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Journal of the Optical Society of America B

Generation of a slow and continuous cesium atomic beam for an atomic clock

Sang Eon Park, Ho Seong Lee, Eun-joo Shin, Taeg Yong Kwon, Sung Hoon Yang, Hyuck Cho

Published Nov 1, 2002• 21 citations
Publisher

Abstract

A thermal atomic beam from a cesium oven was slowed down by use of the Hoffnagle modified white-light cooling technique. In addition, the atomic beam was collimated by use of a two-dimensional optical molasses that was installed transverse to the atomic-beam direction. The flux of the atomic beam was 2×1010 atoms/s, an increase of a factor of 16 as a result of the collimation. The mean longitudinal velocity was ∼24.4 m/s, and the rms velocity spread of the slowed atomic beam was ∼1 m/s. Compared with other methods, we found that the Hoffnagle method is suitable for the generation of slow atomic beams to be used in an atomic clock, which requires an ultralow magnetic field environment. This atomic beam was deflected by an angle of 30° by a one-dimensional optical molasses to separate it from laser light and high-velocity atoms.

DOI
10.1364/josab.19.002595

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Journal of the Optical Society of America B•Apr 1, 2015•7 citations
A thermal atomic beam from a cesium oven was slowed down by use of the Hoffnagle modified white-light cooling technique. In addition, the atomic beam was collimated by use of a two-dimensional optical molasses that was installed transverse to the atomic-beam direction. The flux of the atomic beam was 2×1010 atoms/s, an increase of a factor of 16 as a result of the collimation. The mean longitudinal velocity was ∼24.4 m/s, and the rms velocity spread of the slowed atomic beam was ∼1 m/s. Compared with other methods, we found that the Hoffnagle method is suitable for the generation of slow atomic beams to be used in an atomic clock, which requires an ultralow magnetic field environment. This atomic beam was deflected by an angle of 30° by a one-dimensional optical molasses to separate it from laser light and high-velocity atoms.
By the moving molasses technique we have extracted laser-cooled cesium atoms in a continuous way directly from an optical molasses. The mean launching velocity is precisely tunable from 1 to 12 m/s. The atomic beam has a flux of $1.3\ifmmode\times\else\texttimes\fi{}{10}^{8} \mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}/\mathrm{s}$ at 7 m/s and a longitudinal temperature of $70 \ensuremath{\mu}\mathrm{K},$ which represents the highest flux and lowest velocity spread obtained so far in a continuous beam of cold atoms. These features makes it well suited for atomic fountains. The atomic flux can be slightly increased in a two-dimensional magneto-optical trap operation (+40(+40%).(+40 A simple model accounts for the observed dependence of the flux with the magnetic-field gradient.
Laser-cooled low-velocity (v < 10 m/s) atomic beams have been considered to be feasible cold atom sources in developing compact and highly stable atomic clocks for satellite applications. A cold atomic beam can be realized by laser-accelerating an ensemble of ultracold (T < 1 mK) atoms from a laser-cooled magneto-optical trap (MOT). This technique has the unique advantage of generating a useful cold atomic beam just outside the volume of a MOT and, hence, can greatly reduce the size of the atomic clock physics package. In this paper, we present our design and experimental results towards demonstration of a MOT-based cold Cs beam atomic clock. We have generated a continuous cold Cs beam from a Cs MOT and have determined the typical longitudinal velocity of the Cs beam by Time of Flight (TOF) method to be 7 m/s with a velocity spread of 1 m/s. By adjusting the MOT parameters, the atomic beam velocity can be tuned continuously, with the velocity spread remaining to be 1 m/s. In order to separate the atomic beam from the MOT laser beam, we have used a one-dimensional optical molasses to efficiently deflect the cold Cs beam through an angle of 30 degrees. The 1-D optical molasses is set up in such a way that the molasses laser beams are perpendicular to the final atomic beam propagation path. Thus, the Cs-beam velocity components along the molasses axis are efficiently damped to nearly zero, while the Cs velocity component along the final atomic beam propagation path is unaffected. Our simulation indicates that a deflection efficiency of 100% is achievable for MOT-based cold atomic beams. Our cold Cs beam has an instantaneous atomic flux of 3.6 10 (exp 10) atoms/s when operated in pulsed mode and an estimated continuous beam flux of 2 10 (exp 8) atoms/s.
