High Phase-Space Density of Laser-Cooled Molecules in an Optical Lattice

We report laser cooling and trapping of yttrium monoxide (YO) molecules in an optical lattice. We show that gray molasses cooling remains exceptionally efficient for YO molecules inside the lattice with a molecule temperature as low as 6.1(6) $\mu$K. This approach has produced a trapped sample of 1200 molecules, with a peak spatial density of $\sim 1.2\times {10}^{10}$ cm${}^{-3}$, and a peak phase-space density of $\sim 3.1\times {10}^{-6}$. By adiabatically ramping down the lattice depth, we cool the molecules further to 1.0(2) $\mu$K, twenty times colder than previously reported for laser-cooled molecules in a trap.