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Japanese Journal of Applied Physics

Atom Funnel with a Micron-Sized Outlet Using Evanescent Light

Hiroyuki Kashiwagi, Haruhiko Ito

Published Jan 20, 2010• 3 citations
Publisher

Abstract

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.

DOI
10.1143/jjap.49.012001

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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.
We describe atom deflection with a slit-type deflector as an elemental technique of precise atom control using near-field light. In the demonstration experiments, cold Rb atoms deflected by repulsive near-field light induced in a 200?nm-wide slit are detected by means of two-step photoionization. Then, we consider a near-field-light lens of atomic de Broglie waves by developing the atom deflector. The feasibility of nanofocusing is shown by numerical analyses with Fourier atom optics. Finally, we describe atom funnels with evanescent light to generate a cold atomic beam required for atom manipulation using nanometric near-field light. The flux intensity of cold Rb atoms outputted from a 240??m outlet is estimated to be 7.5 ? 107?atom?cm?2?s?1 at a blue detuning of 1.2?GHz. In order to increase the flux intensity to 1012?atom?cm?2?s?1, we develop a Si-made funnel with a micron-sized outlet.
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 demonstrated an atom funnel using blue-detuned near-field light. Cold Rb atoms generated by a magneto-optical trap (MOT) were introduced into the funnel and the output atoms were counted by an accumulative MOT detection system. The number of cold Rb atoms outputted from a 200-/spl mu/m exit hole was maximal at a blue detuning of 1.2 GHz under Sisyphus cooling. The flux intensity was estimated to be 10/sup 5/ atoms/cm/sup 2/s. To increase the flux intensity up to 10/sup 13/ atoms/cm/sup 2/s, we made an atom funnel with a 2-/spl mu/m exit hole.
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.
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.