Laser cooling diatomic molecules to below the Doppler limit

Ultracold molecules are useful for testing fundamental physics, studying strongly-interacting quantum systems, and exploring collisions and chemistry in the ultracold regime. We produce ultracold CaF by the following steps. First, we produce a beam of CaF molecules, with an average velocity of 140 m/s, by laser ablation of Ca into a flow of cryogenic helium gas mixed with SF$_6$. This beam is slowed via a chirped, counter-propagating laser beam to below the capture velocity of a magneto-optical trap (MOT). The molecules are then trapped and Doppler cooled in the MOT where they reach an equilibrium temperature of 12mK. We cool the molecules further to about 960$\mu$K by decreasing the intensity of the MOT beams. Finally, we load the molecules into a three-dimensional blue-detuned molasses where they cool to 50$\mu$K, well below the Doppler limit.