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Laser cooling of rubidium atoms in a magneto-optical trap

S. A. Hopkins

Published Jan 1, 1996• 1 citation
Publisher

Abstract

This thesis describes theoretical and experimental work concerning radiation forces on atoms, with particular reference to rubidium atoms confined in a magneto-optical trap. After a short history of the field of laser cooling, a review of the semiclassical theory of mechanical interactions between two-level atoms and electromagnetic radiation is given. Different formulations of the semiclassical theory are discussed, including a new formulation in terms of momentum transfer amongst the plane wave modes of the electromagnetic field. Two important applications of light forces on atoms, namely 'optical molasses' and the 'magneto-optical trap', are then described with emphasis on experimental parameters. Three sub-Doppler cooling mechanisms, 'sisyphus cooling', 'motion-induced orientation cooling' and the 'magnetically-assisted sisyphus effect', are described and their role in optical molasses and the magneto-optical trap is discussed. A new study of the polarisation gradients which occur in 3-D monochromatic light fields is presented and quantifies their relative presence in different light field configurations. Polarisation gradient parameters are developed and shown to be directly related to the relativistic spin tensor of the light field. Implications of this polarisation gradient study for laser cooling work are discussed. The design, construction from scratch, operation and testing of a magneto-optical trap for rubidium are described, including novel designs for two vacuum cells. Preliminary experiments to characterise the trap are described and results are presented; they primarily concern the number and distribution of atoms in the trap. Finally. the theory of time domain spectroscopy is reviewed. The construction and testing of a pulsed dye laser for study of coherent transients in samples of laser-cooled atoms and a proposed experiment to measure the temperature of cold atoms using coherent transients are described. Factors expected to influence the shape of coherent transients in cold atoms are discussed.

