Progress towards higher phase space density of trapped SrF molecules

For molecules, rotational closure of an optical cycling transition requires driving `Type-II' transitions, where the ground and excited state angular momenta (J and J' respectively) satisfy J ≥ J'. Such transitions exhibit sub-Doppler heating for red detuning, leading to warm (∼ 1 mK) and large (∼ 1 mm) magneto-optical traps (MOTs). Subsequent loading of optical dipole traps (ODTs), while subject to blue-detuned sub-Doppler cooling, is inefficient (∼ 5%) due to the small volume of the ODT relative to the MOT cloud. This sub-Doppler cooling is typically done after turning the magnetic field off. Here, we demonstrate that, if the magnetic field is left on, blue-detuned light of the correct polarization can also produce a strong confining force on the SrF molecules in our experiment. Switching from the `red-MOT' to this `blue-MOT' enables further cooling and compression, potentially leading to more efficient ODT loading. We also report a scheme for improved slowing of our SrF cryogenic molecular beam, leading to a factor of $sim 10$ increase in the number of trapped molecules in the MOT. This should provide sufficient density to observe SrF-SrF collisions in a bulk gas, which is a key step towards using collisional cooling to reach quantum degeneracy.

We acknowledge funding from AFOSR MURI.