We have built a Zeeman-slower apparatus which produces a slow and cold cesium atomic beam. The atomic beam has a mean velocity in the range 35--120 m/s and a high atomic current of more than $2\ifmmode\times\else\texttimes\fi{}{10}^{10}$ cold atoms/s. A small longitudinal velocity spread was achieved by optimizing the termination of the slowing process. The measured value of less than 1 m/s is consistent with a numerical simulation of the slowing process. With a magnetic lens and a tilted two-dimensional optical molasses stage, the slow atomic beam is transversely compressed, collimated, and deflected. We achieve a transverse temperature below the Doppler limit. The brilliance of this beam has been determined to be $7\ifmmode\times\else\texttimes\fi{}{10}^{23}$ atoms ${\mathrm{s}}^{\ensuremath{-}1}{\mathrm{m}}^{\ensuremath{-}2}$ ${\mathrm{sr}}^{\ensuremath{-}1}.$ By optical pumping the slow atomic beam can be polarized in the outermost magnetic substates ${F=4,m}_{F}=\ifmmode\pm\else\textpm\fi{}4,$ of the cesium ground state. This brilliant beam is an ideal source for experiments in atom optics and atom lithography.
To achieve a primary frequency standard based on a continuous fountain of laser cooled cesium atoms, we report on the production of a bright, slow and cold continuous atomic beam. Atoms are cooled from a thermal vapor in a 45/spl deg/ laser geometry and launched directly from the optical molasses by the moving molasses technique. The effects of the source fluorescence on the continuous beam (average velocity, temperature and flux) have been observed, and ways to optimize simultaneously the atomic capture and the continuous beam are presented. Flux measurements rise to 2/spl times/10/sup 8/ at/s from a 2D magneto-optical trap at 7 m/s. The initial velocity is easily tunable from 1 to 10 m/s. If the longitudinal temperature is about 70 /spl mu/K, additional transverse cooling even reduces the transverse temperature in one dimension below 30 /spl mu/K. By extrapolating the obtained atomic flux to the continuous fountain under construction, we expect a short-term stability of 7/spl times/10/sup -14//spl tau//sup -1/2/. Other applications of continuous beams of cold atoms are suggested.
We describe the design and performance of a slow atom source based on a 2D magneto-optical trap (MOT) that uses an innovative simple optical configuration. Metal-coated retro-reflecting prisms replace mirrors and quarter-wave plates so the optical power of the cooling laser beam is recycled. This source has been characterised for three different configurations: with and without transverse magnetic field gradient, and with a pusher beam to obtain a 2D+-MOT. The longitudinal velocity is of the order of 25 m·s−1, with a transverse velocity spread ≤1 m·s−1, while the typical atomic flux density obtained is up to 1.3×1014 at ·s−1·m−2 for a cesium vapour pressure of ~4×10−8 mbar in the source. We use this slow atom beam, instead of cesium vapour, to load a 3D moving optical molasses that feeds a continuous cold atom fountain. We obtain a gain of a factor ~20 in the atomic flux launched by the fountain.
We investigate optical collimation of an atomic beam using a spectrally broadened white light molasses. We compare this technique with the standard technique using an atomic collimator and find that, under certain conditions, the white light molasses is preferable. Specifically, a white light molasses is suitable for atomic beams with relatively low mean velocities and broad velocity distributions, for example, a beam effusing from a cryogenically cooled reservoir. This result is demonstrated by numerically modeling the atomic trajectories of argon atoms effusing from a cryogenically cooled reservoir. We find that, given sufficient power, a white light molasses captures a larger fraction of atoms when compared to an atomic collimator, and the velocity distribution of the resulting beam is shifted toward slower velocities. Further, an optical molasses is geometrically relatively simple compared to an atomic collimator; therefore, the complexity of the in-vacuum optical arrangement can be significantly reduced.