Not yet reviewed by Pith; the record is open.

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.