  In this article, we address fluorescence of single molecule inside a
plasmonic picocavity by proposing a semi-classical theory via combining the
macroscopic quantum electrodynamics theory and the open quantum system theory.
To gain insights into the experimental results [Nat. Photonics, 14, 693
(2020)], we have further equipped this theory with the classical
electromagnetic simulation of the pico-cavity, formed by single atom decorated
silver STM tip and a silver substrate, and the time-dependent density
functional theory calculation of zinc phthalocyanine molecule. Our simulations
not only reproduce the fluorescence spectrum as measured in the experiment,
confirming the influence of extreme field confinement afforded by the
picocavity, but also reveal Rabi oscillation dynamics and Mollow triplets
spectrum for moderate laser excitation. Thus, our study highlights the
possibility of coherently manipulating the molecular state and exploring
non-linear optical phenomena with the plasmonic picocavity.
