Blue-Detuned Magneto-Optical Trap of Molecules

Direct laser cooling of molecules has reached a phase space density exceeding 10$^{-6}$ in optical traps, but with rather small molecular numbers. To progress towards quantum degeneracy, a mechanism is needed that combines sub-Doppler cooling and magneto-optical trapping (MOT) to facilitate near unity transfer of ultracold molecules from the MOT to a conservative optical trap. Using the unique energy level structure of YO molecules, we demonstrate the first blue-detuned MOT for molecules that is optimized for both gray-molasses sub-Doppler cooling and relatively strong trapping forces. This first sub-Doppler molecular MOT provides an increase of phase-space density by two orders of magnitude over any previously reported molecular MOT.