  The concept of spaser, as the coherent near-field generator, based on
nanometric plasmonic resonators, has been successfully demonstrated in number
of experiments. Here we have developed the theoretical framework for
description of the basic properties of the spaser linewidth and, in particular,
linewidth enhancement. In order to achieve this, we have introduced explicitly
the time dependence in the quasistatic description of localized surface plasmon
resonances via inclusion of the dispersion of a spectral parameter, defining
the localized plasmon resonance wavelength. Linewidth enhancement factor was
estimated for semiconductor gain medium and was found to be of order of 3-6,
strongly depending on carrier density in the active layer, and resulting in
more than order of magnitude broader linewidth compared to that, predicted by
the Schawlow-Townes theory.
