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arXiv:1808.01067 · Physical Review Letters

3D Magneto-Optical Trap of Yttrium Monoxide

Alejandra Collopy, Shiqian Ding, Yewei Wu, Ian A. Finneran, Loïc Anderegg, Benjamin L. Augenbraun, John M. Doyle, Jun Ye

Published Nov 21, 2018• 211 citations
arXivHTMLPDF

Abstract

We report three-dimensional trapping of an oxide molecule (YO), using a radio-frequency magneto-optical trap (MOT). The total number of molecules trapped is ∼1.5×10^{4}, with a temperature of 4.1(5) mK. This diversifies the frontier of molecules that are laser coolable and paves the way for the second-stage narrow-line cooling in this molecule to the microkelvin regime. Futhermore, the new challenges of creating a 3D MOT of YO resolved here indicate that MOTs of more complex nonlinear molecules should be feasible as well.

DOI
10.1103/physrevlett.121.213201

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We report three-dimensional trapping of an oxide molecule (YO), using a radio-frequency magneto-optical trap (MOT). The total number of molecules trapped is ∼1.5×10^{4}, with a temperature of 4.1(5) mK. This diversifies the frontier of molecules that are laser coolable and paves the way for the second-stage narrow-line cooling in this molecule to the microkelvin regime. Futhermore, the new challenges of creating a 3D MOT of YO resolved here indicate that MOTs of more complex nonlinear molecules should be feasible as well.
We demonstrate one- and two-dimensional transverse laser cooling and magneto-optical trapping of the polar molecule yttrium (II) oxide (YO). In a 1D magneto-optical trap (MOT), we characterize the magneto-optical trapping force and decrease the transverse temperature by an order of magnitude, from 25 to 2 mK, limited by interaction time. In a 2D MOT, we enhance the intensity of the YO beam and reduce the transverse temperature in both transverse directions. The approach demonstrated here can be applied to many molecular species and can also be extended to 3D.
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In addition to being suitable for laser cooling and trapping in a magneto-optical trap (MOT) using a relatively broad transition, the molecule YO possesses a narrow-line transition. This forbidden transition between the and states has linewidth kHz. After cooling in a MOT on the 614 nm to (orange) transition, the narrow 690 nm (red) transition can be used to further cool the sample, requiring only minimal additions to the first stage system. We estimate that the narrow line cooling stage will bring the temperature from ∼1 mK to ∼10 μ K, significantly advancing the frontier on direct cooling achievable for molecules.
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.
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 demonstrate a three-dimensional magneto-optical trap (MOT) of a metal hydride molecule, CaH. We are able to scatter ∼\sim∼10410^{4}104 photons with vibrational loss covered up to vibrational quantum number ν=2ν=2ν=2. This allows us to laser slow the molecular beam near zero velocity with a "white-light" technique and subsequently load it into a radio-frequency MOT. The MOT contains 230(40)230(40)230(40) molecules, limited by beam source characteristics and predissociative loss of CaH. The temperature of the MOT is below one millikelvin. The predissociative loss mechanism could, in turn, facilitate controlled dissociation of the molecule, offering a possible route to optical trapping of hydrogen atoms for precision spectroscopy.