  Using a multiple-scale asymptotic approach, we have derived the complex cubic
Ginzburg-Landau equation for amplified and nonlinearly saturated surface
plasmon polaritons propagating and diffracting along a metal-dielectric
interface. An important feature of our method is that it explicitly accounts
for nonlinear terms in the boundary conditions, which are critical for a
correct description of nonlinear surface waves. Using our model we have
analyzed filamentation and discussed bright and dark spatially localized
structures of plasmons.
