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arXiv:2005.09811 · Physical Review Applied

Magneto-Optical Trap with Millimeter Ball Lenses

Cainan S. Nichols, Leo Nofs, Michael A. Viray, Lu Ma, Eric Paradis, Georg Raithel

Published Oct 9, 2020• 10 citations

Abstract

We present a magneto-optical trap (MOT) design based on millimeter ball lenses, contained within a metal cube of 19-mm side length. The ball lenses create highly divergent conical MOT trapping beams. In our experiment and in trajectory simulations, we study the effect of beam divergence and light-intensity gradients on MOT performance. We trap approximately $4.2\ifmmode\times\else\texttimes\fi{}{10}^{5}{\phantom{\rule{0.2em}{0ex}}}^{85}\mathrm{Rb}$ atoms in our ball-lens MOT at a density of $3.2\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ and a loading time of 1.3 s. Experimental measurements and the trajectory model are used to calibrate the trapping efficiency in the highly divergent light fields of the ball lenses. The advantages of the design include small lens and chamber sizes favorable to miniaturization, the absence of large-diameter optical-beam pathways, and a low laser-power requirement. These features are conducive to atom-trap geometries with well-defined electromagnetic boundary conditions, as required for ion and plasma traps, and for efficient shielding of black-body and dc stray fields.

DOI: 10.1103/physrevapplied.14.044013

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We present a magneto-optical trap (MOT) design based on millimeter ball lenses, contained within a metal cube of 19-mm side length. The ball lenses create highly divergent conical MOT trapping beams. In our experiment and in trajectory simulations, we study the effect of beam divergence and light-intensity gradients on MOT performance. We trap approximately

4.2\ifmmode\times\else\texttimes\fi{}{10}^{5}{\phantom{\rule{0.2em}{0ex}}}^{85}\mathrm{Rb}

atoms in our ball-lens MOT at a density of

3.2\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}

and a loading time of 1.3 s. Experimental measurements and the trajectory model are used to calibrate the trapping efficiency in the highly divergent light fields of the ball lenses. The advantages of the design include small lens and chamber sizes favorable to miniaturization, the absence of large-diameter optical-beam pathways, and a low laser-power requirement. These features are conducive to atom-trap geometries with well-defined electromagnetic boundary conditions, as required for ion and plasma traps, and for efficient shielding of black-body and dc stray fields.

As part of an effort to measure parity non-conserving atomic transition rates in a series of Cs radioisotopes, the authors are working on the efficient coupling of a magneto-optical trap (MOT) to a mass separator. They have established a test stand for optical trapping which is used for the off-line development of a high-efficiency MOT. Their trap features a dry-film coated cell with large (5 cm) windows to permit the use of large diameter, high power laser beams to increase the efficiency of the trapping process. Studies of the effect of detuning, magnetic field gradient, laser beam intensity and different beam sizes on the number of trapped atoms are presented.

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.

A three-dimensional magneto-optical trap (MOT), acting as an efficient method for producing cold atoms from room-temperature atomic vapor, has been widely used to develop atomic sensors. Various compact MOTs using a single laser beam have been reported, simplifying apparatuses and leading to miniaturized devices. Here, we propose single-beam MOTs based on back-to-back pyramidal and conical mirrors. In such back-to-back mirrors, a MOT trapping volume is formed by an incident laser beam, a retroreflected beam, and multiple reflections from the mirror surfaces. We present the design of back-to-back mirrors and a series of compact MOT configurations, with the potential of increasing access to the MOT and simultaneously creating multiple MOTs. We demonstrate a MOT in a back-to-back conical mirror, loading \ensuremath{\sim}1 \ifmmode\times\else\texttimes\fi{}

{10}^{7}

{}^{87}\mathrm{Rb}

atoms from background vapor and cooling the atoms to \ensuremath{\sim}7 \textmu{}K using polarization gradients. Single-beam MOTs based on back-to-back mirrors contribute to building compact and scalable cold-atom-based sensors.

We measure the number of atoms N trapped in a conventional vapor-cell magneto-optic trap (MOT) using beams that have a diameter d in the range 1-5 mm. We show that the N is proportional to d(3.6) scaling law observed for larger MOTs is a robust approximation for optimized MOTs with beam diameters as small as 3 mm. For smaller beams, the description of the scaling depends on how d is defined. The most consistent picture of the scaling is obtained when d is defined as the diameter where the intensity profile of the trapping beams decreases to the saturation intensity. Using this definition, N scales as d(6) for d<2.3 mm but, at larger d, N still scales as d(3.6).

We demonstrate a two-dimensional magneto-optical trap (2D MOT) as a beam source for cold

^{6}\text{L}\text{i}

atoms. The source is side loaded from an oven operated at temperatures in the range $600\ensuremath{\lesssim}T\ensuremath{\lesssim}700\text{ }\text{K}$. The performance is analyzed by loading the atoms into a three-dimensional MOT located 220 mm downstream from the source. The maximum recapture rate of $\ensuremath{\sim}{10}^{9}\text{ }{\text{s}}^{\ensuremath{-}1}$ is obtained for $T\ensuremath{\approx}700\text{ }\text{K}$ and results in a total of up to

{10}^{10}

trapped atoms. The recaptured fraction is estimated to be $30\ifmmode\pm\else\textpm\fi{}10%$ and limited by beam divergence. The most-probable velocity in the beam $({\ensuremath{\alpha}}_{z})$ is varied from 18 to 70 m/s by increasing the intensity of a push beam. The source is quite monochromatic with a full width at half maximum velocity spread of 11 m/s at ${\ensuremath{\alpha}}_{z}=36\text{ }\text{m}/\text{s}$, demonstrating that side loading completely eliminates beam contamination by hot vapor from the oven. We identify depletion of the low-velocity tail of the oven flux as the limiting loss mechanism. Our approach is suitable for other atomic species.

The realization of a compact magneto-optical trap (MOT) with a quartz vacuum chamber via ultra-low power of a cooling laser is reported. The dependence of the properties of the MOT on key experimental parameters is investigated. The efficiency of capturing atoms of the novel MOT is compared with that of the metal vacuum chamber. The compact quartz vacuum chamber can achieve a maximum rubidium ( 87 R b ) atomic loading rate of about 8 × 10 9 / s with less than 60 mW of the total power of cooling lasers, while the metal chamber obtains the maximum atomic loading rate of about 3 × 10 9 / s with more than 240 mW of the total power of cooling lasers. The compact apparatus avoids undesired effects found in a metal chamber, such as high power consumption and the magnetic-field eddy current effect. This compact MOT is helpful in the design of miniature atomic gravimeters.