Optical Cycling in ThO, a Molecule with Dense Electronic Structure

Optical cycling, or the ability to interact with an atom or molecule repeatedly using few lasers, lays the foundation for many modern techniques of quantum sensing and control. While simple atoms require only a single laser to optically cycle, molecules and more complex atoms require repumping lasers at additional frequencies to recover population loss caused by rovibrational degrees of freedom and dense electronic structure. Recent work on laser cooling of CaOH and SrOH molecules as well as of atoms like In and Eu, confirms that optical cycling schemes are possible on systems that have either complex rovibrational or electronic structure. Extending photon cycling to more general molecules requires managing both rovibrational degrees of freedom and significantly denser electronic structure than the species hitherto studied. To this end, we present work towards optical cycling in ThO, a diatomic molecule with isotopologues of interest for both electron electric dipole moment and nuclear Schiff moment measurements. Managing both >1% vibrational and electronic branching, we describe a scheme capable of scattering ~100 photons with ~5 lasers. This cycling would be sufficient to achieve optical detection with near-unity efficiency and would match the number of photons scattered in demonstrations of Sisyphus cooling in polyatomic molecules. This work extends the domain of molecular photon cycling and enables enhancements to ongoing and planned tests of fundamental physics.