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Journal of Optics B Quantum and Semiclassical Optics

A tunable doughnut laser beam for cold-atom experiments

S. M. Iftiquar

Published Dec 9, 2002• 8 citations
Publisher

Abstract

A doughnut-shaped light beam has been generated from a Gaussian mode (TEM 00 ). Upon splitting the pump beam into two equal-intensity components and introducing unequal convergences with a phase delay while they are recombined, a doughnut mode results. The output beam is tunable and has a long propagation length. The evanescent field generated by a 360 mW (at 780 nm wavelength) beam creates an optical field of 600 nm decay length with a 5.75 neV repulsive dipole potential. Thus cold Rb atoms (at a temperature of 10 μK or less) released from a magneto-optical trap can be reflected by the funnel surface, with the result that the atoms are ultimately collected at the bottom.

DOI
10.1088/1464-4266/5/1/305

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Journal of the Optical Society of America B•Aug 1, 1998•11 citations
A doughnut-shaped light beam has been generated from a Gaussian mode (TEM 00 ). Upon splitting the pump beam into two equal-intensity components and introducing unequal convergences with a phase delay while they are recombined, a doughnut mode results. The output beam is tunable and has a long propagation length. The evanescent field generated by a 360 mW (at 780 nm wavelength) beam creates an optical field of 600 nm decay length with a 5.75 neV repulsive dipole potential. Thus cold Rb atoms (at a temperature of 10 μK or less) released from a magneto-optical trap can be reflected by the funnel surface, with the result that the atoms are ultimately collected at the bottom.
Doughnut shaped light beam has been generated from Gaussian mode (TEM00TEM_{00}TEM00​) cw-Ti sapphire laser. After splitting the pump beam into two equal intensity components and introducing unequal convergence and phase delay while they are recombined it results in doughnut mode. Such a beam is tunable and have long propagation length. The evanescent field generated by 360 mW (at 780 nm wavelength) of such a beam creates optical field of 600 nm decay length with a 5.75 neV repulsive dipole potential. Thus cold Rb atoms (at 10{μμμ}K or less temperature) released from MOT can be reflected by the surface so that the atoms are collected ultimately at the bottom of the prism. By focussing such doughnut beam with 8 cm focal length converging lens, the dark radius reduces to 22{μμμ}. We also observe such beam to contain azimuthal phase as well as radial phase distribution.
We propose an atomic funnel composed of an evanescent-light mirror produced in a conical hollow prism. The funneling of cold Rb atoms released from a magneto-optical trap is simulated in the scheme of the evanescent-light-induced Sisyphus cooling for compensating the heating by gravity. A geometric method of the conversion of a Gaussian light beam into a doughnut-shaped light beam suitable for the atomic funnel is also experimentally demonstrated by means of a double-cone prism. Under the assumption of a proper optical intensity distribution over the inner surface of a specific conical hollow prism, the Monte Carlo simulation including the van der Waals interaction shows that a half of the released Rb atoms can be collected and the mean temperature of the collected atoms is low enough for the application to be guided with a bent hollow optical fiber.
Atom guiding in an atomic funnel is very useful for many interesting atom optical experiments requiring the precise control of atomic motion. We have obtained a doughnut-shaped laser beam by coupling a Ti:sapphire laser into a hollow core optical fiber and collimating the output laser beam of the fiber with a microscope objective lens. Using such a doughnut-shaped laser beam [2 mm inner diameter and a 5 mm outer diameter at the trapping position (about 1m away from the objective lens)l, we have guided the trapped cold Rb atoms in the atomic funnel.
We propose a simple atomic-beam collimator consisting of a blue-detuned cylindrical-hollow (doughnut) laser beam. Cold atoms from a magneto-optical trap are loaded into the doughnut beam and guided in the propagation direction of the doughnut beam. The cold atoms experience efficient Sisyphus cooling induced by the doughnut beam and a weak repumping beam and are collimated in the transverse direction. In the longitudinal direction the cold atoms experience small accelerations that are due to the absorption-induced heating of the doughnut beam and the repumping beam. The cooling, heating, and loss mechanisms in this atomic collimator are analyzed. Our study shows that an intense cold atomic beam with a minimum transverse velocity of ∼2 cm s-1 (a minimum transverse temperature of ∼1.4 μK) and a brightness of approximately 2×1013 atoms (sr-1 cm-2 s-1) can be obtained from the collimator.