  Controlling the light emitted by individual molecules is instrumental to a
number of novel nanotechnologies ranging from super-resolution bio-imaging and
molecular sensing to quantum nanophotonics. Molecular emission can be tailored
by modifying the local photonic environment, for example by precisely placing a
single molecule inside a plasmonic nanocavity with the help of DNA origami.
Here, using this scalable approach, we show that commercial fluorophores
experience giant Purcell factors and Lamb shifts, reaching values on par with
those recently reported in scanning tip experiments. Engineering of plasmonic
modes enables cavity-mediated fluorescence far detuned from the
zero-phonon-line (ZPL) - at detunings that are up to two orders of magnitude
larger than the fluorescence linewidth of the bare emitter and reach into the
near-infrared. Our results evidence a regime where the emission linewidth is
dominated by the excited state lifetime, as required for indistinguishable
photon emission, baring relevance to the development of nanoscale, ultrafast
quantum light sources and to the quest toward single-molecule cavity-QED. In
the future, this approach may also allow to design efficient quantum emitters
at infrared wavelengths, where standard organic sources have a reduced
performance.
