Optical-dipole-force fiber guiding and heating of atoms

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Abstract
We present an experimental and theoretical investigation characterizing the flux of laser-guided atoms through hollow-core optical fibers and show how it depends on laser detuning from resonance, laser intensity, and fiber curvature. The guiding employs dipole forces arising from the interaction of the atoms with the optical field. Laser light is focused into the hollow region of 40-μm-inner-diam capillary fiber and guided through the fiber by grazing incidence reflection from the glass walls. The lowest-order mode is azimuthally symmetric with maximum intensity on the fiber axis and nearly zero intensity at the walls. Rubidium atoms are attracted to the high-intensity region along the axis when the laser is detuned to the red of resonance and consequently guided through the fiber. Of particular interest is the evolution of the atom-flux versus laser-detuning profile of increasing intensity. At low intensities the dipole guiding potential is purely conservative and the flux profile is roughly dispersion shaped. At high intensity, viscous dipole forces heat the atoms and ``burn a hole'' in the flux-detuning curve. We find that transverse heating of the atoms and the exponential attenuation of optical mode intensity limit the distance atoms may be guided to about 20 cm in a 40-μm-diam fiber. Bending the fiber can reduce the effects of heating on atom flux.