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Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR)

Tunable doughnut light beam for a near-field optical funnel of atoms

S. M. Iftiquar, Hajime Ito, Motoichi Ohtsu

Published Nov 13, 2002• 2 citations
Publisher

Abstract

A doughnut-shaped Laguerre-Gaussian (LG) beam has been generated through interference of two spherical waves. The beam has a 5 mm bright ring and a 250-microns dark center suitable for our atom funnel experiment. The power conversion efficiency from a Gaussian mode to the LG mode is about 50%.
DOI
10.1109/cleopr.2001.968021

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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.
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.