Laser Cooling of Large Polyatomic Molecules

CaOPh-345FThe past several years have been an exciting time in the world of laser-cooled molecules, with diatomic molecules being brought under full quantum control and small triatomic molecules (CaOH) being laser cooled and trapped at temperatures below 1 mK. It is natural to ask: how can we extend this revolutionary degree of control to molecules that offer the complexity and richness that is observed in Nature? Our group has shown theoretically that large molecules, even bent and twisted ones, can likely be cooled using many of the same techniques as are used for atoms and simple molecules [1, 2]. The goal of this experiment is to laser cool large, complex polyatomic molecules and trap these molecules at ultralow temperatures. Our initial focus is on the molecule calcium monophenoxide ("CaOPh") and its fluorinated derivatives.

Motivation

The laser cooling and full quantum control of larger molecules (containing a dozen or more atoms) is at the very frontier of AMO physics, so much so that ideas of what to do with them are just beginning to be explored. Fundamentally, the larger number of atoms in the molecule, the larger the number of vibrational modes and hyperfine states. The concept of "internal motion" also starts to enter, e.g. these molecules can have a spinning ligand. Such modes can naturally be used to store quantum information or probe fundamental symmetries of Nature. With a large enough molecule, one may be able to completely separate the laser cooling and readout section of the molecule (through an "optical cycling center") from the physics end, perhaps containing an exotic atom such as a heavy radioactive species. Thus, one might be able to  realize a "configurable" molecular framework that allows targeted substitution of scientifically interesting components. 

Molecular substitutions increase complexity

"Functionalized" Aromatic Molecules

Our group is investigating molecules built around aromatic rings such as benzene or naphthalene, a set of species we have studied both theoretically and experimentally in collaboration with the Alexandrova, Campbell, Caram, and Hudson groups at UCLA [3-5].

What makes these species interesting and special? One can imagine functionalizing these organic precursors with a CaO bond that acts as a localized optical cycling center. By addressing the optical cycling transition between molecular orbitals localized on the Ca atom, one can feasibly cool and control the entire molecular structure. The aromatic ring possesses numerous other sites that could be used to place other functional groups, e.g. high-Z nuclei for electron EDM experiments, methyl groups that introduce torsional modes with high sensitivity to variation of fundamental constants, or substitution of high-spin nuclei for use as naturally coupled nuclear qubits.   

SpectraGiven that the optical cycling center and the ligand functions can be made to act "orthogonally," this structural motif leads to a highly versatile experimental platform. Indeed, the relative independence of the optically active electron from the structure of the ligand can be experimentally verified, as seen in the spectra shown here, which demonstrate that Ca bonded to -OH,  -OCH3, and a naphthol group all have similar transition properties [3]. In these spectra, the molecules are driven to an electronically excited state and the spontaneously emitted photons that they emit during decay are imaged. For each molecule studied, the prominent peak is due to decays that do not result in a change in vibrational motion-- exactly the type of behavior one seeks in a laser-coolable molecule. The fact that all of the molecules studied show decay primarily on this "vibrationless" transition makes them promising for laser cooling experiments. It is remarkable that molecules ranging in size from three atoms to over a dozen atoms all behave in a way that makes them amenable to optical cycling!

 

References

[1] Kozyryev, Baum, Matsuda, and Doyle, "Proposal for Laser Cooling of Complex Polyatomic Molecules," ChemPhysChem 17(22), 3641-3648 (2016).

[2] Augenbraun, Doyle, Zelevinsky, and Kozyryev, "Molecular Asymmetry and Optical Cycling: Laser Cooling Asymmetric Top Molecules," Phys. Rev. X 10, 031022 (2020).

[3] Dickerson, Guo, Shin, Augenbraun, Caram, Campbell, and Alexandrova, "Franck-Condon Tuning of Optical Cycling Centers by Organic Functionalization," Phys. Rev. Lett. 126, 123002 (2021).

[4] Zhu, Augenbraun, Dickerson, Frim, Lao, Lasner, Mitra, Alexandrova, Campbell, Caram, Doyle, and Hudson, "Functionalizing Aromatic Compounds with Optical Cycling Centers," (submitted 2022).

[5] Mitra, Lasner, Zhu, Dickerosn, Augenbraun, Bailey, Alexandrova, Campbell, Caram, Hudson, and Doyle, "Pathway to Optical Cycling of Functionalized Arenes," (submitted 2022).