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Velocity Tuned Hyperfine Dark State Loading and Cooling in a dipole trap

D. Naik, H. Eneriz-Imaz, Max Carey, Tim Freegarde, F. Minardi, Baptiste Battelier, Philippe Bouyer, Andréa D Bertoldi

Published Sep 9, 2019• 1 citation
OpenAlex

Abstract

We present a novel optical cooling scheme capable of loading and cooling atoms directly inside deep optical dipole traps utilizing hyperfine dark states. In the presence of strong light shifts of the upper excited states, this allows the velocity selective dark-state cooling of atoms into the conservative potential without loss of atoms. We report the lossless optical cooling inside the trap with a seven-fold increase in the number of atoms loaded. Our findings open the door to all-optical cooling of trapped atoms and molecules which lack the closed cycling transitions normally needed to achieve low temperatures and the high initial densities required for evaporative cooling.

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•May 8, 1994•0 citations
We present a novel optical cooling scheme capable of loading and cooling atoms directly inside deep optical dipole traps utilizing hyperfine dark states. In the presence of strong light shifts of the upper excited states, this allows the velocity selective dark-state cooling of atoms into the conservative potential without loss of atoms. We report the lossless optical cooling inside the trap with a seven-fold increase in the number of atoms loaded. Our findings open the door to all-optical cooling of trapped atoms and molecules which lack the closed cycling transitions normally needed to achieve low temperatures and the high initial densities required for evaporative cooling.
We give a systematic analysis of the extension of the one-dimensional velocity-selective coherent population trapping cooling mechanism to atoms with integer angular momenta F>1F>1F>1 using additional light shifts to compensate for the kinetic energy mismatch between the various Zeeman sublevels contributing to the internally decoupled (i.e., dark) state of the atom. Different configurations to achieve the proper light shifts to obtain a very long lifetime of the velocity-selective dark states are discussed and compared. In addition we show that the maximum occupation probability of the resulting quasidark state and its filling time scale strongly depend on the proper choice of detunings to exploit extra precooling mechanisms involving the Zeeman sublevels not contributing to the dark state. We show that additional cooling can also be implemented on the hyperfine repumping transition. This result should prove important for other schemes involving gray states such as, e.g., dark optical lattices and superlattices as well. As a concrete example we consider cesium including all relevant hyperfine levels.
Optical dipole traps can provide strong, state-independent confinement of atoms. Unfortunately, the spatially dependent light shifts of optical transitions in dipole traps can interfere with conventional laser cooling techniques. These limitations may be overcome by cooling trapped atoms with stimulated Raman transitions. In particular, the lights shifts of ground state sublevels can be equal, so that the Raman cooling transition is unperturbed. Cooling in the sideband limit, to the quantum ground state of motion, should be possible. We have confined {sup 85}Rb atoms in a far-detuned optical dipole trap, and indirectly measured extremely low heating rates of the trapped atoms. We have also driven velocity-sensitive stimulated Raman transitions of the trapped atoms. The high resolution of these stimulated Raman resonances should allow us to resolve the motional sidebands and to directly cool the trapped atoms.
Abstract Recent experiments have demonstrated direct cooling and trapping of diatomic and triatomic molecules in magneto-optical traps (MOTs). However, even the best molecular MOTs to date still have density 10−5 times smaller than in typical atomic MOTs. The main limiting factors are: (i) inefficiencies in slowing molecules to velocities low enough to be captured by the MOT, (ii) low MOT capture velocities, and (iii) limits on density within the MOT resulting from sub-Doppler heating (Devlin and Tarbutt 2018 Phys. Rev. A 90 063415). All of these are consequences of the need to drive ‘Type-II’ optical cycling transitions, where dark states appear in Zeeman sublevels, in order to avoid rotational branching. We present simulations demonstrating ways to mitigate each of these limitations. This should pave the way toward loading molecules into conservative traps with sufficiently high density and number to evaporatively cool them to quantum degeneracy.
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.
Optical trapping and cooling of atoms at high densities is limited by strong absorption of both the incident laser light and the emitted fluorescence. Reabsorption of the emitted fluorescence gives rise to repulsive forces between the trapped atoms and also to additional heating thus limiting both density and ultimate temperature. We have recently demonstrated a method to avoid these effects by shelving most of the atoms being trapped or cooled in a dark hyperfine state. At any instant, about one percent of the atoms are in the bright state where the trapping and cooling forces are exerted. This scheme ensures transparency of the atom cloud at very high optical densities.