Eric Norrgard

Laser cooling in a magneto-optical trap (MOT) is the workhorse technique for atomic physics in the ultracold regime, serving as the starting point in applications from optical clocks to quantum-degenerate gases. It was recently shown that optical cycling, and thus laser cooling, should be possible for a class of at least 40 molecular species, using just three (or fewer) lasers. In this work, we demonstrate the first laser slowing and first magneto-optical trapping of a molecule, strontium monofluoride (SrF).

In our experiments, a laser-slowed molecular beam is used to load a MOT. The rotational structure in molecules prevents cycling on a two-level system (as is typical with atoms) and leads to populating states which are dark to the confining laser. In the first molecular MOT demonstration, the confining forces were very weak compared to typical atomic MOTs. In a second iteration, with the aid of recent theoretical insights into the origin of the restoring force in a MOT, we optimized the confining force for static fields in the MOT in lieu of the dark states. We then demonstrated a trap which eliminates dark states by trapping in time-varying fields. This method allows for capture of a large sample of molecules at ultracold temperatures and high phase space densities with long trap lifetimes. The trapped population is sufficiently cold to proceed with many proposed experiments posed to expand the frontiers of knowledge, such as operation of molecular fountains and study of ultracold molecule-atom collisions.