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 n₀≈3×10¹⁰ cm⁻³ and temperature T≈40 μK. We observe two-body loss with rate coefficient β=2.7-0.8+1.2×10⁻¹⁰ cm³ s⁻¹. Achieving this density and temperature opens a path to evaporative cooling towards quantum degeneracy of laser-cooled molecules.