  Photoassociation (PA) is a powerful technique to synthesize molecules
directly and continuously from cold and ultracold atoms into deeply bound
molecular states. In freespace, however, PA efficiency is constrained by the
number of spontaneous decay channels linking the initial excited molecular
state to a sea of final (meta)stable rovibronic levels. Here, we propose a
novel scheme based on molecules strongly coupled to a guided photonic mode in a
photonic crystal waveguide that turns PA into a powerful tool for near
deterministic formation of ultracold molecules in their ground rovibrational
level. Our example shows a potential ground state molecule production
efficiency $> 90\%$, and a saturation rate $>10^6$ molecules per second. By
combining state-of-the-art cold atomic and molecular physics with nanophotonic
engineering, our scheme presents a novel experimental package for trapping,
cooling, and optical manipulation of ultracold molecules, opening up new
possibilities in the direction of ultracold chemistry and quantum information.
