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Atom guiding in free-space light beams and photonic crystal fibres

J. Livesey

Published Jan 1, 2007• 0 citations
OpenAlex

Abstract

In this thesis I describe experimental work and present data on the guiding of \nRubidium atoms along free-space propagating light beams as well as within hollow \ncore glass fibres, namely photonic crystal fibres. I describe experiments, laser systems \nand vacuum trap assemblies designed to facilitate this guiding. \nThese experiments are intended to aid progression within the field of cold \natom guidance wherein narrow diameter, long distance hollow-fibre guides are a \ncurrent goal. Realisation of these guides could lead to promising applications such as \natom interferometers and spatially accurate, multi-source, atom depositors. \nHerein, guided fluxes are observed in free-space guiding experiments for \ndistances up to 50mm and up to 10GHz red-detuning from resonance. Additionally \nhollow-core, Kagome structured, quasi- and true-photonic crystal fibres are \ncharacterised. Finally a number of detailed fibre-guiding magneto-optic traps are \ndeveloped. \nBoth cold atomic-beams and cold atomic clouds are reliably positioned above \nfibre entrance facets in conjunction with a guiding laser beam coupled into the fibre \ncore. \nIssues regarding optical flux detection outwith fibre confinement appear to \nhave hindered observation of guided atoms. A far more sensitive detection system has \nbeen developed for use in current, ongoing fibre-guide experiments.

