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Physical Review A

Atom-number amplification in a magneto-optical trap via stimulated light forces

Tara Cubel Liebisch, E. Blanshan, Elizabeth A. Donley, John E. Kitching

Published Jan 10, 2012• 16 citations
Publisher

Abstract

We decelerated an atomic beam of ${}^{87}$Rb using a stimulated-emission slowing technique that employs a bichromatic standing light wave of high intensity, and increased the atom number in a magneto-optical trap (MOT) of low capture velocity by up to a factor of 14, and the load rate into the MOT by a factor of 20. We performed the slowing over distances under 1.5 cm with 5.2 mW of total power in the bichromatic beams. The average stimulated force was 2.2 times the maximum spontaneous emission force, and could be made even stronger in a smaller apparatus.

DOI
10.1103/physreva.85.013407

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We decelerated an atomic beam of 87{}^{87}87Rb using a stimulated-emission slowing technique that employs a bichromatic standing light wave of high intensity, and increased the atom number in a magneto-optical trap (MOT) of low capture velocity by up to a factor of 14, and the load rate into the MOT by a factor of 20. We performed the slowing over distances under 1.5 cm with 5.2 mW of total power in the bichromatic beams. The average stimulated force was 2.2 times the maximum spontaneous emission force, and could be made even stronger in a smaller apparatus.
We demonstrate atom number enhancement in a magneto-optical trap (MOT) by use of bichromatic cooling to slow an atomic beam that is loaded into a MOT. Bichromatic cooling employs stimulated emission to apply strong cooling forces that are not limited by spontaneous emission. We demonstrate a factor of 3.5 increase in atom number captured from the atomic beam for a 1.5 cm cooling length. For a 1.5 cm effective cooling length, our technique yields MOT atom number enhancement that is about three times higher than the enhancement achieved via spontaneous emission.
For cold samples of laser-cooled atoms to be useful in emerging technologies such as compact atomic clocks and sensors, it is necessary to achieve small sample sizes while retaining a large number of cold atoms. Achieving large atom numbers in a small system is a major challenge for producing miniaturized laser-cooled atomic clocks, since the number of captured atoms in a vapor-cell magneto-optical trap (MOT) scales as the fourth power of the laser beam diameter. This strong dependence on size is fundamentally set by the maximum spontaneous light force ħkγ/2, where ħk is the photon momentum and γ/2 is the maximum spontaneous photon scatter rate of a saturated transition of linewidth γ. We are attempting to surmount the limit imposed by spontaneous emission by using bichromatic cooling - a technique that uses stimulated emission to slow the atoms. We have built a table-top experiment that uses stimulated-emission bichromatic cooling to pre-cool rubidium atoms and dramatically enhance the trappable atom number in a small MOT. The apparatus lets us test how bichromatic cooling scales with miniaturization. Here we report on our first experimental results of cooling a thermal beam of rubidium atoms down to MOT capture velocities.
We produced a slow atomic beam of rubidium with a low-velocity intense source (LVIS) and loaded a magneto-optical trap (MOT) in a connected chamber with the beam. The hole for the atomic beam allowed us to keep the MOT chamber at a much lower pressure than that of the LVIS. The LVIS produced an atomic beam with a continuous flux of (4.3±0.4)×109/s and an average velocity of only 11±3 m/s. The MOT captured the atoms with 67±15% efficiency, which was limited by the atomic-beam divergence. The MOT held 6×109 atoms with a typical trap-confinement time of 8 s.
We demonstrate a novel dual color magneto—optical trap (MOT), which uses two sets of overlapping laser beams to cool and trap 87 Rb atoms. The volume of cold cloud in the dual color MOT is strongly dependent on the frequency difference of the laser beams and can be significantly larger than that in the normal MOT with single frequency MOT beams. Our experiment shows that the dual color MOT has the same loading rate as the normal MOT, but much longer loading time, leading to threefold increase in the number of trapped atoms. This indicates that the larger number is caused by reduced light induced loss. The dual color MOT is very useful in experiments where both high vacuum level and large atom number are required, such as single chamber quantum memory and Bose—Einstein condensation (BEC) experiments. Compared to the popular dark spontaneous-force optical trap (dark SPOT) technique, our approach is technically simpler and more suitable to low power laser systems.
We analyzed bichromatic and polychromatic stimulated forces for laser cooling and trapping of Yb atoms using only the narrow ${}^{1}{S}_{0}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}{\phantom{\rule{4pt}{0ex}}}^{3}{P}_{1}$ transition. Our model is based on numerical solutions of optical Bloch equations for two-level atoms driven by multiple time-dependent fields combined with Monte Carlo simulations, which account for realistic experimental conditions such as atomic beam divergence, geometry, and Gaussian laser modes. Using 1 W of laser power, we predict a loading rate of $\ensuremath{\approx}\phantom{\rule{4pt}{0ex}}{10}^{8}$ atoms/s into a 556-nm magneto-optical trap (MOT) with a slowing force of $\ensuremath{\approx}60{F}_{\text{rad}}$. We show that a square-wave modulation can produce similar stimulated forces with almost twice the velocity range and improve the MOT loading rate of Yb atoms by up to 70%.