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Physical Review A

Atomic funnel with evanescent light

Hajime Ito, K. Sakaki, Wonho Jhe, Motoichi Ohtsu

Published Jul 1, 1997• 77 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.56.712

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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.
We propose an atomic funnel consisting of a dark hollow, divergent laser beam and an evanescent light in a short micron-sized hollow optical fiber. The cold atoms extracted from a magneto-optical trap experience effective Sisyphus cooling in the dark hollow beam. We calculate the potential barriers for atomic guiding in the evanescent light and the dark hollow beam, and analyze the atomic losses during the guiding and funneling process. Our results show that the equilibrium three-dimensional root-mean-square momentum of about $4.5\ensuremath{\Elzxh}k$ (temperature $\ensuremath{\sim}2.5\ensuremath{\mu}\mathrm{K}$) and the funneling efficiency of about 95% could be obtained. A low-velocity, intense coherent atomic beam can be generated in this funnel scheme and its potential applications in atom optics are briefly discussed.
We investigate theoretically the Sisyphus cooling of three-level atoms due to interaction with the evanescent field developed on the surface of a conical-hollow atom trap. We treat the total processes in a fully random way so that quantitative and realistic results are obtained by Monte Carlo simulation. We show that the trapped atoms, when released inside the hollow region, are cooled down to the final temperature of 30 $\ensuremath{\mu}$K, independent of the intial conditions, with negligible loss (about 1%) of the atoms. We also find that the mean height of the collected atoms is about 1 mm from the apex of the conical hollow with the trap radius of 0.5 mm. This system may be useful to realize an atomic funnel or a cold atomic beam for atom optical experiments such as atom interferometer and atom lithography.
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.
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 a novel gravito-optical atom trap composed of a blue-detuned, conical dark-hollow-beam propagating upward and the gravity field. When cold atoms are loaded into the conical dark-hollow-beam from a magnetic-optical trap and bounce inside the hollow beam, they experience an efficient hollow-beam induced Sisyphus cooling and repumping-beam induced geometric cooling. Our investigation shows that an ultracold and dense 87 Rb atomic sample with an equilibrium 3D temperature of about 1 μK can be obtained and the appearance of an optically-trapped Bose-Einstein condensation may be possible in this pure optical trap.