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arXiv:2107.06422 · Physical Review Letters

High Phase-Space Density of Laser-Cooled Molecules in an Optical Lattice

Yewei Wu, Justin J. Burau, Kameron Mehling, Jun Ye, Shiqian Ding

Published Dec 21, 2021• 55 citations
arXivHTMLPDF

Abstract

We report laser cooling and trapping of yttrium monoxide molecules in an optical lattice. We show that gray molasses cooling remains exceptionally efficient for yttrium monoxide molecules inside the lattice with a molecule temperature as low as 6.1(6) μK. This approach has produced a trapped sample of 1200 molecules, with a peak spatial density of ∼1.2×10^{10} cm^{-3}, and a peak phase-space density of ∼3.1×10^{-6}. By ramping down the lattice depth, we cool the molecules further to 1.0(2) μK, 20 times colder than previously reported for laser-cooled molecules in a trap.

DOI
10.1103/physrevlett.127.263201

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We report laser cooling and trapping of yttrium monoxide molecules in an optical lattice. We show that gray molasses cooling remains exceptionally efficient for yttrium monoxide molecules inside the lattice with a molecule temperature as low as 6.1(6) μK. This approach has produced a trapped sample of 1200 molecules, with a peak spatial density of ∼1.2×10^{10} cm^{-3}, and a peak phase-space density of ∼3.1×10^{-6}. By ramping down the lattice depth, we cool the molecules further to 1.0(2) μK, 20 times colder than previously reported for laser-cooled molecules in a trap.
Complex molecular structure demands customized solutions to laser cooling by extending its general set of principles and practices.Compared with other laser-cooled molecules, yttrium monoxide (YO) exhibits a large electron-nucleus interaction, resulting in a dominant hyperfine interaction over the electron spinrotation coupling.The YO ground state is thus comprised of two manifolds of closely spaced states, with one of them possessing a negligible Landé g factor.This unique energy level structure favors dual-frequency dc magneto-optical trapping (MOT) and gray molasses cooling (GMC).We report exceptionally robust cooling of YO at 4 μK over a wide range of laser intensity, detunings (one-and two-photon), and magnetic field.The magnetic insensitivity enables the spatial compression of the molecular cloud by alternating GMC and MOT under the continuous operation of the quadrupole magnetic field.A combination of these techniques produces a laser-cooled molecular sample with the highest phase space density in free space.
We present new techniques in cooling 39 K atoms using laser light close to the D1 transition.First, a new compressed-MOT configuration is taking advantage of gray-molasses-type cooling induced by blue-detuned D1 light.It yields an optimized density of atoms.Then, we use pure D1 gray molasses to further cool the atoms to an ultra-low temperature of 6 μK.The resulting phase-space density is 2 × 10 -4 and will ease future experiments with ultracold potassium.As an example, we use it to directly load up to 3 × 10 7 atoms in a far detuned optical trap, a result that opens the way to the all-optical production of potassium degenerate gases.
We report optical trapping of laser-cooled molecules at sufficient density to observe molecule-molecule collisions for the first time in a bulk gas. SrF molecules from a red-detuned magneto-optical trap (MOT) are compressed and cooled in a blue-detuned MOT. Roughly 30% of these molecules are loaded into an optical dipole trap with peak number density n_{0}≈3×10^{10} cm^{-3} and temperature T≈40 μK. We observe two-body loss with rate coefficient β=2.7_{-0.8}^{+1.2}×10^{-10} cm^{3} s^{-1}. Achieving this density and temperature opens a path to evaporative cooling towards quantum degeneracy of laser-cooled molecules.
We demonstrate loading of SrF molecules into an optical dipole trap (ODT) via in-trap $\mathrm{\ensuremath{\Lambda}}$-enhanced gray molasses cooling. We find that this cooling can be optimized by a proper choice of relative ODT and cooling beam polarizations. In this optimized configuration, we observe molecules with temperatures as low as $14(1)\text{ }\text{ }\ensuremath{\mu}\mathrm{K}$ in traps with depths up to $570\text{ }\text{ }\ensuremath{\mu}\mathrm{K}$. With optimized parameters, we transfer $\ensuremath{\sim}5%$ of molecules from our radio-frequency magneto-optical trap into the ODT, at a density of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$, a phase space density of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$, and with a trap lifetime of $\ensuremath{\sim}1\text{ }\text{ }\mathrm{s}$.
Direct laser cooling of molecules has reached a phase-space density exceeding 10^{-6} in optical traps but with rather small molecular numbers. To progress toward quantum degeneracy, a mechanism that combines sub-Doppler cooling and magneto-optical trapping would facilitate near unity transfer of ultracold molecules from the magneto-optical trap (MOT) to a conservative optical trap. Using the unique energy level structure of YO molecules, we demonstrate the first blue-detuned MOT for molecules that is optimized for both gray-molasses sub-Doppler cooling and relatively strong trapping forces. This first sub-Doppler molecular MOT provides an increase of phase-space density by 2 orders of magnitude over any previously reported molecular MOT.
We report magneto-optical trapping (MOT) of a polyatomic molecule, calcium monohydroxide (CaOH). The MOT contains 2.0(5)×1042.0(5)\times 10^42.0(5)×104 CaOH molecules at a peak density of 3.0(8)×1063.0(8)\times10^{6}3.0(8)×106 cm−3^{-3}−3. CaOH molecules are further sub-Doppler laser cooled in an optical molasses, to a temperature of 110(4) μμμK. The temperatures and densities achieved here make CaOH a viable candidate for a wide variety of quantum science applications, including the creation of optical tweezer arrays of CaOH molecules. This work also suggests that laser cooling and magneto-optical trapping of many other polyatomic species will be both feasible and practical.