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Optics Letters

Stable radiation-pressure particle traps using alternating light beams

Arthur Ashkin

Published Oct 1, 1984• 42 citations
Publisher

Abstract

A new type of stable alternating-beam light trap is proposed for confinement of neutral atoms and macroscopic dielectric particles. This trap, based only on the scattering force of radiation pressure, overcomes the limitations of the optical Earnshaw theorem. Trapping of ~10(7) sodium atoms in large volumes (100 cm(3)) seems possible with well depths >1 K and with optical cooling close to the Purcell limit of ~10(-4) K. Trapping at points of zero light intensity is considered.

DOI
10.1364/ol.9.000454

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A new type of stable alternating-beam light trap is proposed for confinement of neutral atoms and macroscopic dielectric particles. This trap, based only on the scattering force of radiation pressure, overcomes the limitations of the optical Earnshaw theorem. Trapping of ~10(7) sodium atoms in large volumes (100 cm(3)) seems possible with well depths >1 K and with optical cooling close to the Purcell limit of ~10(-4) K. Trapping at points of zero light intensity is considered.
Radiation pressure forces have been proposed for making stable optical traps for neutral atoms and other dielectric particles. Recently introduced optodynamic or alternating beam traps based on the scattering force are compared with cw traps based on the gradient force. Trapping experiments on micron-sized dielectric particles are described which demonstrate the basic principles of alternating beam light trapping. The relationship of these traps to the optical Earnshaw theorem is considered. The implications for trapping of recently demonstrated sources of slow atoms are also discussed.
The new principle of radiation-pressure trapping of neutral dielectric particles by means of alternating light beams was demonstrated in an optical levitation experiment. Stable trapping of micron-sized spheres was observed due to alternating scattering-force fields under conditions where stable cw trapping was not possible. Application of this principle to neutral-atom trapping as a means of circumvention of the optical Earnshaw theorem is suggested. A new stable cw optical trap for spheres was also discovered.
We report the confinement and cooling of an optically dense cloud of neutral sodium atoms by radiation pressure. The trapping and damping forces were provided by three retroreflected laser beams propagating along orthogonal axes, with a weak magnetic field used to distinguish between the beams. We have trapped as many as 107{10}^{7}107 atoms for 2 min at densities exceeding 1011{10}^{11}1011 atoms ${\mathrm{cm}}^{\ensuremath{-}3}$. The trap was \ensuremath{\simeq}0.4 K deep and the atoms, once trapped, were cooled to less than a millikelvin and compacted into a region less than 0.5 mm in diameter.
We show that the optical Earnshaw theorem does not always apply to atoms and that it is possible to confine atoms by spontaneous light forces produced by static laser beams. A necessary condition for such traps is that the atomic transition rate cannot depend only on the light intensity. We give several general approaches by which this condition can be met and present a number of specific trap designs illustrating these approaches. These traps have depths on the order of a kelvin and volumes of several cubic centimeters.