  We study the coherent properties of plasmon polaritons optically excited on
periodic nanostructures. The gold grains are coupled to a single mode photonic
waveguide which exhibits a dramatically reduced transmission originating from
the derived quantum interference. With a nonequilibrium description of
Floquet-dressed polaritons we demonstrate the switching of light transmission
through the waveguide due to sheer existence of intraband transitions in gold
from right above the Fermi level driven by the external laser light.
