Combined action of a resonant cavity and metal nanocomposites is shown to result in dramatic enhancement of spontaneous emisson rate in the optical spectral range. The cavity leads to modification of the optical density of states (Purcell effect), whereas the metal nanoparticles lead to giant local-field effects in spontaneous emission occuring because of surface-plasmon-enhanced vacuum field fluctuations. It is also shown that the surface-plasmon resonances may result in a full photonic gap in a metal-dielectric composite, even for a completely random structure of the composite. In our experiments, lasing at extremely low pump intensities, below 1 mW was observed for a novel class of optical materials, microcavities doped with fractal aggreates of metal nanoparticles.
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