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Conference on Lasers and Electro-Optics Pacific Rim (CLEO-PR)

Theory of atomic wave-guidance in dark hollow beam

Xinye Xu, Yuzhu Wang, Wonho Jhe

Published Jan 20, 2003• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cleopr.1999.817883

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Journal of the Optical Society of America B•Jan 1, 1998•86 citations
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 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%.
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.
This thesis is concerned with the guiding of cold atoms using optical forces, which is of great importance in the field of atom optics. Atomic beams can be used for precision sensor equipment, building nano-scale structures, construction of quantum computers and to further the understanding of the properties of atoms. Atoms are guided along light beams using the dipole force; there are two regimes under which this force works. Typically red-detuned guides are used (atoms are attracted towards the light) such guides, however, require large detuning and high powers. In this thesis we investigate the use of blue-detuned (atoms are repelled from the light) hollow light beams of moderate power (a few hundred mW) and confine atoms in the dark centre of the beams. Several magneto-optical traps (MOTs) have been constructed to exploit different guiding geometries. Hollow beams have been generated using a variety of methods; in particular the use of a computer controlled spatial light modulator (SLM) has provided great versatility and simplicity to the experimental arrangements. First, experiments were performed with a low-velocity intense source (LVIS) of atoms. A co-linear LG beam significantly enhances the observed flux, however, considerable difficulties are encountered loading atoms into oblique guides. Imaging a hole in the walls of the light tube was used to improve the loading efficiency. Second, guiding a free-falling atom cloud is performed using a non-diffracting Bessel beam. It is found that while the potential of the Bessel beam is steeper than equivalent LG beams the power distribution across the beam severely limits its usefulness. The next study investigated higher-order LG guide beams generated with an SLM. High order modes have a narrower profile so confine the atoms with less interaction with the guide beam, leading to a more natural guide (as opposed to a pushing force). Finally the SLM was used to create non-trivial beam shapes for beam splitters and interferometers.
A theoretical investigation of atomic trapping and guiding by quasi-dark hollow beams is presented in this paper. By quasi-dark hollow beams we refer to beams of which the axial intensity is lower than the surrounding intensity, but not equal to zero. A new theoretical model of quasi-dark hollow beams is applied, which is appropriate to describe some kinds of practical hollow beams. The dependences of the corresponding atom trap resolution, the maximum possible detuning, the trap darkness factor and the ensemble-averaged spontaneous photon scattering rate on the beam parameters are analysed in detail. The relations among optical beam waist size, guide distance, darkness ratio and minimum detuning are obtained. The results obtained can not only explain the existed trapping experiments, but also predict the requirements on the beam quality to achieve particular atomic trapping and guiding.
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.