DOI
10.21954/ou.ro.00004be7

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•Jan 1, 2007•1 citation
This thesis describes theoretical and experimental work concerning radiation forces on atoms, with particular reference to rubidium atoms confined in a magneto-optical trap. After a short history of the field of laser cooling, a review of the semiclassical theory of mechanical interactions between two-level atoms and electromagnetic radiation is given. Different formulations of the semiclassical theory are discussed, including a new formulation in terms of momentum transfer amongst the plane wave modes of the electromagnetic field. Two important applications of light forces on atoms, namely 'optical molasses' and the 'magneto-optical trap', are then described with emphasis on experimental parameters. Three sub-Doppler cooling mechanisms, 'sisyphus cooling', 'motion-induced orientation cooling' and the 'magnetically-assisted sisyphus effect', are described and their role in optical molasses and the magneto-optical trap is discussed. A new study of the polarisation gradients which occur in 3-D monochromatic light fields is presented and quantifies their relative presence in different light field configurations. Polarisation gradient parameters are developed and shown to be directly related to the relativistic spin tensor of the light field. Implications of this polarisation gradient study for laser cooling work are discussed. The design, construction from scratch, operation and testing of a magneto-optical trap for rubidium are described, including novel designs for two vacuum cells. Preliminary experiments to characterise the trap are described and results are presented; they primarily concern the number and distribution of atoms in the trap. Finally. the theory of time domain spectroscopy is reviewed. The construction and testing of a pulsed dye laser for study of coherent transients in samples of laser-cooled atoms and a proposed experiment to measure the temperature of cold atoms using coherent transients are described. Factors expected to influence the shape of coherent transients in cold atoms are discussed.
This thesis reports on the design and construction of, and results from, an optical-dipole trapping apparatus developed to confine ultracold rubidium atoms in a conservative, large period, optical-dipole trap. An ultra-high vacuum system was designed and constructed to create a very low- background pressure. A new technique of viewport construction was developed, allowing for the fabrication of economical, high-quality windows for transmission of mid-infra-red laser radiation. The construction of a magneto-optical trap (MOT) and an optical molasses, and the subsequent characterisation, are discussed. A theory for the ac-Stark shift of atoms in a far-detuned laser field was developed. The nature of the scalar and tensorial light-shifts of the ground and first excited states of alkali atoms - the 5s(^2)S1/2 and 5p(^2)P3/2 states of Rb - has been examined. The effect of the differential light-shifts between these states on the operation of efficient laser cooling is discussed. A quasi-electrostatic dipole trap (QUEST) was formed from 50 W of CO2 laser power (٢ = 10.6 μm), focussed to <100 μm. The transfer of ultra-cold atoms from the MOT and optical molasses to the QUEST have been examined. Single beam and standing wave geometries of the QUEST have been implemented, with lifetimes of many seconds. The theory for the ac-Stark effect due to a single laser field has been further developed to consider orthogonally polarised fields with independent wavelengths. The use of an auxiliary laser field, a Nd:YAG laser at λ = 1.064 μm, to enhance the number and density of atoms loaded into the QUEST has been proposed and realised.
ENGLISH ABSTRACT: A Magneto Optical Trap (MOT) is a configuration formed by three orthogonal pairs of counterpropagating circularly polarized laser beams and a magnetic field gradient. A MOT is used to cool, capture and trap large numbers of atoms in vacuum at very low temperature in K range. In this thesis the development of an experimental setup for realising a MOT of 87Rb atoms is presented. The atomic structure of Rb and the theoretical background of laser cooling and magneto optical trapping was reviewed. The influence of rubidium background pressure in the vacuum system, the laser beam size and the power and frequency on the number of the trapped atoms were studied in literature. The trapping and repumping lasers were characterised experimentally. Six circularly polarised trapping beams with equal power were formed and properly aligned to intersect at the center of the trapping cell. Two optical setups were designed and exploited to investigate and optimise the trapping beam polarisation. The repumping laser beam was successfully aligned and colinearly combined into all the trapping beams. Three different experimental setups for saturated absorption spectroscopy were developed. Saturated absorption spectra showing the hyperfine structure of both 85Rb and 87Rb isotopes were measured and are discussed. Using two saturated absorption spectroscopy setups, the frequencies of both lasers were successfully locked to the trapping and repumping transitions of 87Rb respectively. A rectangular trapping cell was designed and attached to the vacuum system. A pressure of about 10­¯7 mbar was achieved. The magnetic field coils were characterised and affixed on both sides of the cell in an anti-Helmholtz configuration. Setups for imaging and quantification of the 87Rb atoms in the MOT were designed. Finally, the procedures for demonstrating a MOT are presented. In conclusion, the current status of the project is reported, with recommendations for the future work.
We have measured the spring constant, friction parameter, and temperature of the motion of caesium atoms in a magneto-optical trap. The results are very different from those predicted by models based on the scattering force, which have been used up to now. We show that the behaviour can be understood in terms of the sub-Doppler cooling mechanisms which have been observed in molasses, involving atomic motion through a polarization gradient. These mechanisms can operate in a trap because the atoms congregate in a region of low magnetic field. The lowest temperature measured was (30 ± 15) μK.
This thesis reports on the generation of MOT for laser cooling and trapping of Rubidium 87 atoms. Probe-pump technique have been used in Rb gas cell and doppler free absorption peaks has been observed. Peak at the crossover frequency will be locked for laser cooling and repump beams. This locked freuqency used in MOT to trap Rb atoms. The technique of optical molasses is used to cool the atoms and quadrupole magnetic field applied to confine or trap atoms at the center of MOT. Two chambers are constructed for 2D and 3D MOT with different pressure. Rb atoms loaded efficiently from the high flux cold atom beam generated in a compact 2D MOT. This is the first step of Bose Einstein Condensate (BEC). A performance of 2D MOT was tested by another main chamber of 3D MOT contains 3 pairs of counter-propagating beams. Over 5x10⁶ atoms trapped in a 3D MOT with loading rate of 1:8x10⁶ atoms/sec. This experiment provided an excellent starting point for future experiment on Rydberg atoms in dense ultracold quantum gases.
"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."
This thesis presents work on a laser cooling experiment designed for trapping Rb atoms in a CO(_2) laser optical trap. Some emphasis is placed on experimental features designed to allow the future implementation of a neutral atom quantum computation scheme. The experiment was built from scratch and includes the development of stable and reliable lasers for laser cooling and the construction of a double-chamber ultra-high vacuum system. The construction of a magneto-optical trap and optical molasses are discussed and results presented. The search for a signature of atoms trapped in the CO(_2) laser optical trap is described but so far no such signature has been observed. Possible reasons for this difficulty are presented Numerical modeling of the optical potential expected from the CO(_2) laser lattice has been performed and the expected experimental parameters of trap depth and oscillation frequency deduced from them.