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Annalen der Physik

Spatio‐Temporal Monte Carlo Modelling of Collisional Processes in Loading and Decay of an Optical Dipole Trap

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

Published Nov 25, 2025• 0 citations
Publisher

Abstract

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.

DOI
10.1002/andp.202500538

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Physical Review Letters•Jun 19, 2002•280 citations
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.
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.
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.
The process of collisional trap loss in magneto-optically trapped 7Li shows a rich dependence on the depth of the trapping potential, ET. When the trap is shallow enough to allow the escape of atoms with kinetic energy equal to one-half the fine-structure splitting, ΔEfs, fine-structure changing collisions [FS] dominate the collisional trap loss rate. However when the trap is deeper than 1 2 Δ E FS, only the radiative escape mechanism [RE] contributes to trap loss. In this regime, Julienne and Vigue predicted that the collisional loss rate will depend on ET as ~ET-17/6.1 We have measured the collisional loss rate, β, of 7Li from a beam-loaded magneto-optical trap.2 The circles in Fig. 1 show the data for four different trap laser detunings, Δ, and a range of trap laser intensities, 1. The solid line in Fig. 1 is the result of a semi-classical, optical Bloch equation calculation of the RE trap loss rate3 using as input ET, calculated from a truly three-dimensional model.4 The trap depth model predicts that ET is anisotropic. The dashed line in Fig. 1 shows the intensity at which the model predicts E T > 1 2 Δ E FS in all directions, for each detuning. The theory predicts the magnitude of the RE trap loss rate accurately near the point at which the fine structure mechanism turns off, where the trap depth is well known. However at larger detunings and intensities the calculations deviate from the measured values. This suggests that the trap depth model overestimates E, at higher intensities and detunings, or that the trap depth dependence of the RE loss rate deviates from ET-17/6.
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.
We analyze the operating regimes of a very small optical dipole trap, loaded from a magneto-optical trap, as a function of the atom loading rate, i.e., the number of atoms per second entering the dipole trap. We show that, when the dipole trap volume is small enough, a "collisional blockade" mechanism locks the average number of trapped atoms on the value 0.5 over a large range of loading rates. We also discuss the "weak loading" and "strong loading" regimes outside the blockade range, and we demonstrate experimentally the existence of these three regimes.