  We study the effect of off-resonant plasmon modes on spaser threshold in
nanoparticle-based spasers. We develop an analytical semiclassical model and
derive spaser threshold condition accounting for gain coupling to higher-order
plasmons. We show that such a coupling originates from inhomogeneity of gain
distribution near the metal surface and leads to an upward shift of spaser
frequency and population inversion threshold. This effect is similar, albeit
significantly weaker, to quenching of plasmon-enhanced fluorescence near metal
nanostructures due to excitation of off-resonant modes with wide spectral band.
We also show that spaser quenching is suppressed for high gain concentrations
and establish a simple criterion for quenching onset, which we support by
numerical calculations for spherical geometry.
