  Plasmon-mediated superradiance for molecules around metallic nanospheres was
proposed ten years ago. However, its demonstration has not been achieved yet
due to the experimental difficulty of positioning molecules, and the
theoretical limitation to the enhanced collective rate of low excited
molecules. In this Letter, we propose that the ultrafast plasmon-mediated
superradiant pulses can be observed with strongly excited methylene blue
molecules standing vertically inside gold nanoparticle-on-mirror nanocavities.
Our simulations indicate that in this system the molecules could interact with
each other via plasmon- and free-space mediated coherent and dissipative
coupling. More importantly, the coherent coupling mediated by short-ranged
propagating surface plasmons cancel largely the direct dipole-dipole coupling
mediated by the free-space field, and the dominated dissipative coupling
mediated by relatively long-ranged gap plasmons enables the ultrafast
superradiant pulses within picosecond scale. Our study opens up the possibility
of studying the rich superradiant effects from the quantum emitters in a
sub-wavelength volumn by engineering the plasmonic environments.
