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arXiv:2301.12620 · Physical Review Research

Dynamics of a buffer-gas-loaded, deep optical trap for molecules

A. Singh, Lothar Maisenbacher, Ziguang Lin, Jeremy J. Axelrod, Cristian Daniel Panda, Holger Müller

Published Jul 5, 2023• 9 citations
arXivHTMLPDF

Abstract

We describe an approach to optically trapping small, chemically stable molecules at cryogenic temperatures by buffer-gas loading a deep optical dipole trap. The $\ensuremath{\sim}10\phantom{\rule{4pt}{0ex}}\mathrm{K}$ trap depth will be produced by a tightly focused, 1064-nm cavity capable of reaching intensities of hundreds of $\mathrm{GW}/{\mathrm{cm}}^{2}$. Molecules will be directly buffer-gas loaded into the trap using a helium buffer gas at $1.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The very far-off-resonant, quasielectrostatic trapping mechanism is insensitive to a molecule's internal state, energy level structure, and its electric and magnetic dipole moment. Here, we theoretically investigate the trapping and loading dynamics, as well as the heating and loss rates, and conclude that ${10}^{4}$--${10}^{6}$ molecules are likely to be trapped. Our trap would open new possibilities in molecular spectroscopy, studies of cold chemical reactions, and precision measurement, amongst other fields of physics.

