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arXiv:0908.0263 · Physical Review A

Imaging-based parametric resonance in an optical dipole-atom trap

Salim Balik, Aye Win, Mark D. Havey

Published Aug 6, 2009• 17 citations
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Abstract

We report sensitive detection of parametric resonances in a high-density sample of ultracold $^{87}\text{R}\text{b}$ atoms confined to a far-off-resonance optical dipole trap. Fluorescence imaging of the expanded ultracold atom cloud after a period of parametric excitation shows significant modification of the atomic spatial distribution and has high sensitivity compared with traditional measurements of parametrically driven trap loss. Using this approach, a significant shift of the parametric resonance frequency is observed and attributed to the anharmonic shape of the dipole trap potential.

DOI
10.1103/physreva.80.023404

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We report sensitive detection of parametric resonances in a high-density sample of ultracold 87Rb^{87}\text{R}\text{b}87Rb atoms confined to a far-off-resonance optical dipole trap. Fluorescence imaging of the expanded ultracold atom cloud after a period of parametric excitation shows significant modification of the atomic spatial distribution and has high sensitivity compared with traditional measurements of parametrically driven trap loss. Using this approach, a significant shift of the parametric resonance frequency is observed and attributed to the anharmonic shape of the dipole trap potential.
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We demonstrate a direct, fluorescence-free measurement of the oscillation frequency of cold atoms in an optical dipole trap based on a high-finesse optical cavity strongly coupled to atoms. The parametric heating spectra of the trapped atoms are obtained by recording the transmitted photons from the cavity with the trap depth is modulated by different frequency. Moreover, in our method the oscillation can be observed directly in the time scale. Being compared to the conventional fluorescence-dependent method, our approach avoids uncertainties associated with the illuminating light and auxiliary imaging optics. This method has the potential application of determining the motion of atoms with stored quantum bits or degenerate gases without destroying them.
We perform fluorescence imaging of a single 87 Rb atom after its release from an optical dipole trap. The time-of-flight expansion of the atomic spatial density distribution is observed by accumulating many single atom images. The position of the atom is revealed with a spatial resolution close to 1 μm by a single-photon event, induced by a short resonant probe. The expansion yields a measure of the temperature of a single atom, which is in very good agreement with the value obtained by an independent measurement based on a release-and-recapture method. The analysis presented in this paper provides a way of calibrating an imaging system useful for experimental studies involving a few atoms confined in a dipole trap.
We present a novel imaging system for ultracold quantum gases in expansion. After release from a confining potential, atoms fall through a sheet of resonant excitation laser light and the emitted fluorescence photons are imaged onto an amplified CCD camera using a high numerical aperture optical system. The imaging system reaches an extraordinary dynamic range, not attainable with conventional absorption imaging. We demonstrate single-atom detection for dilute atomic clouds with high efficiency where at the same time dense Bose-Einstein condensates can be imaged without saturation or distortion. The spatial resolution can reach the sampling limit as given by the 8 μm pixel size in object space. Pulsed operation of the detector allows for slice images, a first step toward a 3D tomography of the measured object. The scheme can easily be implemented for any atomic species and all optical components are situated outside the vacuum system. As a first application we perform thermometry on rubidium Bose-Einstein condensates created on an atom chip.
We study the loading dynamics of an optical dipole trap and an optical lattice both experimentally and theoretically. A simple power dependence for the number of trapped atoms (N∝P3∕2) is revealed in both cases. We then study the parametric excitation of the trapped atoms. High order parametric resonance is observed close to but lower than 4ν in a CO2 laser optical lattice. The existence of the high harmonic and its shift toward lower frequency are attributed to the anharmonicity of the optical trapping potential. We investigate the discrepancy between experimentally measured optical lattice frequencies and the theoretical calculation for several published experiments and present corrected formula for trap frequency calculations. Using this corrected formula and accounting for the off-center lattice sites, we present a modified formula for calculating the optical lattice frequency and obtain better agreement between experimental measurement and theoretical calculation.
We report on light-shift tomography of a cloud of 87Rb^{87}\mathrm{Rb}87Rb atoms in a far-detuned optical-dipole trap at 1565nm1565\phantom{\rule{0.3em}{0ex}}\mathrm{nm}1565nm. Our method is based on standard absorption imaging, but takes advantage of the strong light shift of the excited state of the imaging transition, which is due to a quasiresonance of the trapping laser with a higher excited level. We use this method to (i) map the equipotentials of a crossed optical-dipole trap and (ii) study the thermalization of an atomic cloud by following the evolution of the potential energy of atoms during the free-evaporation process.