Ultracold molecules can be used for a wide range of applications, including quantum chemistry,
tests of fundamental physics, and quantum simulation. These goals hinge on the ability to
produce a gas of polar molecules at high phase-space density. Sympathetic cooling offers a
promising route to bring laser-cooled molecules into this regime.
In this thesis, we make progress towards sympathetic cooling of CaF molecules with ultracold
Rb atoms. Both species are loaded into a magnetic trap and we study collisions between them.
Rotationally-excited molecules are found to collide inelastically with the atoms with a rate
coefficient close to the universal limit. In the rotational ground state, no inelastic collisions are
observed but an upper bound is placed on the inter-species inelastic loss rate coefficient. An
attempt was made to observe thermalization between the two species is observed, which places
an upper limit on the elastic cross section.
Transverse cooling of the molecular beam used for loading the MOT is implemented, increasing
the number of molecules captured by a factor of 3.5. Benefiting from this, both species are
loaded into a 1D optical lattice - demonstrating the first mixture of atoms and laser-cooled
molecules in an optical trap. We observe rapid loss of molecules when cotrapped with the
atoms, even for molecules in the rotational ground state. This is attributed to elastic collisions
with the atoms, which have a temperature close to the trap depth for molecules. To circumvent
this, it is necessary to cool the atoms further in a deep optical trap. Cooling of Rb atoms in a
lattice and an optical dipole trap (ODT) is demonstrated using a Λ-enhanced gray molasses and
we succeed in cooling the atoms down to 18 μK in an ODT of depth 1mK. At this temperature,
sympathetic cooling of the molecules in the ODT should be observable.