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Journal of the Optical Society of America B

Doughnut-beam-induced Sisyphus cooling in atomic guiding and collimation

Jianping Yin, Yifu Zhu

Published Aug 1, 1998• 11 citations
Publisher

Abstract

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.

DOI
10.1364/josab.15.002235

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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 $\ensuremath{\sim}1.2\ensuremath{\mu}\mathrm{K},$ a final longitudinal mean velocity of 0--4.41 m/s, and a guiding efficiency of 65--95 % may be obtained.
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.
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We propose a trap scheme for cold alkali-metal atoms with an efficient intensity-gradient induced Sisyphus cooling in a weak standing-wave hollow-beam gravito-optical trap, which is composed of the interference of two well collimated, counterpropagating doughnut hollow beams with an intensity difference and a plug beam. We calculate the intensity distribution of the weak standing-wave hollow-beam field and its intensity gradient one, and find that such an optical dipole trap with an extremely high intensity gradient is desirable to realize an efficient intensity-gradient cooling for alkali-metal atoms in the trap. We also calculate the optical potentials, instantaneous dipole forces and spontaneous emission rates for a three-level dressed atom and study the dynamic process of intensity-gradient cooling of 87Rb^{87}\mathrm{Rb}87Rb atoms in the weak standing-wave hollow-beam trap by Monte Carlo simulations. Our study shows that the minimum optical potential at each node in our dipole trap is high enough to trap almost all cold atoms with a temperature of $120\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ released from a standard magneto-optical trap, and an ultracold 87Rb^{87}\mathrm{Rb}87Rb atomic sample with a temperature of $\ensuremath{\sim}0.73\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ can be obtained in the trap. Starting from this stage, an all-optical Bose-Einstein condensation (BEC) could be realized by using the optical-potential evaporative cooling.
We propose a novel gravito-optical atom trap composed of a blue-detuned, conical dark-hollow-beam propagating upward and the gravity field. When cold atoms are loaded into the conical dark-hollow-beam from a magnetic-optical trap and bounce inside the hollow beam, they experience an efficient hollow-beam induced Sisyphus cooling and repumping-beam induced geometric cooling. Our investigation shows that an ultracold and dense 87 Rb atomic sample with an equilibrium 3D temperature of about 1 μK can be obtained and the appearance of an optically-trapped Bose-Einstein condensation may be possible in this pure optical trap.
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.