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 toward quantum degeneracy, a mechanism that combines
sub-Doppler cooling and magneto-optical trapping would facilitate near unity transfer of ultracold
molecules from the magneto-optical trap (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 2 orders of magnitude over any
previously reported molecular MOT.
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 toward quantum degeneracy, a mechanism that combines
sub-Doppler cooling and magneto-optical trapping would facilitate near unity transfer of ultracold
molecules from the magneto-optical trap (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 2 orders of magnitude over any
previously reported molecular MOT.