Ultracold polar molecules provide an exciting platform for the study of many subfields ofphysics. Some of the studies include spin-models, quantum information, strongly correlated and exotic phases of matter, ultracold chemistry, and beyond-standard model searches. This makes them a very versatile testbed as any individual molecule could potentially be used for all the above-mentioned studies.

In this thesis, I will describe my work on the direct laser-cooling of molecules, specifically theYO molecule. The work spans over different magneto-optical-trapping mechanisms, sub-Doppler cooling, conservative trapping, and collisional studies. Some of the highlights of my work are the demonstration of the first sub-Doppler molecular MOT, and the study of bulk gas collisions in a true single partial wave regime. In the culmination of this work, we achieve a record phase space density of PSD = 2.5 × 10−5 in a bulk dilute gas of YO molecules pushing laser-cooled molecules into the ultracold regime.