High Phase Space Density for Laser Cooled YO Molecules in an Optical Lattice

We report the highest phase space density of molecules by direct laser cooling. Our experiment begins with a dual-frequency magneto optical trap of yttrium monoxide (YO) loaded from a cryogenic buffer gas beam. Next, our molecules are further cooled via gray molasse and compressed via a compression sequence tailored for the special structure of YO, achieving a density of 5.4 x 10 cm^-3 and temperature of 4 µK. Molecules are then loaded and cooled into a 1 dimensional optical lattice. Utilizing effective sub-Doppler cooling inside the lattice, we create a molecular sample with a peak phase space density of 3 x 10^-6. With a long lattice lifetime of 850(70) ms, we are poised to explore molecular interaction effects.