Polarization enhanced cooling of SrF molecules in a deep Optical Dipole Trap

Creating quantum degenerate molecular gases by `direct' cooling and trapping of molecules (as opposed to assembly from ultracold atoms), once a far-off dream, now seems like a very realistic goal. One step towards achieving this dream is demonstrating the ability to load molecules into conservative traps, such as optical dipole traps (ODTs), while retaining very cold temperatures. We demonstrate that, by choosing optimal polarizations of the ODT laser and of intensity imbalanced counter-propagating $\Lambda$-enhanced gray molasses lasers, we can load SrF molecules with temperatures as low as 10$\mu$K into a 420$\mu$K deep ODT. Unlike alkali atoms, SrF molecules in the $X^{2}\Sigma_{1/2}$ state have tensor and vector polarizabilities, so a dependence on ODT polarization was expected. However, the interplay between the polarization of the ODT and the $\Lambda$-cooling light, and the intensity imbalance of the latter, was unexpected, and requires further exploration. Using this technique, we have loaded $\sim 4$\% of the $N\sim 4000$ SrF molecules trapped in our rfMOT and reached densities of $6.5\times 10^{8}$\,cm$^{-3}$. We are currently working on techniques for loading more molecules and for further cooling of SrF.