pressure by a factor of six. This is quite encouraging and opens the possibility to do a rapid evaporation. In a second experiment, we have designed a compact system based on a stainless steel chamber to trap either ⁷Li or ⁶Li atoms in a MOT loaded from alkali-metal dispensers without the need of conventional oven-Zeeman slower. This setup can also load ⁸⁷Rb atoms, allowing future projects to simultaneously produce degenerate quantum gases of bosonic ⁸⁷Rb and fermionic ⁶Li atoms.
The long term aim of the work is to study non-equilibrium and symmetry-breaking phenomena for a Bose-Einstein condensate in time-dependent optical traps. The work presented in this thesis is mostly devoted to the design, construction and testing of the apparatus, in which condensation was successfully achieved. The first part of this thesis describes the design and construction of a hybrid setup for the production of a Bose-Einstein condensate (BEC) of 87 Rb in the |F = 2, m F = +2 > state. The vacuum system is composed of a first chamber, where a low-velocity intense source (LVIS) of atoms is produced to provide a pre-cooled atomic beam to the science chamber, where the magneto-optical trap (MOT) is produced, trapping more than 3 x 10 8 atoms at a temperature of roughly 400 μK. Forced radio- frequency evaporation is performed in a quadrupole magnetic trap and the atoms are transferred to a hybrid far-detuned optical trap in a crossed-beams configuration; evaporation in the optical potential leads to reliable production of a BEC every 50 s, composed of 10^5 atoms at a temperature of 10-30 nK. To ensure fine control on the optical potential depth and high intensity and pointing stability, a stabilisation system has been developed and characterised. The second part of this thesis describes the realisation of a 1D optical lattice, produced by two phase-locked laser beams emitting at 780 nm, with a red detuning of approximately 3.5 GHz, in counter-propagating configuration. Oscillating forces obtained by phase-modulating the optical potential are applied.
We describe several novel techniques for the optical trapping of ultracold atoms and for the production of wavelength-stabilised, coherent light at the frequencies required for use in atomic physics experiments. The greater part of this thesis deals with work towards the creation of regular arrays of microscopic optical dipole traps formed at the foci of truncated spherical cavities in a metallic film, in which the inter-site spacing can be set anywhere between one and several hundred micrometers. Arrays of such cavities are synthesised and structurally characterised via optical and electron microscopy, and numerical simulations of the light intensity distribution near the foci of such cavities under normal illumination are used to confirm their suitability for dipole trap production. A method for the construction of arrays of magneto-optical traps based on such structures is proposed and theoretically examined, and some preliminary experimental work towards the synthesis of the required microstructures is also described. Possible approaches to the loading of such traps and the imaging of the atoms contained therein are discussed - experimental work towards ballistic atom transfer from a specialised form of magneto-optical trap that can be formed close to a microstructured surface is carried out, and the efficacy of wavelength-selective fluorescence imaging as a means of reducing the effects of background scatter from the surface is experimentally demonstrated. Further work described herein includes the proposal and experimental demonstration of two novel techniques for the removal of the carrier wave from a phase-modulated laser beam, one of which is based on a fiber-optic Mach-Zehnder interferometer that is shown to be an effective device for splitting or combining beams of nearly equal frequencies. A spontaneous-force based atom trapping mechanism that does not rely on the use of a magnetic field, but rather on spatially-dependent optical pumping between different metastable atomic states, is also proposed, and a proof of principle experiment is carried out to demonstrate the validity of the suggested mechanism. We find that this trapping scheme allows the spatial dependence of the trapping force to be tailored with a greater degree of flexibility than is usually possible with magneto-optical trapping, and that it is also capable of producing traps with stronger spring constants than are typically achievable with magneto-optical trapping under realistic experimental constraints.