This book represents the first detailed description‎,‎ including both theoretical aspects and experimental methods‎,‎ of the interaction of rare‎-earth ions with surface plasmon polariton from the point of view of collective plasmon‎-photon interactions via resonance modes ‎(‎metal nanoparticles or nanostructure arrays‎)‎ with quantum emitters ‎(‎rare‎-earth ions‎)‎‎.‎ These interactions are of particular interest for applications to optical telecommunications‎,‎ optical displays‎,‎ and laser solid state technologies‎.‎ Thus‎,‎ our 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 e‎.‎‎.‎‎.‎