High density loading and collisional loss of laser cooled molecules in an optical trap

We report optical trapping of laser-cooled molecules at sufficient density to observe molecule-molecule collisions for the first time in a bulk gas. SrF molecules from a red-detuned magneto-optical trap (MOT) are compressed and cooled in a blue-detuned MOT. Roughly 30% of these molecules are loaded into an optical dipole trap with peak number density $n_0\approx 3\times {10}^{10}{\text{ cm}}^{-3}$ and temperature $T\approx 40$ $\mu$K. We observe two-body loss with rate coefficient $\beta =2.7_{-0.8}^{+1.2}\times {10}^{-10}{\text{ cm}}^3{\text{ s}}^{-1}$. Achieving this density and temperature opens a path to evaporative cooling towards quantum degeneracy of laser-cooled molecules.