  We consider light propagation in a silicon dioxide substrate with implanted
${\mathrm{Er}}^{3+}$ ions and silver nanoparticles that are randomly and
homogeneously distributed in the substrate. When their densities are large
enough, the medium can have a negative refractive index over a certain range of
frequencies, within which the following exotic property ensues: increasing the
electric and magnetic plasma frequencies, the medium transparency is augmented.
