We report on a novel mechanism that allows for laser cooling of atoms that do not have a closed cycling transition. This mechanism is observed in a magneto-optical trap for erbium, an atom with a very complex energy level structure with multiple pathways for optical pumping losses. We observe surprisingly high trap populations of over 106 atoms and densities of over 1011 atoms cm-3, despite the many potential loss channels. A model based on recycling of metastable and ground state atoms held in the quadrupole magnetic field of the trap explains the high trap population, and agrees well with time-dependent measurements of MOT fluorescence. The demonstration of trapping of a rare-earth atom such as erbium opens a wide range of new possibilities for practical applications and fundamental studies with cold atoms.
Physical Review Letters