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Quantum Electronics and Laser Science Conference

Atom motion in Laguerre-Gaussian doughnut beams

M. E. J. Friese, J.A. Andersen, Halina H. Rubinsztein-Dunlop

Published Nov 13, 2002• 0 citations
Publisher

Abstract

Summary form only given. Preparation of slow atomic beams has applications in experiments such as lithography, atom interferometry and in general for transporting atoms, for example from one vacuum chamber to another. Optical methods together with hollow fibers have been successfully employed to transport atoms, and all-optical methods using far off-resonant dipole forces have recently attracted much interest. Such all-optical methods have advantages in that far off-resonant dipole traps have potentially very low scattering rates, especially blue detuned hollow beam traps where atoms are restricted to low intensity regions where perturbations due to the light field are minimum. Optical atom guides also have practical advantages in that they can be switched, and can be easily introduced into high vacuum apparatus. Blue detuned hollow laser beams generated from a hollow optical fiber have been used to guide atoms (Xu et al, 1999; Babiker et al, 1994), and holographically generated doughnut beams have been used to guide and focus a cold beam of metastable neon atoms. In our experiments, we investigate the motion of cold rubidium atoms in far-detuned and near-resonant doughnut beams.

DOI
10.1109/qels.2001.962108

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Summary form only given. Preparation of slow atomic beams has applications in experiments such as lithography, atom interferometry and in general for transporting atoms, for example from one vacuum chamber to another. Optical methods together with hollow fibers have been successfully employed to transport atoms, and all-optical methods using far off-resonant dipole forces have recently attracted much interest. Such all-optical methods have advantages in that far off-resonant dipole traps have potentially very low scattering rates, especially blue detuned hollow beam traps where atoms are restricted to low intensity regions where perturbations due to the light field are minimum. Optical atom guides also have practical advantages in that they can be switched, and can be easily introduced into high vacuum apparatus. Blue detuned hollow laser beams generated from a hollow optical fiber have been used to guide atoms (Xu et al, 1999; Babiker et al, 1994), and holographically generated doughnut beams have been used to guide and focus a cold beam of metastable neon atoms. In our experiments, we investigate the motion of cold rubidium atoms in far-detuned and near-resonant doughnut beams.
This thesis reports on the design, setup, and operation of a new atom trap apparatus, which is developed to confine few rubidium atoms in ultrahigh vacuum by means of taylored and dynamically variable light potentials (dipole traps), thus making the atoms available for spectroscopy and controlled manipulations. To maintain low background pressure, atoms of a vapour cell are transferred into a cold atomic beam by laser cooling techniques, and accumulated by a magneto-optic trap (MOT) in a separate part of the vacuum system. The laser cooled atoms are then transferred into dipole traps made of focused far-off-resonant laser fields in single or crossed-beam geometry, which are superimposed with the center of the MOT. Gaussian as well as hollow Laguerre-Gaussian (LG01) beam profiles are used with red-detuned and blue-detuned light. Microfabricated dielectric phase objects allow the efficient and robust mode conversion of Gaussian into Laguerre-Gaussian laser beams. Trap geometries can easily be changed due to the highly flexible experimental setup. The dipole trap laser beams have powers up to several hundred milliwatts and are focused to around 10 m. At a detuning of several tens of nanometers from the atomic resonance, the resulting traps are a few millikelvin deep, and the trapped atoms exhibit motional frequencies near 30 kHz, trap light scattering rates of a few hundred photons per second per atom, and storage times of several seconds. The number of dipole-trapped atoms ranges from more than ten thousand to below ten. The dipole-trapped atoms are detected either by a photon counting system with efficient straylight discrimination, or by recapture into the MOT, which is imaged onto a sensitive photodiode and a CCD-camera. Due to the strong AC-Stark shift imposed by the high-intensity trapping light, energy-selective resonant excitation and detection of the stored atoms is possible. The measured energy distribution is consistent with a harmonic potential shape and allows for the determination of temperatures and heating rates. From initial measurements, the thermal energy is found to be about 10 % of the trap depth. Using a crossed-beam geometry with red-detuned laser light, efficient