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Physics

A New Way to Laser-Cool Molecules

Nicholas R. Hutzler

Published Jun 3, 2020• 0 citations
Publisher

Abstract

A New Way to Laser-Cool MoleculesResearchers exploit the peculiar structure of yttrium monoxide to cool the gas to ultralow temperatures and record-breaking densities.
DOI
10.1103/physics.13.89

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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.
The electronic energy level structure of yttrium monoxide (YO) provides a long-lived, low-lying 2 Δ state ideal for high-precision molecular spectroscopy, narrowline laser cooling at the single photon-recoil limit, and studying dipolar physics with unprecedented interaction strength. High-resolution laser spectroscopy of ultracold laser-cooled YO molecules is used to study the Stark effect in the A ′ 2 Δ 3 / 2 J = 3 / 2 state. An immediate onset of the linear Stark effect is observed in the presence of weak applied electric fields due to the near-degenerate Λ doublet and the large electric dipole moment. By applying a small electric field the Stark-insensitive state is spectroscopically isolated and the absolute transition frequency to the X 2 Σ + electronic ground state is determined with a fractional frequency uncertainty of 9 × 10 − 12 . This electric field control is necessary to implement a quasi-closed photon-cycling scheme that preserves parity. With this scheme the first narrowline laser cooling of a molecule is demonstrated, reducing the temperature of sub-Doppler cooled YO in two dimensions.