In recent years great progress was made in the spatial control of light with dynamic phase and amplitude modulators such as spatial light modulators and digital micromirror devices. In this work we describe the theory and practice of light shaping with such devices, detailing the spatial control of amplitude, phase and polarisation of coherent laser beams. We use our expertise in generating and measuring light fields with spatially dependent polarisation structures to characterise the correlations between spatial modes and polarisation in such light fields. We do this by adapting concurrence, a quantum measure of entanglement, to these classical correlations. One of the most promising application of coherent laser light is in the control of atomic media via atom-light interactions. In this work we describe the construction of simple external cavity diode lasers designed for the generation of resonant light for atomic physics applications. We exploit these lasers and spatial light modulators to create and measure three-dimensional atomic population structures in a warm rubidium vapour. We also implement a magneto-optic and a dynamic dark spontaneous-force optical trap for rubidium. These traps produce dense (~ 10^11 cm^-3) and cold (~100 uK) clouds of rubidium atoms. We develop the theory of spatially dependent electromagnetically induced transparency in such traps using rate equations. We find that the absorption of linearly polarised light depends on the relative direction of a magnetic field and the light polarisation. We use the cold atom clouds to measure the direction of magnetic fields by using this dependence and laser beams with structured polarisation.
In recent years great progress was made in the spatial control of light with dynamic phase and amplitude modulators such as spatial light modulators and digital micromirror devices. In this work we describe the theory and practice of light shaping with such devices, detailing the spatial control of amplitude, phase and polarisation of coherent laser beams. We use our expertise in generating and measuring light fields with spatially dependent polarisation structures to characterise the correlations between spatial modes and polarisation in such light fields. We do this by adapting concurrence, a quantum measure of entanglement, to these classical correlations. One of the most promising application of coherent laser light is in the control of atomic media via atom-light interactions. In this work we describe the construction of simple external cavity diode lasers designed for the generation of resonant light for atomic physics applications. We exploit these lasers and spatial light modulators to create and measure three-dimensional atomic population structures in a warm rubidium vapour. We also implement a magneto-optic and a dynamic dark spontaneous-force optical trap for rubidium. These traps produce dense (~ 10^11 cm^-3) and cold (~100 uK) clouds of rubidium atoms. We develop the theory of spatially dependent electromagnetically induced transparency in such traps using rate equations. We find that the absorption of linearly polarised light depends on the relative direction of a magnetic field and the light polarisation. We use the cold atom clouds to measure the direction of magnetic fields by using this dependence and laser beams with structured polarisation.
Whether in art or physics, two dimensions are better than one. And in this context, we consider the spatial shaping of optical fields and atomic polarisations. This work begins with a comprehensive review of how to accurately and arbitrarily modulate transverse beam profiles using six different methods. The methods are presented in the context of a general complex input beam and the shaping and measuring of the input beam with a single SLM is discussed and demonstrated. A simple setup is then considered that allows for the rapid switching between arbitrary profiles, using only a single holo- graphic component and an acousto-optical modulator (AOM). In this setup, a switching speed of approximately 10 kHz is demon- strated explicitly, limited by the current detection system, but where speeds of up to 20 MHz are estimated to be possible. The following discussion then concentrates on the spatial structure of atoms, reviewing the conceptual tools needed to measure and interpret an atomic cloud in the presence of a mag- netic field from fundamental concerns, both in theory and in practice: assuming a classical light field and a quantum atomic system. The specific implementation of both a magneto-optical trap and a holographically-shaped dynamic dark SpOT follows. The crux of the work then concerns how polarisation-structured light can be used to create spatial patterns of transparency (spa- tial EIT) within an atomic cloud. Beginning with a review of EIT in general, with particular focus on an F = 1 → F = 0 transition, the spatial modulation of absorption, dispersion, polarisation rotation and change in ellipticity is predicted for systems in the presence of a transverse magnetic field. The depth of this mag- netic field dependency is then explored in some detail, where the relationship between observed patterns and applied magnetic field suggests the possibility of a visual magnetometer. The final section then considers how linearly polarised light and a q-plate was used to demonstrate spatially dependent transparency in a real atomic cloud.
