Collective plasmon modes in gain media : quantum emitters and plasmonic nanostructures Main goal is to give a more precise overview of the rapidly emerging field of nanophotonics by means of the study of the quantum properties of light interaction with matter at the nanoscale. In this way collective plasmon modes in a gain medium result from the interaction coupling between a quantum emitter (created by rare earth ions) with a metallic surface inducing different effects such as the polarization of the metal electrons (so called surface plasmon polariton SPP) a field enhancement sustained by resonance coupling or transfer of energy due to non resonant coupling between the metallic nanostructure and the optically active surrounding medium.