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Sword•Jan 1, 2008•0 citations
In this thesis I describe experimental work and present data on the guiding of \nRubidium atoms along free-space propagating light beams as well as within hollow \ncore glass fibres, namely photonic crystal fibres. I describe experiments, laser systems \nand vacuum trap assemblies designed to facilitate this guiding. \nThese experiments are intended to aid progression within the field of cold \natom guidance wherein narrow diameter, long distance hollow-fibre guides are a \ncurrent goal. Realisation of these guides could lead to promising applications such as \natom interferometers and spatially accurate, multi-source, atom depositors. \nHerein, guided fluxes are observed in free-space guiding experiments for \ndistances up to 50mm and up to 10GHz red-detuning from resonance. Additionally \nhollow-core, Kagome structured, quasi- and true-photonic crystal fibres are \ncharacterised. Finally a number of detailed fibre-guiding magneto-optic traps are \ndeveloped. \nBoth cold atomic-beams and cold atomic clouds are reliably positioned above \nfibre entrance facets in conjunction with a guiding laser beam coupled into the fibre \ncore. \nIssues regarding optical flux detection outwith fibre confinement appear to \nhave hindered observation of guided atoms. A far more sensitive detection system has \nbeen developed for use in current, ongoing fibre-guide experiments.
Light can exert optical forces on matter. In the macroscopic world these forces are minuscule, but on the microscopic or atomic scale, these forces are large enough to trap and manipulate particles. They may even be used to cool atoms to a fraction of a degree above absolute zero. \n \nThis thesis details a number of experiments concerned with the optical manipulation of atoms and micron-size particles using free space light beams and photonic crystal fibres. \n \nTwo atom guiding experiments are described. In the first experiment, a spatial light modulator is used to generate higher blue-detuned azimuthal Laguerre-Gaussian LG) beams, which are annular beams with a hollow core. These LG beams are then used to guide laser cooled rubidium-85 atoms within the dark core over a distance of 30 mm. The second atom guiding experiment involves attempting to guide laser cooled and thermal rubidium atoms through a hollow-core photonic crystal fibre using red-detuned light. Hollow-core photonic crystal fibres are fibres that are able to guide light with low attenuation within a hollow core. For this experiment a hot wire detection system was designed, along with a number of complex vacuum systems. \n \nThe first dual-beam fibre trap for micron-size particles constructed using endlessly single-mode photonic crystal fibre (ESM-PCF) is described. The characteristics of dual-beam fibre traps are governed by the fibres used. As ESM-PCF has considerably different properties in comparison to conventional single- or multimode fibres, this dual beam ESM-PCF trap exhibits some novel characteristics. I show that the dual beam ESM-PCF trap can form trapping, repulsive and line potentials; an interference-free ‘white light’ trap; and a dual-wavelength optical conveyor belt.
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.
This thesis presents the first demonstration of a guide for cold atoms based on a miniature structure of four current-carrying wires. The four wires are embedded within a hollow silica fibre. Atoms are guided along the centre of a fifth hole on the axis of the fibre by the Stern-Gerlach force. A vapour cell Magneto Optical Trap (MOT), formed 1 cm above the mouth of the waveguide is the source of cold "8"5Rb atoms. After cooling the atoms to 25 #mu#K in optical molasses they fall under the influence of gravity through a magnetic funnel into the waveguide. After propagating for 2 cm, the atoms are reflected by the field of a small pinch coil wound around the base of the guide. The atoms then travel back up the fibre and out into the funnel, where they can be imaged either in fluorescence or by recapturing in the MOT. A video sequence of atoms falling into the guide and re-emerging after reflection from the pinch coil graphically illustrates the operation of the guide. The coupling efficiency and transverse temperature of the atoms is measured experimentally and in a Monte-Carlo simulation. We find an optimum coupling efficiency of 12% and we measure the spatial extent of the cloud within the fibre to be of order 100 #mu#m. We find good agreement between experimental data and results from the numerical simulation. We have also been able to observe different thresholds for the reflection of different positive m_F levels. In another experiment we are able to trap the atoms in an elongated Ioffe trap for up to two seconds, increasing the distance over which the atoms are guided. We are able to guide the atoms over distances of 40 cm with a loss rate indistinguishable from the free space loss rate. (author)
This thesis reports on the design and fabrication of an experimental platform for in-fibre laser cooling of Rb and atom optics. By in-fibre laser cooling, we mean the long term aim of laser cooling thermal Rb atoms of a Photonic MicroCell (PMC), and subsequently developing what would be cold-atom photonic crystal fibre (PCF). The platform was designed to harbor several experiments on cold and thermal atom guidance and in-fibre spectroscopy so to address several open questions related for example to the effect of the core inner-wall surface on the atom energy structure and on selective fibre mode excitation for atom trapping and cooling. The completed platform comprises a specific and large ultra-high vacuum (UHV) chamber and a set of lasers for both atom cooling and atom guiding inside highly tailored hollow-core PCF (HC-PCF). The UHV chamber was designed to accommodate several HC-PCFs and two magneto-optical traps (MOT). The HC-PCF were designed, fabricated and post-processed to exhibit different core diameter, modal content and core inner surface material. For example, the mode field diameters range from ~30 µm to ~80 µm for the fundamental Gaussian-like core mode, and the surface materials include pure silica, a layer of Aluminosilicate or a layer of PDMS so to provide a large parameter space in assessing the effect of surface on the fibre-confined atoms. The system has been constructed and characterized. The cooling/repumping laser was frequency-stabilized, with measured Allan variance deviation of σ(τ)=3.8×10^(-11)/√τ. With the system we generated MOT with both isotopes of the Rb atom, with a cooling temperature as low as 7 µK. The platform is now operational to undertake the first atom guidance and explore the feasibility of atom cooling inside a HC-PCF.
We have developed an atom-guiding technique that loads 3×1063\times 10^63×106 cold rubidium atoms into hollow-core optical fibre, an order-of-magnitude larger than previously reported results. This achievement was guided by a physically realistic simulation that led to an experimental loading efficiency of 3%3\%3%, giving a peak optical depth of over 600600600. The loading efficiency was limited only by the geometric overlap of the atom cloud an an optical guide beam, and we have identified techniques that would allows us to exceed this limit. We also present a fast and minimally destructive probing technique that enables us to follow the dynamics of the cold atom cloud as it falls into the fibre - this strengthens the confidence in our detailed model, and enables adjustment of the experiment into an optimal state. We are able to observe the effect of light-assisted cold atom collisions as well as background gas collisions in the fibre. The combination of these observations and physical understanding from the simulation allows us to estimate the ultimate limits to loading cold atoms into hollow-core fibres.
"The work presented in this thesis is concerned with the manipulation of laser- cooled rubidium atoms and the detection of small numbers of cold atoms using very sensitive light detectors. Laser-cooled atoms are atoms that have been slowed to very low velocities, corresponding to temperatures of the order of 100 //K, and they can be trapped in magnetic and/or optical traps. The advantage of using cold atoms for experiments in atomic and quantum physics stems from the fact that these atoms are effectively confined within a region of space and this provides longer interaction and observation times compared to atoms at room temperature. Hence, it is possible to explore and observe a number of effects that would otherwise be extremely challenging. The achievement of laser cooling and trapping of neutral atoms was a major breakthrough in the mid 80’s and has resulted in significant advances being made in modern spectroscopy, the realisation of Bose-Einstein condensation, atomic clocks, and quantum information technologies amongst others. In particular, the work focuses on two theoretical proposals for developing microtraps for cold rubidium atoms involving (i) ultrathin optical fibres and (ii) wavelength sized apertures in thin films. In addition, a number of experimental techniques are explored with emphasis on the detection of small numbers of atoms near ultrathin optical fibres using both avalanche photodiodes and single photon detectors. A study on the interactions between atoms and photons is crucial for a full understanding of quantum mechanical processes at the few atom scale and detection schemes sensitive to very low levels of fluorescence from the laser-cooled atoms are vital for attaining this goal. Finally, an experimental study on the design and fabrication of arrays of silica microdisks, that could be used for facilitating interactions between cold atoms and photons, is presented."