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Journal of the Optical Society of America B

Theory of atom guidance in a hollow laser beam: dressed-atom approach

Xinye Xu, Yuzhu Wang, Wonho Jhe

Published Jun 1, 2000• 67 citations
Publisher

Abstract

We present a general theory of atom guiding in a blue-detuned hollow laser beam. Using the dressed-atom approach, we obtain the mean dipole gradient force, the radiation pressure force, and the momentum diffusion coefficients for three-level Λ-type cold atoms. Using Monte Carlo simulation, we calculate the guiding efficiencies and the final velocity distributions of atoms for various conditions. We find that the guiding efficiency depends not only on the intensity and detuning of the guiding hollow beam but also on the atom-guiding direction with respect to the propagation direction of the hollow laser beam. Comparing our analyses with recent experimental results, we find that they are mutually consistent. The results that we present can also be applied to atom guiding by hollow optical fibers.

DOI
10.1364/josab.17.001039

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Chinese Physics Letters•Jan 1, 2005•0 citations
We present a general theory of atom guiding in a blue-detuned hollow laser beam. Using the dressed-atom approach, we obtain the mean dipole gradient force, the radiation pressure force, and the momentum diffusion coefficients for three-level Λ-type cold atoms. Using Monte Carlo simulation, we calculate the guiding efficiencies and the final velocity distributions of atoms for various conditions. We find that the guiding efficiency depends not only on the intensity and detuning of the guiding hollow beam but also on the atom-guiding direction with respect to the propagation direction of the hollow laser beam. Comparing our analyses with recent experimental results, we find that they are mutually consistent. The results that we present can also be applied to atom guiding by hollow optical fibers.
The theory of atom guiding in a far blue-detuned hollow laser beam (HLB) is developed for the dipole interaction scheme described by a three-level $\ensuremath{\Lambda}$ model. The complete kinetic description of atomic motion based on the Fokker-Planck equation for the atomic distribution function is presented. The dipole gradient force, radiation pressure force, and momentum diffusion tensor are then derived. It is found that even for a far-detuned laser beam, the optical potential for a three-level $\ensuremath{\Lambda}$ atom is not generally reduced to a sum of two independent potentials associated with the two two-level interactions in the $\ensuremath{\Lambda}$ scheme. The theory developed here is also compared with the experimental guiding of cold 85Rb{}^{85}\mathrm{Rb}85Rb atoms in the HLB. The experimental results are found to be in good agreement with the Monte Carlo simulations based on the three-level $\ensuremath{\Lambda}$ model. We observe that the guiding efficiency depends strongly on the intensity and the detuning of the HLB and the initial temperature of atoms. In particular, the experimental results show that, at small detunings, the guiding efficiency is deteriorated strongly by the radiation pressure force. The Monte Carlo simulations also indicate that the efficiency of guiding versus detuning depends strongly on the direction of the HLB propagation with respect to that of atomic motion. Under optimal conditions, the guiding efficiency was found to be about 20%.
Firstly the interaction of a three-level /spl Lambda/-type atom with a blue-detuning dark hollow beam is described in the dressed-atom picture. The dipole potential and radiation pressure forces are derived, and the spontaneous emission processes between both adjacent manifolds are analyzed in detail. Then through Monte Carlo simulation, the relations between the guiding efficiency and the detuning in varieties of conditions are acquired. We found that the guiding efficiency also depends on guiding direction relative to the propagation direction of the hollow beam except for the intensity and configuration of the hollow beam and pressure of the chamber, when both directions are opposite, the optimal detuning is in high detuning region, the value is similar to the experimental value. Furthermore, we calculated the final velocity distribution of atoms guided by the hollow beam, we found that in same direction case, even though in low detuning region the guiding efficiency is high, but the radial and longitudinal coherence are seriously destroyed, the most atoms are much heated. It is not useful in application to atom optics experiments. The main factors are the radiation pressure forces and photon scattering effects. Finally we found that the atoms can only be guided in the high detuning region, and their coherence will be held whether same or opposite direction.
We present an experimental and theoretical study of efficient optical guiding of trapped cold atoms by a blue-detuned hollow laser beam. We have measured the atomic guiding efficiency with respect to the propagation direction of the hollow beam, which is in good agreement with numerical analysis. In particular, using a copropagating guide laser, we have obtained 20-fold enhancement of atom guiding with a guiding efficiency of 50%. Simulation shows that the efficiency can be increased to 80% with an improved hollow beam mode. The possibility of coherent atom guiding is also discussed.
We propose and analyze theoretically a scheme for atomic guiding and cooling in a dark hollow laser beam generated from the collimated output beam of a LP01{\mathrm{LP}}_{01}LP01​ mode in a micrometer-sized hollow optical fiber. In the scheme, cold atoms from a magneto-optical trap are loaded into the blue-detuned dark hollow beam, move down in the dark hollow beam, and experience transverse Sisyphus cooling induced by the dark hollow beam and a weak repumping beam. In the longitudinal direction, the guided atoms experience heating from the gravity field and cooling from the upward-propagating dark hollow beam and repumping beam. We estimate the transverse two-dimensional equilibrium temperature, the final longitudinal mean velocity, and the total loss of the guided atoms. Our calculations show that a transverse equilibrium temperature of $\ensuremath{\sim}1.2\ensuremath{\mu}\mathrm{K},$ a final longitudinal mean velocity of 0--4.41 m/s, and a guiding efficiency of 65--95 % may be obtained.
We analyze the optical potential for atom guidance in a dark hollow laser beam and compare it with that generated by the evanescent wave in a micrometer-sized hollow optical fiber. We calculate dependence of the optical potential on various parameters: radial position r, laser–atom detuning Δ, beam power P0, and propagation distance Z. We experimentally generated a dark hollow laser beam from a hollow optical fiber with a 7-μm hole diameter and obtained a coupling efficiency of ∼50% for the LP01 mode. Our study shows that a dark hollow laser beam can produce an optical potential deep enough to guide cold atoms from a magneto-optical trap.
Optical guiding of ^Rb atoms in a magneto-optical trap (MOT) by a blue-detuned horizontal hollow laser beam is demonstrated experimentally. The guiding efficiency and the velocity distribution of the guided atoms are found to have strong dependence on the detuning of the guiding laser. In particular, the optimum guiding occurs when the blue detuning of the hollow laser beam is approximately equal to the hyperfine structure splitting of the ^85Rb ground states, in good agreement with the theoretical analysis based on a three-level model.