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Journal of Experimental and Theoretical Physics Letters

Focusing of an atomic beam by a two-dimensional magneto-optical trap

Павел Николаевич Мелентьев, P. A. Borisov, S. N. Rudnev, Anton E. Afanasiev, V. I. Balykin

Published Mar 1, 2006• 9 citations
Publisher

Abstract

A method for focusing neutral atoms based on the light-pressure force in a nonuniform magnetic field is proposed and analyzed. Its particular scheme is realized by means of a two-dimensional magneto-optical trap using a thermal beam of Rb atoms. A feature of this focusing method is the linear dependence of the focal length on the longitudinal velocity of atoms in contrast to the quadratic dependence in the known methods of focusing material-particle beams. The minimum size of the waist of the focused atomic beam is equal to 270 µm. Owing to focusing by means of the two-dimensional magneto-optical trap, the velocity monochromatization of a thermal atomic beam is realized: the width of the distribution of the longitudinal atomic velocities in the beam is reduced from 350 to 60 m/s.

DOI
10.1134/s0021364006010048

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A method for focusing neutral atoms based on the light-pressure force in a nonuniform magnetic field is proposed and analyzed. Its particular scheme is realized by means of a two-dimensional magneto-optical trap using a thermal beam of Rb atoms. A feature of this focusing method is the linear dependence of the focal length on the longitudinal velocity of atoms in contrast to the quadratic dependence in the known methods of focusing material-particle beams. The minimum size of the waist of the focused atomic beam is equal to 270 µm. Owing to focusing by means of the two-dimensional magneto-optical trap, the velocity monochromatization of a thermal atomic beam is realized: the width of the distribution of the longitudinal atomic velocities in the beam is reduced from 350 to 60 m/s.
We present a two-dimensional (2D) magneto-optical trap (MOT) setup for the production of a continuous collimated beam of cold 87Rb{}^{87}\mathrm{Rb}87Rb atoms out of a vapor cell. The underlying physics is purely two-dimensional cooling and trapping, which allows for a high flux of up to $6\ifmmode\times\else\texttimes\fi{}{10}^{10}$ atoms/s and a small divergence of the resulting beam. We analyze the velocity distribution of the 2D MOT. The longitudinal velocity distribution of the atomic beam shows a broad feature (full width at half maximum $\ensuremath{\simeq}75 \mathrm{m}/\mathrm{s}),$ centered around 50 m/s. The dependence of the flux on laser intensity, on geometry of the trapping volume, and on pressure in the vapor cell was investigated in detail. The influence of the geometry of the 2D MOT on the mean velocity of the cold beam has been studied. We present a simple model for the velocity distribution of the flux based on rate equations describing the general features of our source.
The fine control of a neutral atomic beam by radiation pressure has opened a new field called atom lithography. In particular, a direct writing method, which focuses an atomic beam with a standing-wave light field into narrow lines, has been demonstrated. A velocity-controlled and isotope-selected atomic beam is essential for improving the performance of this lithography method. An atomic deflection technique using radiation pressure is indispensable for producing such an atomic beam. Here we report on an atomic beam guide using a one-dimensional magneto-optical trap (1D-MOT). First we decelerate atoms whose velocities lie in the range between 250 and 320 m/s. A diode laser (780 nm) is used as the light source for deceleration and its injection current is modulated at 2 MHz. Then the decelerated beam is deflected by a tilted 1D-MOT, which consists of four copper rods and four rectangular mirrors.
We discuss the implementation and characterization of the source of a slow, intense, and collimated beam of rubidium atoms. The cold atomic beam is produced by two-dimensional magneto-optical trapping in directions transverse to the atomic beam axis and unbalanced Doppler cooling in the axial direction. The vacuum design allows use of relatively low laser power and a considerably simplified assembly. The atomic beam has a high flux of about $2\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}\mathrm{atoms}∕\mathrm{s}$ at a total cooling laser power of 55mW55\phantom{\rule{0.3em}{0ex}}\mathrm{mW}55mW. It has a narrow longitudinal velocity distribution with mean velocity 15m∕s15\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}15m∕s with full width at half maximum 3.5m∕s3.5\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}3.5m∕s and has a low divergence of 26mrad26\phantom{\rule{0.3em}{0ex}}\mathrm{mrad}26mrad. The high flux enables ultrafast loading of about 1010{10}^{10}1010 atoms into a three-dimensional (3D) magneto-optical trap within 500 ms. The variation of the atomic beam flux was studied as a function of the rubidium vapor pressure, cooling laser power, transverse cooling laser beam length, detuning of the cooling laser, and relative intensities of the cooling beams along the atomic beam axis. We also discuss a detailed comparison of our measurements of the cold atomic beam with a 3D numerical simulation.
A beam source is proposed for the production of an intense cold cesium atomic beam that can be used in cesium beam atomic clocks. The source is based on a two-dimensional magneto-optical trap (2D-MOT), but introduces hollow cooling and pushing lights in the axial direction to create a 2D+-MOT, which separates the cooling and pushing functions while the low-power pushing light pushes atoms out to form a cold atomic beam. This cold cesium atomic beam source reduces the light shift due to leakage light and retains longitudinal cooling to increase the flux of the cold atomic beam compared with that of the conventional 2D+-MOT scheme. The specifics of the design are investigated, the atomic velocity and beam flux are calculated, and the results are experimentally verified. The results demonstrate that when the power of the pushing light is 180 µW and when its frequency resonates with the 4 → 5′ transition of the Cs D2 line, the most probable longitudinal velocity of the outgoing cold atomic beam, the width of velocity distribution, and the atomic beam flux are 19.38 m/s, 8.1 m/s, and 1.7 × 1010 atoms/s, respectively.
A method has been proposed to increase the rate of loading of atoms in a U-magneto-optical trap near an atom chip. The method is based on the focusing of a slow atomic beam into the localization region of the atom chip. The overdamped focusing regime has been considered. In this case, the focal length is independent of the initial transverse velocity of atoms. It has been shown that the focusing of the atomic beam makes it possible to increase the loading rate in the localization region 250 μm in diameter by a factor of 160.
We experimentally study the use of two-dimensional magneto-optical trapping (2D-MOT) for the generation of slow beams of cold atoms out of a vapor cell. A particularly high flux of $9\ifmmode\times\else\texttimes\fi{}{10}^{9}$ rubidium atoms/s at a mean velocity of 8 m/s is obtained using a combination of magneto-optical trapping in two dimensions and Doppler cooling in the third dimension $({2\mathrm{D}}^{+}\ensuremath{-}\mathrm{MOT}).$ The resulting width of the velocity distribution is 3.3 m/s [full width at half maximum (FWHM)] with a beam divergence of 43 mrad (FWHM). We investigate the total flux as a function of vapor cell pressure and determine the velocity distribution of our slow atom sources. For comparison, we also realized a low-velocity intense source (LVIS), first reported by Lu et al. [Phys. Rev. Lett. 77, 3331 (1996)]. We find that the ${2\mathrm{D}}^{+}\ensuremath{-}\mathrm{MOT}$ yields a significantly higher flux than the LVIS, even when used with an order of magnitude less laser power.