  Metallic nanostructures provide a toolkit for the generation of coherent
light below the diffraction limit. Plasmonic based lasing relies on the
population inversion of emitters (such as organic fluorophores) along with
feedback provided by plasmonic resonances. In this regime, known as weak light
matter coupling, the radiative characteristics of the system can be described
by the Purcell effect. Strong light matter coupling between the molecular
excitons and electromagnetic field generated by the plasmonic structures leads
to the formation of hybrid quasi-particles known as plasmon exciton polaritons
(PEPs). Due to the bosonic character of these quasi particles, exciton
polariton condensation can lead to laser-like emission at much lower threshold
powers than in conventional photon lasers. Here, we observe PEP lasing through
a dark plasmonic mode in an array of metallic nanoparticles with a low
threshold in an optically pumped organic system. Interestingly, the threshold
power of the lasing is reduced by increasing the degree of light matter
coupling in spite of the degradation of the quantum efficiency of the active
material, highlighting the ultrafast dynamic responsible for the lasing, i.e.,
stimulated scattering. These results demonstrate a unique roomtemperature
platform for exploring the physics of exciton polaritons in an open cavity
architecture and pave the road toward the integration of this on-chip lasing
device with the current photonics and active metamaterial planar technologies.
