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Quantum Electronics and Laser Science Conference

Observations of density variations in trapped atoms

Mara Goff Prentiss, A. Cable

Published Apr 24, 1989• 0 citations
OpenAlex

Abstract

We have demonstrated that a continuous supply of stow atoms can be produced using a strong standing wave laser field collinear with an atomic beam. Using an interaction length of 27 cm, we typically obtained an atomic beam with a density of ~1 × 10−8cm−3, a most probable velocity of 12 m/s, and a width in velocity space of <12 m/s. We used such a cooled atomic beam to load a magnetic molasses trap.1 We found that the motion of atoms in the trap is not always diffusive but can be confined to very narrow channels, which can be curved and can extend over lengths of several millimeters. In addition, the fluorescence due to the trapped atoms was examined using an optical microscope, and we observed that there were strong spatial variations in the fluorescence. The variations had two distinct components: very regular striations due to interference between the trapping light fields and very irregular variations with a characteristic size scale, which was not strongly correlated with that of the interference fringes.

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•Jan 1, 1994•1 citation
We have demonstrated that a continuous supply of stow atoms can be produced using a strong standing wave laser field collinear with an atomic beam. Using an interaction length of 27 cm, we typically obtained an atomic beam with a density of ~1 × 10−8cm−3, a most probable velocity of 12 m/s, and a width in velocity space of <12 m/s. We used such a cooled atomic beam to load a magnetic molasses trap.1 We found that the motion of atoms in the trap is not always diffusive but can be confined to very narrow channels, which can be curved and can extend over lengths of several millimeters. In addition, the fluorescence due to the trapped atoms was examined using an optical microscope, and we observed that there were strong spatial variations in the fluorescence. The variations had two distinct components: very regular striations due to interference between the trapping light fields and very irregular variations with a characteristic size scale, which was not strongly correlated with that of the interference fringes.
We have demonstrated a new slowing and cooling technique which uses a standing-wave laser field collinear with a thermal Na atomic beam to produce a continuous supply of slow atoms, with most probable velocities ranging from -40 to + 80 m/sec. The velocity spread can be \ensuremath{\sim}10 m/sec (150 mK), the density >108{>10}^{8}>108, and the peak density in phase space \ensuremath{\sim}100 times that at the peak of the original Maxwell-Boltzmann distribution.
We have continuously stopped thermal sodium atoms with laser light and loaded them into a 0.1-K deep superconducting magnetic trap. The trap is formed by a local minimum of the magnetic field produced by two superconducting coils and a radially confining octupole field. The atoms are slowed and stopped in two stages: The atoms are first slowed to ~200 m/s using a counterpropagating resonant laser beam and tapered magnetic field to compensate for the changing Doppler shift as the atoms are slowed.1 A second laser beam and tapered magnetic field, actually located within the trap, are used to stop the atoms in the trap. This continuous loading process has allowed us to accumulate up to 1 × 109 trapped atoms, and trapping times of several minutes have been achieved. Our trap has the feature of having a uniform magnetic field at its bottom, opening up the possibility of precision spectroscopy of the trapped atoms. The fluorescence from the trapped atoms has been studied, and they have been cooled to ~1 mK using 1-D optical molasses.
In two recent experiments Verkerk et al. and our group have observed quantization of the center-of-mass motion of laser cooled atoms in sub-wavelength sized potential wells formed by the light field of a 1-dimensional optical molasses. Our 1-dimensional optical molasses consists of a pair of counterpropagating laser beams of orthogonal, linear polarization, creating a series of potential wells separated by {lambda}/4. Polarization gradient laser cooling reduces the thermal energy of the atoms until they are trapped in well resolved vibrational levels near the bottom of the wells. Transitions between these vibrational levels are seen in a high resolution spectrum of fluorescence, where they give rise to a Rayleigh peak centered at the laser frequency, and well resolved Raman sidebands shifted by a frequency {plus_minus}{omega}, typically on the order of 100 kHz. We find quantitative agreement between the measured level separation h{omega} and the results of a calculation of the bandstructure in the periodic optical potential. Based on the observed ratio of the strength of the blue to the red sideband we determine the temperature of the atoms, and find agreement with theoretical predictions. Based on the level spacing and temperature we find the atoms to be localized to {approx}/15, withmore » up to 50-60% of the population of the trapped atoms in the vibrational ground state. We observe intriguing differences between fluorescence radiated by localized atoms, and the fluorescence radiated by non-localized atoms in a 1-dimensional optical molasses composed of a pair of counterpropagating laser beams of opposite circular polarization. The prospect of observing quantum motion and spatial order in 2 and 3 dimensions, and the consequences for the spectrum of fluorescence, will be discussed.« less
We demonstrate experimentally the continuous and pulsed loading of a slow and cold atomic beam into a magnetic guide. The slow beam is produced using a vapor loaded laser trap, which ensures two-dimensional magneto-optical trapping, as well as cooling by a moving molasses along the third direction. It provides a continuous flux larger than 10910^9109 atoms/s with an adjustable mean velocity ranging from 0.3 to 3 m/s, and with longitudinal and transverse temperatures smaller than 100μ100 \mu100μK. Up to 31083 10^83108 atoms/s are injected into the magnetic guide and subsequently guided over a distance of 40 cm.
The interaction of a beam of two level atoms with a standing wave laser field has been the subject of extensive investigation in recent times. A standing wave perpendicular to an atomic beam allows the manipulation of the transverse velocity distribution. The interaction of an atomic beam with a strong standing wave laser field has been systematically investigated by both theoretical simulation and experiment in this thesis. We have developed a numerical model based on a continued fraction solution for the light pressure forces to simulate the near perpendicular interaction of an atomic beam with a strong standing wave laser field. The reshaping of the transverse spatial profile of an atomic beam by a perpendicular, near resonant, standing wave laser field is investigated for red detunings using a longitudinal velocity selective detection system. The experimental dependence of the reshaped profile on the longitudinal and transverse atomic velocities agrees well with a continued fraction solution for the light force. In particular, a regime is found where channelling of the atoms near the antinodes of the standing wave produces a central peak in the spatial distribution of the atomic beam in the far field which have also been observed elsewhere.