This thesis is concerned with the guiding of cold atoms using optical forces, which is of great importance in the field of atom optics. Atomic beams can be used for precision sensor equipment, building nano-scale structures, construction of quantum computers and to further the understanding of the properties of atoms. Atoms are guided along light beams using the dipole force; there are two regimes under which this force works. Typically red-detuned guides are used (atoms are attracted towards the light) such guides, however, require large detuning and high powers. In this thesis we investigate the use of blue-detuned (atoms are repelled from the light) hollow light beams of moderate power (a few hundred mW) and confine atoms in the dark centre of the beams. Several magneto-optical traps (MOTs) have been constructed to exploit different guiding geometries. Hollow beams have been generated using a variety of methods; in particular the use of a computer controlled spatial light modulator (SLM) has provided great versatility and simplicity to the experimental arrangements. First, experiments were performed with a low-velocity intense source (LVIS) of atoms. A co-linear LG beam significantly enhances the observed flux, however, considerable difficulties are encountered loading atoms into oblique guides. Imaging a hole in the walls of the light tube was used to improve the loading efficiency. Second, guiding a free-falling atom cloud is performed using a non-diffracting Bessel beam. It is found that while the potential of the Bessel beam is steeper than equivalent LG beams the power distribution across the beam severely limits its usefulness. The next study investigated higher-order LG guide beams generated with an SLM. High order modes have a narrower profile so confine the atoms with less interaction with the guide beam, leading to a more natural guide (as opposed to a pushing force). Finally the SLM was used to create non-trivial beam shapes for beam splitters and interferometers.
Recent years have seen vast progress in the generation and detection of structured light, with potential applications in high capacity optical data storage and continuous variable quantum technologies. Here we measure the transmission of structured light through cold rubidium atoms and observe regions of electromagnetically induced transparency (EIT), using the phase profile as control parameter for the atomic opacity. With q plates we generate a probe beam with azimuthally varying phase and polarization structure, and its right and left circular polarization components provide the probe and control of an EIT transition. We observe an azimuthal modulation of the absorption profile that is dictated by the phase and polarization structure of the probe laser. Conventional EIT systems do not exhibit phase sensitivity. We show, however, that a weak transverse magnetic field closes the EIT transitions, thereby generating phase-dependent dark states which in turn lead to phase-dependent transparency, in agreement with our measurements.
In recent years there has been great interest in the applications of the interaction of the phase and intensity of laser light in atomic vapours. The generation of light beams with arbitrary phase and intensity patterns can now be easily achieved by using Spatial Light Modulators (SLMs). The transformation of quantized units of phase information to atomic vapours has implications in the fields of quantum optics for the realisation of sources of entangled photons, optical switching, and quantum information storage. Spatially shaped beams with non-trivial intensity geometries have found use in single atom quantum well traps and for the enhancement of density of standard Magneto Optical Traps. The work in this thesis is focussed around two main pro jects, involving the interaction of holographically shaped light beams with cold trapped atoms and with a hot atomic vapour respectively. An enhancement to the previously investigated technique of a SPontaneous force Optical Trap SPOT of 87 Rb is presented here which aims to solve various issues which naturally arise from compressing cold atoms in a Magneto Optical Trap (MOT) such as unavoidable heating during the compression. High density/high atom number traps are highly sought after in many experiments for more efficient transfer of atoms to Bose Einstein Condensates and for improved quantum storage capabilities in cold atom traps. The highest density achieved in our SPOT was 2.5 × 10^12 atoms cm−3 for 2 × 10^8 atoms at a temperature of approximately 100µK. This represents almost 2 orders of magnitude increase in density from the standard MOT setup with no adverse heating of the trap while maintaining 75% of the atoms. In the second part of this work hot atomic vapours are utilized for the efficient transfer of orbital angular momentum information from near infra-red pump fields, driving from 5S_1/2 to 5D_5/2 on a two-photon transition, to a cascade from 5D_5/2 to 6P_3/2 to 5S_1/2 generating 5230nm light and a coherent blue, 420nm, beam respecitively. This generation is performed using four wave mixing in 85 Rb. We observe the complete conversion of all input quantum information, the Orbital Angular Momentum (OAM) from the pump fields to the blue. In addition we show the additional phase coherence effects of this experiment through the use of simple superpositions of Laguerre-Gaussian (LG) modes showing that the process is indeed quantum in nature. A theoretical basis for the transfer of all OAM information to only the 420nm beam is also discussed here.
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.
At Glasgow we routinely use a variety of techniques to imprint structure in the spatial amplitude, phase and polarization of a light beam. This allows us to generate vector vortex beams at high fidelity, with applications in polarimetry and strong focusing. Vector vortex beams can also induce magnetic dipole moments in an atomic medium, in our case rubidium atoms at 100 micro Kelvin, with an orientation that depends on the local polarization direction. This affects, in a nontrivial manner, the propagation of the structured light field through the atomic medium, dependent on an external magnetic field. We show that we can retrieve the orientation of the magnetic field from the absorption of the structured light - an atomic compass.