transfer of atoms between the beams is observed. Optimum transfer occurs when the two beams cross at a radial offset, which can qualitatively be understood when the particle energy and geometrical properties of the two-beam trapping potential are considered. Numerical simulations reproduce the general features of the measured transfer efficiency vs. radial beam offset. This experiment is a first step towards dynamically variable potentials. Atoms have been radially confined in a blue-detuned hollow beam, and their decay time has been determined. This configuration is currently extended to a three-dimensionally confining blue-detuned dipole trap. For advanced laser cooling, state manipulation and spectroscopy of trapped atoms, a system of two diode lasers has been set up which are phase-coherently locked with a difference frequency near 6.834 GHz, to drive Raman transitions between the hyperfine-split ground states of 87Rb atoms. Resonances of such a transition have been recorded in a buffer gas loaded rubidium vapour cell with linewidths below 100 Hz.
Summary form only given. One of the more promising proposals for guiding and focusing neutral atoms involves dark hollow laser beams. When detuned blue of resonance, the dipole force the atoms feel in the light confines them to the dark core of the beam where they can be transported with minimal interaction with the light. Dark hollow beams show promise for long distance transport of large numbers of ultra cold atoms, making them a strong competitor with hollow-core fiber atom guides. In this presentation we describe a simple way to construct an all-light atom tunnel with a series of axicons and a moderately powered diode laser beam.
One of the more promising proposals for guiding and focusing neutral atoms involves dark hollow laser beams. When the frequency of the laser is detuned to the blue of resonance, the dipole force the atoms feel in the light confines them to the dark core where the atoms can be transported with minimal interaction with the light. The ability of the all-light atom guides to transport large number of ultracold atoms for long distances without physical walls leads to the possibility of a versatile tool for atom lithography, atom interferometry, atomic spectroscopy as well as for transporting and manipulating Bose-Einstein condensates. Furthermore since the atoms transported in all-light atom guides do not come into contact with matter, they can in principle be used to transport antimatter as well. In this work a hollow beam atom tunnel is demonstrated by guiding magneto-optically trapped ultracold cesium atoms with a hollow core laser beam. The 1mm diameter diffractionless hollow beam used to construct the atom tunnel was generated from a TEM00 mode diode laser at 852 nm using a series of axicons and simple lenses. The axicon generated hollow beam has a dark core extended for most of its diameter and
Summary form only given. Recent advances in cold atom physics have seen the emergence of the area of atom guiding along hollow core optical fibres and also along light beams. Both employ the optical dipole force which for suitably tuned light can restrict the atoms to the dark hollow region of the fibre or the light beam. In this work, we discuss and analyse a novel scheme of channelling cold atoms into a high-order Bessel beam (via a high-order Laguerre-Gaussian beam) and demonstrate an efficient method of producing such beams.
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.
We theoretically investigate the process of coupling cold atoms into the core of a hollow-core photonic-crystal optical fiber using a blue-detuned Laguerre-Gaussian beam. In contrast to the use of a red-detuned Gaussian beam to couple the atoms, the blue-detuned hollow beam can confine cold atoms to the darkest regions of the beam, thereby minimizing shifts in the internal states and making the guide highly robust to heating effects. This single optical beam is used as both a funnel and a guide to maximize the number of atoms into the fiber. In the proposed experiment, Rb atoms are loaded into a magneto-optical trap (MOT) above a vertically oriented optical fiber. We observe a gravito-optical trapping effect for atoms with high orbital momentum around the trap axis, which prevents atoms from coupling to the fiber: these atoms lack the kinetic energy to escape the potential and are thus trapped in the laser funnel indefinitely. We find that by reducing the dipolar force to the point at which the trapping effect just vanishes, it is possible to optimize the coupling of atoms into the fiber. Our simulations predict that by using a low-power (2.5 mW) and far-detuned (300 GHz) Laguerre-Gaussian beam with a 20-$\ensuremath{\mu}$m-radius core hollow fiber, it is possible to couple 11% of the atoms from a MOT 9 mm away from the fiber. When the MOT is positioned farther away, coupling efficiencies over 50% can be achieved with larger core fibers.