  We develop a general perturbation theory to treat small parameter changes in
dispersive plasmonic nanostructures and metamaterials. We specifically apply it
to dielectric refractive index, and metallic plasma frequency modulation in
metal- dielectric nanostructures. As a numerical demonstration, we verify the
theory's accu- racy against direct calculations, for a system of plasmonic rods
in air where the metal is defined by a two-pole fit of silver's dielectric
function. We also discuss new optical behavior related to plasma frequency
modulation in such systems. Our approach provides new physical insight for the
design of plasmonic devices for biochemical sensing and optical modulation, and
future active metamaterial applications.
