  Organic dye molecules have been used in great many scientific and
technological applications, but their wider use in quantum optics has been
hampered by transitions to short-lived vibrational levels, which limit their
coherence properties. To remedy this, one can take advantage of optical
resonators. Here we present the first results on coherent molecule-resonator
coupling, where a single polycyclic aromatic hydrocarbon molecule extinguishes
38\% of the light entering a microcavity at liquid helium temperature. We also
demonstrate four-fold improvement of single-molecule stimulated emission
compared to free-space focusing and set the ground for coherent mechanical
manipulation of the molecular transition. Our experimental approach based on a
microcavity of low mode volume and low quality factor paves the way for the
realization of various nonlinear and collective quantum optical effects with
molecules.
