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arXiv:1906.10797 · Physical Review A

Improvements on type-II Zeeman slowing of molecules through polarization selectivity

Qian Liang, Wenhao Bu, Yuhe Zhang, Tao Chen, Bo Yan

Published Nov 6, 2019• 10 citations
arXivHTMLPDF

Abstract

We propose a general Zeeman slower scheme applicable to the majority of the laser-coolable molecules. Different from previous schemes, we apply a careful modulation on the repump laser and guarantee the velocity selectivity through the polarization selection rules. Only atoms or molecules with the right velocity will be laser slowed. Such a scheme is more feasible for molecules with complex energy structures. We apply this scheme for molecules with a large Land\'e $g$ factor of the excited states, i.e., ${g}_{e}>0.1$, and polyatomic molecules with one-dimensional numerical simulations. The results show a better slowing efficiency with about 62% of the BaF molecules and 99% of the SrOH molecules being successfully slowed down to a velocity below $15\phantom{\rule{0.28em}{0ex}}\mathrm{m}/\mathrm{s}$ without the consideration of transverse divergence.

DOI
10.1103/physreva.100.053402

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We propose a general Zeeman slower scheme applicable to the majority of the laser-coolable molecules. Different from previous schemes, we apply a careful modulation on the repump laser and guarantee the velocity selectivity through the polarization selection rules. Only atoms or molecules with the right velocity will be laser slowed. Such a scheme is more feasible for molecules with complex energy structures. We apply this scheme for molecules with a large Land'e ggg factor of the excited states, i.e., ge>0.1{g}_{e}>0.1ge​>0.1, and polyatomic molecules with one-dimensional numerical simulations. The results show a better slowing efficiency with about 62% of the BaF molecules and 99% of the SrOH molecules being successfully slowed down to a velocity below 15m/s15\phantom{\rule{0.28em}{0ex}}\mathrm{m}/\mathrm{s}15m/s without the consideration of transverse divergence.
We present a novel slowing scheme for beams of laser-coolable diatomic molecules reminiscent of Zeeman slowing of atomic beams. The scheme results in efficient compression of the one-dimensional velocity distribution to velocities trappable by magnetic or magneto-optical traps. We experimentally demonstrate our method in an atomic testbed and show an enhancement of flux below v = 35 m s −1 by a factor of ≈20 compared to white light slowing. 3D Monte Carlo simulations performed to model the experiment show excellent agreement. We apply the same simulations to the prototype molecule 88 Sr 19 F and expect 15% of the initial flux to be continuously compressed in a narrow velocity window at around 10 m s −1 . This is the first experimentally shown continuous and dissipative slowing technique in molecule-like level structures, promising to provide the missing link for the preparation of large ultracold molecular ensembles.
We present a novel slowing scheme for beams of laser-coolable diatomic molecules reminiscent of Zeeman slowing of atomic beams. The scheme results in efficient compression of the one-dimensional velocity distribution to velocities trappable by magnetic or magneto-optical traps. We experimentally demonstrate our method in an atomic testbed and show an enhancement of flux below v = 35 m s-1 by a factor of ≈20 compared to white light slowing. 3D Monte Carlo simulations performed to model the experiment show excellent agreement. We apply the same simulations to the prototype molecule 88Sr19F and expect 15% of the initial flux to be continuously compressed in a narrow velocity window at around 10 m s-1. This is the first experimentally shown continuous and dissipative slowing technique in molecule-like level structures, promising to provide the missing link for the preparation of large ultracold molecular ensembles. © 2018 The Author(s). Published by IOP Publishing Ltd on behalf of Deutsche Physikalische Gesellschaft.
Cold molecules are an invaluable tool for modern physics. Cold atoms have already revolutionized precision metrology, allowed for spectacular findings in quantum physics and opened the door for the creation and observation of novel states of matter. Cold molecules bring with them additional degrees of freedom which give rise to unique properties that enable new kinds of experiments. For these experiments it is of vital importance to start with a sufficiently large number of cold molecules. There are multiple paths for the slowing and cooling of atoms and most of the slowing techniques utilizing lasers for direct cooling have been transferred to the molecular case with its specific requirements. The Zeeman slowing technique is advantageous for loading a trap with cold particles but had not yet been transferred from atoms to the molecular case. In this thesis I will show the first realization of the Zeeman slowing technique for diatomic molecules. This thesis will discuss several preparatory steps: Our group has had some ideas to collimate the molecular beam with a conical nozzle outlet at the buffer gas cell which I investigated on potassium atoms generated by the ablation of 