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arXiv:2505.04134

Spatio-temporal Monte Carlo modeling of loading and decay dynamics in an optical dipole trap

Sankalp Shandilya, Gunjan Verma, Kavish Bhardwaj, Surjya Prakash Ram, Vibhuti Bhushan Tiwari, S. R. Mishra

Published May 7, 2025• 0 citations
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Abstract

Here, we present our studies on intra-trap dynamics of an optical dipole trap (ODT) loaded from a Magneto-optical trap (MOT) of $^{87}$Rb atoms. A spatio-temporal Monte Carlo (MC) simulation approach has been employed in conjunction with the semi-classical theory based calculations for estimation of various intra-trap two-body loss parameters in the ODT. The temporal MC method based on estimated parameters from spatial MC, gives a close estimation of the experimentally observed atom-number evolution in ODT. We also find that, the decay rate of atoms in an ODT is dominated by momentum-transfer elastic collisions in the absence of MOT beams. However, in presence of MOT beams, the radiative escape process is the dominant two-body loss mechanism in the trap, which surpasses the fine-structure changing and hyperfine changing collisional losses by nearly an order of magnitude.

DOI
10.48550/arxiv.2505.04134

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Here, we present our studies on intra-trap dynamics of an optical dipole trap (ODT) loaded from a Magneto-optical trap (MOT) of 87^{87}87Rb atoms. A spatio-temporal Monte Carlo (MC) simulation approach has been employed in conjunction with the semi-classical theory based calculations for estimation of various intra-trap two-body loss parameters in the ODT. The temporal MC method based on estimated parameters from spatial MC, gives a close estimation of the experimentally observed atom-number evolution in ODT. We also find that, the decay rate of atoms in an ODT is dominated by momentum-transfer elastic collisions in the absence of MOT beams. However, in presence of MOT beams, the radiative escape process is the dominant two-body loss mechanism in the trap, which surpasses the fine-structure changing and hyperfine changing collisional losses by nearly an order of magnitude.
ABSTRACT We present our studies on intra‐trap dynamics of an optical dipole trap loaded from a Magneto‐optical trap of atoms. A spatio‐temporal Monte Carlo simulation approach has been employed in conjunction with the semi‐classical theory based calculations for the estimation of various intra‐trap two‐body loss parameters in the optical dipole trap. Our methodology gives a close estimation of the experimentally observed the atom‐number evolution in the optical dipole trap. We also find that, the decay rate of atoms in an optical dipole trap is dominated by momentum‐transfer elastic collisions due to the absence of Magneto‐optical trap beams. However, in presence of Magneto‐optical trap beams, the radiative escape process is the dominant two‐body loss mechanism in the trap, which surpasses the fine‐structure changing and hyperfine changing collisional losses by nearly an order of magnitude.
We present a detailed experimental study of the physics involved in transferring atoms from a magneto-optical trap (MOT) to an optical dipole trap. The loading is a dynamical process determined by a loading rate and a density dependent loss rate. The loading rate depends on cooling and the flux of atoms into the trapping volume, and the loss rate is due to excited state collisions induced by the MOT light fields. From this study we found ways to optimize the loading of the optical dipole trap. Key ingredients for maximum loading are found to be a reduction of the hyperfine repump intensity, increased detuning of the MOT light, and a displacement of the optical dipole trap center with respect to the MOT. A factor of 2 increase in the number of loaded atoms is demonstrated by using a hyperfine repump beam with a shadow in it. In this way we load $8\ifmmode\times\else\texttimes\fi{}{10}^{6}{}^{85}\mathrm{Rb}$ atoms into a 1 mK deep optical dipole trap with a waist of $58 \ensuremath{\mu}\mathrm{m},$ which is 40% of the atoms initially trapped in the MOT.
We investigate two-body loss in an optical dipole trap for 87Rb{}^{87}\mathrm{Rb}87Rb atoms. In the presence of additional near-resonant light, such as from a magneto-optical trap during the trap loading, the two-body loss is strongly enhanced by long-range radiative escape. We suppressed this loss by a factor of 15 by adding a sideband to the optical dipole trap laser. This allows more atoms to be loaded into the optical dipole trap.
Abstract We study the cold collisions of mixed atoms in an optical dipole trap (ODT), which are loaded from a dark magneto-optical trap (MOT). A comprehensive, phenomenological rate equation is presented to derive the ultracold homonuclear and heteronuclear collision rates in loading and holding procedures. Our results show that the cold atoms in the dark MOT can provide a much better stable, initial atomic sample than MOT. The dependence of the heteronuclear collision rate on the trap depth is attributed to the hyperfine-changing collision by the ODT laser with a broad linewidth. The processes of deriving the collision rate are also universal for other kinds of atoms or even molecules.
In this work, a broadband 1071 nm crossed optical dipole trap was loaded simultaneously from a two-atomic-species K-Rb magneto-optical trap. The number of atoms loaded into the dipole trap, for both species, was characterized as a function of different laser parameters and loading procedures. The best observed loading procedure for K is explained by the ac Stark shift of the hyperfine states involved in the D2{D}_{2}D2​ transition. For the optimum conditions, the dipole trap has about $3\ifmmode\times\else\texttimes\fi{}{10}^{6}$ 39{}^{39}39K atoms at an atomic density of $7\ifmmode\times\else\texttimes\fi{}{10}^{12}$ cm${}^{\ensuremath{-}3}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ Rb atoms at an atomic density of $5\ifmmode\times\else\texttimes\fi{}{10}^{12}$ cm${}^{\ensuremath{-}3}$, with temperatures of 22 and 10 $\ensuremath{\mu}$K, respectively. Finally, by studying the time evolution of both species trapped in the crossed optical dipole trap, we have observed a nonexponential decay associated with heteronuclear two-body losses. Such losses may be due to KRb photoassociation by the broadband 1071 nm laser.
In this work, we load a 1070 nm crossed optical dipole trap (ODT) from a 133Cs dark spontaneous emission optical trap. Obvious improvements in the loading rate and atomic lifetime are demonstrated, comparing with the values of the atoms which are loaded directly from a normal magneto-optical trap. We analyze the atomic number as a function of time in the ODT’s loading and holding processes based on the constant density and the constant volume approximations, respectively. We also demonstrate that the atomic temperature in holding process is related to the atomic collision, which consists of two-body collision and background gas induced collision. Finally the relationship between atomic collision and optical trap depth is measured and analyzed. The atomic loading way we present can be easily expanded to other species to obtain an atomic sample with large number and high density.