1. A modulator for varying an optical signal by varying an electrical signal or varying an electrical signal by varying an optical signal, the modulator comprising: an ultra-long-range surface plasmon (ULRSP) waveguide comprising Bragg mirror structures disposed at both ends thereof and forming a linear resonant Fabry-Perot structure, a metal layer; said metal layer being sufficiently thin to couple surface plasmons propagating along each surface of said metal layer; a first layer of a dielectric material contacting a first side of said metal layer and having a first refractive index; a second layer of a dielectric oxide, a first side of the second layer contacting a side of said metal layer opposite to the first side of said metal layer, wherein said second layer is electrically insulating and has a second refractive index lower than the first refractive index and a thickness effective to achieve ULRSP propagation; a third layer of a semiconductor material contacting a side of said second layer opposite to the first side of said second layer, the third layer having a third refractive index, the first and third refractive indices being substantially matched; wherein said metal, second, and third layers form a metal-oxide-semiconductor (MOS) capacitor; and means for inducing a selected voltage across said capacitor; wherein inducing the selected voltage across the capacitor controls transmission efficiency in the Fabry-Perot structure; and an optical waveguide coupled to the ULRSP waveguide, wherein the optical waveguide is a dielectric waveguide comprising a dielectric core layer, the core layer not forming any of the first, second, or third layer of the ULRSP waveguide.