Abstract
A laser oscillation assisted by localized surface plasmons of nanostructured metals is described. A
random laser is readily achieved in media where metallic nanoparticles are randomly dispersed.
Surprisingly, laser oscillation is feasible even in such a medium where the scattering mean free path is as
long as 200 mm, indicating that plasmon-enhanced scattering is not the main origin for the laser
oscillation. An experiment using nanoparticles composed of metallic core and oxide shell shows that the
separation between the metal and an emission center determines the lasing threshold and suggests that
the excitation rate, which is enhanced by the increased electric field due to the surface plasmons, is
dominant in the low-threshold laser oscillation. We also numerically demonstrate that an energy flow in
only one direction is realized for a dielectric nanosphere that contains an optical gain with a metallic
semi-shell, which is one kind of optical microresonator.
