  Active nanoplasmonic metamaterials support bright and dark modes that compete
for gain. Using a Maxwell-Bloch approach incorporating Langevin noise we study
the lasing dynamics in an active nano-fishnet structure. We report that lasing
of the bright negative-index mode is possible if the higher-Q dark mode is
discriminated by gain, spatially or spectrally. The nonlinear competition
during the transient phase is followed by steady-state emission where bright
and dark modes can coexist. We analyze the influence of pump intensity and
polarization and explore methods for mode control.