DOI
10.1103/physrevresearch.5.033008

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We describe an approach to optically trapping small, chemically stable molecules at cryogenic temperatures by buffer-gas loading a deep optical dipole trap. The \ensuremath∼10K\ensuremath{\sim}10\phantom{\rule{4pt}{0ex}}\mathrm{K}\ensuremath∼10K trap depth will be produced by a tightly focused, 1064-nm cavity capable of reaching intensities of hundreds of GW/cm2\mathrm{GW}/{\mathrm{cm}}^{2}GW/cm2. Molecules will be directly buffer-gas loaded into the trap using a helium buffer gas at 1.5K1.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}1.5K. The very far-off-resonant, quasielectrostatic trapping mechanism is insensitive to a molecule's internal state, energy level structure, and its electric and magnetic dipole moment. Here, we theoretically investigate the trapping and loading dynamics, as well as the heating and loss rates, and conclude that 104{10}^{4}104--106{10}^{6}106 molecules are likely to be trapped. Our trap would open new possibilities in molecular spectroscopy, studies of cold chemical reactions, and precision measurement, amongst other fields of physics.
Ultracold and cold molecules have enormous potential for yielding a wealth of new physics and new technologies. Molecules possess several features that greatly increase the possibilities for new interactions, collective quantum states and effects, collisional processes, fundamental symmetry tests and chemical processes. This is due to 1) the strong interactions between the dipole moments of polar molecules, 2) the rotational and vibrational internal structure of all molecules, and 3) the easily orientable internal electric field of many molecules. The promise of cold and ultracold molecules will only be realized when samples can be prepared with at least the same ease in which we now prepare atomic samples. Several approaches towards trapping of molecules, the key first step toward studying ultracold molecules at high density, have already succeeded: direct cooling of molecules via a buffer gas, mechanical slowing of a pulsed molecular beam with electric fields and photoassociation of alkali atoms. In the ICAP 2002 session on molecules I described some of the current theoretical results and predictions about ultracold molecular systems. Profs. Meijer and Heinzen described results on pulsed beam slowing and photoassociation. This contribution describes some of the key results on buffer-gas loading of molecules. A.
The advent of buffer-gas loaded magnetic traps for atoms has opened the possibility of trapping paramagnetic molecules. We survey our results on the loading, trapping and spectroscopy of Eu atoms that demonstrated the technique. The principles governing molecular trapping considering in particular the O2 and NO systems are outlined. The trapping of molecules should prove particularly useful in spectroscopy, especially ultra-high resolution spectroscopy that requires cold (slow), trapped (long interaction time) samples. Similar to cold atoms, cold molecules could be interrogated at a level of detail which is likely to provide new insights into their structure and interactions.
A new technique for buffer gas loading is described. This technique greatly extends the range of atoms and molecules that may be magnetically trapped at low temperature. The advance is made possible by the rapid removal of the buffer gas on a time scale one hundred times greater than in previous buffer gas loading experiments. A new cryogenic valve is developed and employed. The following benchmarks were attained in our first experimental run; approximately 10 atoms with effective magnetic moments μeff & 3μB were trapped and thermally isolated with near unit efficiency, ~ 10 μeff = 2μB atoms were trapped and thermally isolated, and ~ 10 μeff = 1μB atoms were trapped, but without thermal isolation. For comparison, all previous buffer gas loading experiments that achieved thermal isolation after trapping were done with atoms having a magnetic moment of at least 6μB. In our second run of the experiment, better temperature management allowed us to increase the number of trapped and thermally isolated μeff = 2μB atoms to ~10 and trapped μeff = 1μB atoms to ~ 10. The performance of the present apparatus is limited by the presence of a desorbing helium film that compromises the vacuum in the cell after the bulk of the buffer gas is removed. Future improvements to address this problem are suggested that will likely allow for efficient trapping and thermal isolation of atoms and molecules with magnetic moments as low as 1μB. Analysis of the trapping and pumping dynamics is presented.
Cryogenic buffer gas cooled beams and cells can be used to study many species, from atoms and polar molecules to biomolecules. We report on recent applications of this technique to improve the limit on the electron electric dipole moment [1], load polar molecules into a magnetic trap through optical pumping [2], perform chirally sensitive microwave spectroscopy on polyatomic molecules [3], progress towards magneto-optical trapping of polar molecules [4], and studies of atom-molecule sticking [5]. [1] The ACME Collaboration: J. Baron et al., Science 343, p. 269 (2014) [2] B. Hemmerling et al., arXiv:1310.2669, to appear in Phys. Rev. Lett. [3] D. Patterson, M. Schnell, \& J. M. Doyle, Nature 497, p. 475 (2013) [4] H. Lu et al., arXiv:1310.3239, to appear in New. J. Phys. [5] J. Piskorski et al., under preparation
Molecular magneto-optical traps (MOTs) are loaded from cryogenic buffer gas beam (CBGB) sources, which produce a large flux of cold molecules. However, only a small fraction of this flux is slowed and captured. Here, we investigate and optimize one of these CBGB sources through a combination of experiment and simulation to maximize the number of molecules loaded into a MOT of CaF molecules. We experimentally determine the range of molecule velocities from the source that are successfully slowed and captured in the MOT, and perform a systematic study of buffer gas source geometries to identify designs that maximize MOT loading. Our observations agree well with Monte Carlo simulations, which provide physical insight into the dynamics within the source and explain how source geometry influences the subset of molecules that are ultimately trapped. These results provide practical design guidelines for optimizing cryogenic buffer gas beam sources used in laser cooling experiments.
Cold gases of atoms and molecules provide a system for the exploration of a diverse set of physical phenomena. For example, cold gasses of magnetically and electrically polar atoms and molecules are ideal systems for quantum simulation and quantum computation experiments, and cold gasses of large polar molecules allow for novel spectroscopic techniques. Buffer-gas cooling is a robust and widely applicable method for cooling atoms and molecules to temperatures of approximately 1 Kelvin. In this thesis, I present novel applications of buffer-gas cooling to obtaining gases of trapped, ultracold atoms and diatomic molecules, as well as the study of the cooling of large organic molecules. In the first experiment of this thesis, a buffer-gas beam source of atoms is used to directly load a magneto-optical trap. Due to the versatility of the buffer-gas beam source, we obtain trapped, sub-milliKelvin gases of four different lanthanide species using the same experimental apparatus. In the second experiment of this thesis, a buffer-gas beam is used as the initial stage of an experiment to directly laser cool and magneto-optically trap the diatomic molecule CaF. In the third experiment of this thesis, buffer-gas cooling is used to study the cooling of the conformational state of large organic molecules. We directly observe conformational relaxation of gas-phase 1,2-propanediol due to cold collisions with helium gas. Lastly, I present preliminary results on a variety of novel applications of buffer-gas cooling, such as mixture analysis, separation of chiral mixtures, the measurement of parity-violation in chiral molecules, and the cooling and spectroscopy of highly unstable reaction intermediates.