Molecular Asymmetry and Optical Cycling: Laser Cooling Asymmetric Top Molecules

Abstract

We present a practical roadmap to achieve optical cycling and laser cooling of asymmetric top molecules (ATMs). Our theoretical analysis describes how reduced molecular symmetry, as compared to diatomic and symmetric non-linear molecules, plays a role in photon scattering. We present methods to circumvent limitations on rapid photon cycling in these systems. We calculate vibrational branching ratios for a diverse set of asymmetric top molecules and find that many species within a broad class of molecules can be effectively cooled with a manageable number of lasers. We also describe methods to achieve rotationally closed optical cycles in ATMs. Despite significant structural complexity, laser cooling can be made effective using extensions of the current techniques used for linear molecules. Potential scientific impacts of laser-cooled ATMs span frontiers in controlled chemistry, quantum simulation, and searches for physics beyond the Standard Model.

Establishing a nearly closed cycling transition in a polyatomic molecule

“…While the measurements presented here are unique to CaOH, the experimental methods and calculations outline a general framework that could be used to predict and confirm VBRs in other molecular candidates for direct laser cooling. The insights gained here for CaOH can be generalized to support recent proposals extending laser cooling to symmetric and asymmetric top molecules [20,24,25] and even molecules with multiple cycling centers [54][55][56][57] .…”

“…These are closely related to the Franck-Condon factors (FCFs) of the molecule, defined by the overlap integral between vibrational wavefunctions in the ground and excited states (see section III). CaOH is an example of a broad class of polyatomic molecules that have been identified as amenable to laser cooling due to strong electronic transitions and near-diagonal FCFs [21,24,25].…”

Direct Laser Cooling of a Symmetric Top Molecule

“…However, the same complexity that provides these advantages makes laser cooling challenging for these molecules. Recent theoretical proposals have nonetheless suggested that laser cooling of polyatomic molecules, even nonlinear ones like symmetric tops, is a practical possibility [8][9][10].…”

“…This would offer an ideal starting point for realizing new polyatomic quantum simulation and computation platforms [5,6]. Laser cooling could also be extended to asymmetric tops, including biochemically relevant chiral molecules [9,10,48,49]. Finally, laser cooling and trapping of the heavier symmetric top molecule YbOCH 3 would allow precise searches for time-reversal violating interactions at a previously inaccessible energy scale, while ultracold chiral molecules such as YbOCHDT could enable precision probes of fundamental parity violation [7,30].…”