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Physical Review A

Atom guiding and cooling in a dark hollow laser beam

Jianping Yin, Yifu Zhu, Wonho Jhe, Zuzhu Wang

Published Jul 1, 1998• 107 citations
Publisher

Abstract

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 ${\mathrm{LP}}_{01}$ 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.

DOI
10.1103/physreva.58.509

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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∼1.2\ensuremathμK,\ensuremath{\sim}1.2\ensuremath{\mu}\mathrm{K},\ensuremath∼1.2\ensuremathμK, a final longitudinal mean velocity of 0--4.41 m/s, and a guiding efficiency of 65--95 % may be obtained.
We propose a simple atomic-beam collimator consisting of a blue-detuned cylindrical-hollow (doughnut) laser beam. Cold atoms from a magneto-optical trap are loaded into the doughnut beam and guided in the propagation direction of the doughnut beam. The cold atoms experience efficient Sisyphus cooling induced by the doughnut beam and a weak repumping beam and are collimated in the transverse direction. In the longitudinal direction the cold atoms experience small accelerations that are due to the absorption-induced heating of the doughnut beam and the repumping beam. The cooling, heating, and loss mechanisms in this atomic collimator are analyzed. Our study shows that an intense cold atomic beam with a minimum transverse velocity of ∼2 cm s-1 (a minimum transverse temperature of ∼1.4 μK) and a brightness of approximately 2×1013 atoms (sr-1 cm-2 s-1) can be obtained from the collimator.
Summary form only given.We present a novel gravito-optical trap (GOT) consisting of a dark hollow-divergent laser beam (DHB) either from a collimated and focused doughnut beam or from the output beam of the LP/sub 01/ mode in a hollow optical fiber. The cold atoms are loaded into the DHB from a magneto-optical trap (MOT). As the atoms bounce in the DHB, they experience Sisyphus cooling and geometrical cooling induced by the blue detuned DHB and a very weak repumping beam propagating downward into the DHB. The coupling of the three-level atom and the DHB forms dressed states.
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.
Presents work on the realization of Bose-Einstein condensates (BEC) in optical and electro-magnetic traps. We have done some theoretical and experimental work on the project. We have proposed an optical funnel or a hollow beam, which are produced from a short micron-sized hollow optical fiber or a set-up of lenses. The cold atoms extracted from an magneto-optical trap experience effective Sisyphus cooling in the dark hollow beam. We calculate the potential barriers for atomic guiding in the dark hollow beam, and analyze the atomic losses during the guiding process. Inside the hollow beam atoms experience an efficient Sisyphus cooling and repumping-beam induced geometric cooling. We have estimated the total loss rate of the cold atoms, and it shows that an ultracold and dense Rb atomic sample with a 3D temperature of /spl sim/1 K can be obtained and the appearance of an optically-trapped Bose-Einstein condensation may be possible in this optical trap. We have also made a proposal of a micro-wave electro-magnetic trap, which is working at the frequency which is very far from the optical transition of the cold atoms. The trapping force comes from the interaction between the electro-magnetic induced magnetic dipole moment with the electro-magnetic field. Theoretical calculation shows that the force can be used to trap the cold atoms in the trap.
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.
We propose a dark gravito-optical dipole trap, for alkali atoms, consisting of a blue-detuned, pyramidal-hollow laser beam propagating upward and the gravity field. When cold atoms from a magneto-optical trap are loaded into the pyramidal-hollow beam and bounce inside the pyramidal-hollow beam, they experience efficient Sisyphus cooling and geometric cooling induced by the pyramidal-hollow beam and the weak repumping beam propagating downward. Our study shows that an ultracold and dense atomic sample with an equilibrium 3D momentum of � 3 ¯ and an atomic density above the point of Bose-Einstein condensation may be obtained in this pure optical trap. PACC: 3280P; 3380P; 4250