  Experimental approaches to manipulate light-matter interaction at nanoscale
have quickly advanced in recent years, leading to the demonstration of spaser
(surface plasmon amplification by stimulated emission of radiation) in
plasmonic nanocavities. Yet, a well-understood analytical theory to better
understand and quantitatively explain the connotation of spaser system is
urgently needed. Here we develop an all-analytical semiclassical theory to
investigate the energy exchange between active materials and fields and the
spaser performance in a plasmonic nanocavity. The theory can be commonly used
in understanding and designing all novel microlaser, nanolaser, and spaser
systems.
