  Plasmonic waveguides offer the unique possibility to confine light far below
the diffraction limit. Past room temperature experiments of single quantum
emitters coupled to such waveguides have mainly focused on efficient
channelling of the spontaneous emission into waveguide modes after a linear
optical excitation. However, only the simultaneous interaction of the emitter
with multiple plasmonic fields would lead to functionality in a plasmonic
circuit. Here, we demonstrate the nonlinear optical interaction of a single
molecule and propagating plasmons. An individual terrylene diimide (TDI)
molecule is placed in the nanogap between two single-crystalline silver
nanowires. A visible wavelength pump pulse and a red-shifted depletion pulse
travel along the waveguide, leading to stimulated emission depletion (STED) in
the observed fluorescence. The efficiency increases by up to a factor of 50
compared to far-field excitation. Our study thus demonstrates remote nonlinear
four-wave mixing at a single molecule with propagating plasmons. It paves the
way towards functional quantum plasmonic circuits and improved nonlinear
single-molecule spectroscopy.
