Evanescent-wave guiding of atoms in hollow optical fibers.

Abstract

An atom placed in a near-resonant laser field is either attracted to or repelled from regions of high intensity depending on the sign of the laser’s detuning from atomic resonance. We have demonstrated that the optical forces induced by laser light guided in a fiber may be used to reflect atoms from the inner wall of a hollow-core optical fiber in a recent work [1,2]. In that demonstration, light was coupled to the lowest-order grazing incidence mode [3] and the laser frequency was tuned to the red side, so that atoms were attracted to the high-intensity region at the center of the fiber. Atoms guided in this way undergo a series of lossless oscillations in the transverse plane and unconstrained motion along the axis. Atoms can also be guided by the evanescent light field of the glass surface surrounding a hollow fiber. With a detuning on the blue side of resonance, atoms are expelled from the high-intensity-field region near the fiber wall. The intensity in the evanescent field is significant at a distance of .=X/2.rr into the hollow region. Consequently, the atoms are nearly specularly reflected from the potential walls. Atom propagation through the fiber in this case is similar to the propagation of light in a multimode, step-index fiber. Evanescent guiding has several advantages over guiding by grazing incidence modes: heating of the atoms due to spontaneous scattering of photons is small in the evanescent case because the atoms spend most of the time in a dark region away from the high laser intensity at the wall. In the grazing incidence configuration, atoms are guided in the high-intensity region, and consequently the spontaneous scattering rate is relatively high. Furthermore, in evanescentwave guiding, the optical potential is generated by light traveling in lossless guided modes. Small-diameter atomic guides of very long length may be practical. By contrast, grazing incidence optical modes decay exponentially with distance [3], effectively limiting the guiding distance to a