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arXiv:physics/0306021

Mode conversion in optical beam in order to create funnel shaped optical near field to collect cold Rb atoms from MOT

S. M. Iftiquar

Published Jun 3, 2003• 0 citations
arXivHTMLPDF

Abstract

Doughnut shaped light beam has been generated from Gaussian mode ($TEM_{00}$) 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{$μ$}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{$μ$}. We also observe such beam to contain azimuthal phase as well as radial phase distribution.

DOI
10.48550/arxiv.physics/0306021

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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.
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.
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 process a silicon-on-insulator substrate into a funnel structure with a 2-µm-side square-shaped outlet by photolithography, anisotropic chemical etching, sputtering, and drilling with a focused ion beam. The funnel element is designed to generate a cold atomic beam with a high flux intensity of 1 atom/nm 2 ·s using repulsive evanescent light. The evanescent light is produced in TE modes propagating through a 1-µm-thick TiO 2 layer deposited on the slope faces in the case of coupling near-infrared lights for collecting Rb atoms released from a magneto-optical trap. The decay length of evanescent light is measured with a fiber probe in the collection mode.