  Lasing at the nanometre scale promises strong light-matter interactions and
ultrafast operation. Plasmonic resonances supported by metallic nanoparticles
have extremely small mode volumes and high field enhancements, making them an
ideal platform for studying nanoscale lasing. At visible frequencies, however,
the applicability of plasmon resonances is limited due to strong ohmic and
radiative losses. Intriguingly, plasmonic nanoparticle arrays support
non-radiative dark modes that offer longer life-times but are inaccessible to
far-field radiation. Here, we show lasing both in dark and bright modes of an
array of silver nanoparticles combined with optically pumped dye molecules.
Linewidths of 0.2 nanometers at visible wavelengths and room temperature are
observed. Access to the dark modes is provided by a coherent out-coupling
mechanism based on the finite size of the array. The results open a route to
utilize all modes of plasmonic lattices, also the high-Q ones, for studies of
strong light-matter interactions, condensation and photon fluids.
