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The resonant behaviour of single plasmonic nanoparticles in contact with gain media is studied theoretically, with a focus on the issue of loss compensation, resonance amplification and modification. Two approaches are presented. The first description is macroscopic and based on a simple, quasi-static continuum description of the gain medium, and shows that gain-assisted particle display unusual plasmonic responses, with features depending notably on the nature of the metal used (gold or silver). Results are discussed in two geometries, metallic nanoparticles in an infinite gain medium, and metal core-gain shell nanoparticles. The second model is microscopic and describes the situation of a metallic gold nanoparticle, surrounded with a corona of dye molecules or nanocrystals. A full analytical electromagnetic model is proposed, taking into account interactions, through a Green function formalism, between dye and particle, and also between dye and dye, both directly and mediated via the nanoparticle. The modes of the particle-and-dye system are studied, demonstrating that some modes, either emissive or absorptive, can be strongly amplified and dominate the response of the ensemble.

active metamaterials; gain; loss compensation; plasmonics