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arXiv:0802.1213 · Physical Review A

Cold-atom confinement in an all-optical dark ring trap

Spencer E. Olson, Matthew L. Terraciano, Mark Bashkansky, Fredrik K. Fatemi

Published Dec 28, 2007• 91 citations
arXivHTMLPDF

Abstract

We demonstrate confinement of $^{85}\mathrm{Rb}$ atoms in a dark, toroidal optical trap. We use a spatial light modulator to convert a single blue-detuned Gaussian laser beam to a superposition of Laguerre-Gaussian modes that forms a ring-shaped intensity null bounded harmonically in all directions. We measure a $1∕e$ spin-relaxation lifetime of $\ensuremath{\approx}1.5\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ for a trap detuning of $4.0\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. For smaller detunings, a time-dependent relaxation rate is observed. We use these relaxation rate measurements and imaging diagnostics to optimize trap alignment in a programmable manner with the modulator. The results are compared with numerical simulations.

DOI
10.1103/physreva.76.061404

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We demonstrate confinement of 85Rb^{85}\mathrm{Rb}85Rb atoms in a dark, toroidal optical trap. We use a spatial light modulator to convert a single blue-detuned Gaussian laser beam to a superposition of Laguerre-Gaussian modes that forms a ring-shaped intensity null bounded harmonically in all directions. We measure a 1∕e1∕e1∕e spin-relaxation lifetime of \ensuremath≈1.5s\ensuremath{\approx}1.5\phantom{\rule{0.3em}{0ex}}\mathrm{s}\ensuremath≈1.5s for a trap detuning of 4.0nm4.0\phantom{\rule{0.3em}{0ex}}\mathrm{nm}4.0nm. For smaller detunings, a time-dependent relaxation rate is observed. We use these relaxation rate measurements and imaging diagnostics to optimize trap alignment in a programmable manner with the modulator. The results are compared with numerical simulations.
We demonstrate the generation of single-beam dark toroidal optical intensity distributions, which are of interest for neutral atom storage and atom interferometry. We demonstrate experimentally and numerically optical potentials that contain a ring-shaped intensity minimum, bounded in all directions by higher intensity. We use a spatial light modulator to alter the phase of an incident laser beam, and analyze the resulting optical propagation characteristics. For small toroidal traps (< 50 &mgr;m diameter), we find an optimal superposition of Laguerre-Gaussian modes that allows the formation of single-beam toroidal traps. We generate larger toroidal bottle traps by focusing hollow beams with toroidal lenses imprinted onto the spatial light modulator.
A scheme to trap atoms in a blue-detuned optical dipole trap, formed with a single laser beam and a holographic phase plate, is demonstrated. 105{10}^{5}105 rubidium atoms are trapped for (1/e lifetime) \ensuremath{\sim}300 msec, at temperatures of \ensuremath{\sim}24 \ensuremath{\mu}K and a density of $\ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3},$ for a trapping beam detuning of 0.1--30 nm. The time for a trapped atom to scatter one photon off the trapping beam is measured and found to be linear in the trapping laser detuning. At a detuning of 0.5 nm from resonance this time was measured to be \ensuremath{\sim}100 msec, indicating that the atoms are exposed on average only to \ensuremath{\sim}1/700 of the maximal light intensity in the trap. The use of a single laser beam allows for simple dynamical changes of the potential and large adiabatic compressions, while keeping the atoms mainly in the dark.
We have constructed a novel optical trap for neutral atoms by using a Laguerre-Gaussian (doughnut) beam whose frequency is blue detuned to the atomic transition. Laser-cooled rubidium atoms are trapped in the dark core of the doughnut beam with the help of two additional laser beams which limit the atomic motion along the optical axis. About 108{10}^{8}108 atoms are initially loaded into the trap, and the lifetime is 150 ms. Because the atoms are confined at a point in a weak radiation field in the absence of any external field, ideal circumstances are provided for precision measurements. The trap opens the way to a simple technique for atom manipulation, including Bose-Einstein condensation of gaseous atoms.
Summary form only given. Blue-detuned optical traps confine neutral atoms mostly in the dark, therefore reducing the perturbations induced by the trapping light on the atoms. Recently, traps based on a single laser beam were demonstrated, providing greater experimental simplicity and enabling dynamical changes of the trap geometry and strength. However, all these traps have a large difference in the potential strength in the radial and axial directions, resulting in a very inefficient use of the available laser power, and a poor loading efficiency from the magneto-optical trap. In this work we present a novel optical scheme to generate an optimal trap, which maximizes the trap depth, and greatly reduces the light-induced perturbations to the atoms. This is achieved by surrounding a dark volume with a light envelope having (a) an almost minimal surface area for a given volume, (b) the minimal wall thickness allowed by diffraction, and (c) an almost constant wall height on every point on the envelope. The stiffness of the walls, combined with the large detuning allowed by the efficient distribution of light intensity, yield a very low photon scattering rate for the trapped atoms.
We demonstrated trapping single neutral Rb atoms in micro traps of an optical ring lattice formed by superposing the +/-l components of the Laguerre-Gaussian mode, and generated by reflecting a single laser beam from a computer controlled spatial light modulator. A single atom in one trap or two atoms with one each in two traps were identified by observing the fluorescence. The trap array loaded with single atoms was rotated by dynamically displaying the hologram animation movie on the modulator. The modulation period in the fluorescence indicates the rotation of one or two single atoms in the lattice.
We propose a versatile optical ring lattice suitable for trapping cold and quantum degenerate atomic samples. We demonstrate the realisation of intensity patterns from pairs of Laguerre-Gauss (exp(i??) modes with different ? indices. These patterns can be rotated by introducing a frequency shift between the modes. We can generate bright ring lattices for trapping atoms in red-detuned light, and dark ring lattices suitable for trapping atoms with minimal heating in the optical vortices of blue-detuned light. The lattice sites can be joined to form a uniform ring trap, making it ideal for studying persistent currents and the Mott insulator transition in a ring geometry.