39K35Cl inside the buffer gas cell. I also realized a fluorescence spectroscopy of the |XΣ1/2, N = 1, v = 0⟩ → |AΠ1/2, N = 0, v = 0⟩ transition in magnetic fields up to 300 G and confirmed simulations regarding our two laser cycling scheme in the Paschen-Back regime by measuring the scattering rate of light on one molecule. Finally, I created pre-cooled 40Ca19F with a forward velocity of around 150 m s−1 from a 40Ca19F2 target with laser ablation inside a cryogenic buffer gas cell. With the Zeeman slowing technique I slowed the molecules down to 25 m s−1, a velocity where trapping can be possible. I have also shown the characteristic compression into specific velocity classes for not yet trappable velocities. Together these two findings demonstrate that molecular Zeeman slowing is possible and a real promising tool for future experiments.
We describe a Zeeman slowing method reported in [New J. Phys. 20, 042001 (2018)] and compare it to conventional radiative beam-slowing schemes. The scheme is designed to work on a type-II level structure, making it particularly attractive for radiative beam slowing of molecules. Working on the D1{D}_{1}D1​ line of atomic 39K^{39}\mathrm{K}39K, we demonstrate efficient slowing of an atomic beam from $400\phantom{\rule{4pt}{0ex}}\mathrm{m}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ down to $35\phantom{\rule{4pt}{0ex}}\mathrm{m}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ with a final flux of $3.3\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. We give experimental details and compare our results to other established radiative slowing schemes in atomic and molecular physics. We find type-II Zeeman slowing to outperform white-light slowing commonly used in molecular beam slowing and to be comparably as efficient as traditional type-I Zeeman slowing being the standard beam-slowing technique in atomic physics.
Slowing down atoms using laser cooling, together with techniques like magnetic trapping and evaporative cooling, has led to numerous exciting results over the last decade [1], with the realization of Bose-Einstein condensation as the most spectacular achievement [2]. Laser-cooling schemes are applicable to atoms since closed multilevel systems can be realized and many consecutive absorptionemission cycles can be induced. This leads to a significant momentum transfer from the photons of the laser to the atom, resulting in a change of the velocity of the atom. There currently is a great interest in producing cold molecules for the study of cold molecule-molecule collisions, molecular quantum collective effects, Doppler-free spectroscopy, frequency standards, as well as many other applications already realized for atoms [3]. Slowing down of neutral molecules, a prerequisite for being able to trap them, has proven to be considerably more difficult to be achieved, however. The above-mentioned laser-cooling scheme cannot be applied to slow down molecules as no simple closed-level systems can be found; the unavoidable off-resonant radiative decay of molecules from the excited state to other vibrational levels in the electronic ground state hampers efficient momentum transfer [4]. Once molecules are present at sufficiently low kinetic energies, below a fraction of a cm21, they can be trapped in various ways. Trapping of neutral molecules in inhomogeneous magnetic [3] and electric fields [5] and trapping based on the use of evanescent fields close to the dielectricvacuum interface have been proposed [6]. It has also been suggested that extremely high laser powers might be used to induce a dipole moment in polarizable molecules, leading to trapping of these molecules in the laser beam [7]. Other methods to produce slow molecules have been put forward and have recently been successfully applied to produce the first samples of trapped molecules. By injecting molecules in a cold buffer gas, sufficiently low kinetic energies of the injected molecules can be obtained after thermalization that they can subsequently be trapped in an inhomogeneous magnetic field. This has recently been demonstrated for CaH injected in 3He buffer gas at temperatures of around 0.3 K [8]. Associative collisions between ultracold trapped atoms [9] have proven to be
Atoms can be cooled and trapped efficiently with the help of lasers. So-called Doppler cooling takes advantage of momentum transfer upon absorption and emission of photons and of Doppler shifts to facilitate effectively closed optical absorption-emission loops, by which atoms are slowed down and cooled. Due to the wealth of internal degrees of freedom accessible in molecules, it was assumed for a long time that similarly closed optical loops cannot be realised for molecules. After an early theoretical proposal by Di Rosa for diatomic molecules, such cooling has been achieved in this decade for SrF, YO, CaF and YbF. It has been outlined recently that also polyatomic molecules should be coolable with lasers and classes of molecules expected to be amenable to this have been proposed by the present authors. Experimental success in laser cooling of SrOH has been reported. The status of cooling polyatomic molecules with lasers and the prospects for obtaining ultracold chiral molecules is reviewed herein.