Patent ID: 6914999

Claim:
Modulation means comprising input means ( 107 ), output means ( 109 ) and a waveguide structure therebetween: the waveguide structure formed by a thin metallic strip ( 106 ) surrounded by material having a relatively low free charge carrier density, the surrounding material comprising first ( 102 ) and second ( 104 ) distinct portions with the metallic strip extending at an interface between respective juxtaposed surfaces ( 102 ″, 104 ″) of the first and second distinct portions, the metallic strip having finite width and thickness, the width being greater than the thickness, dimensioned such that optical radiation emitted by the input means having a wavelength in a predetermined range couples to the metallic strip and, when said first and second distinct portions are substantially index-matched, propagates along the length of the metallic strip as a plasmon-polariton wave with its transverse electric field substantially perpendicular to the width of the metallic strip; the input means being arranged to couple said optical radiation to one end of the metallic strip so as to excite said plasmon-polariton wave and said output means being coupled to the opposite end of the metallic strip so as to receive the plasmon-polariton wave; at least the first distinct portion ( 102 ) comprising an electro-optic material having a preferred axis along which its refractive index changes in response to an applied electric field; electrode means ( 100 ) extending adjacent, preferably substantially longitudinally parallel to, the metallic strip, at least part of the first distinct portion extending between said electrode means and said metallic strip; and control means ( 112 ) for applying a voltage (V T ) to the electrode means so as to establish an electric field (E) in said first distinct portion, the orientation of said preferred axis of the electro-optic material relative to said metallic strip, and the positioning of the electrode means relative to said metallic strip both being such that variation of said refractive index is in a direction that extends transversely of, and preferably is substantially perpendicular to, said width of the metallic strip; said control means ( 112 ) being operable to modulate said voltage so as to vary said refractive index of said first distinct portion relative to that of said second distinct portion, such that a plasmon-polariton wave propagating along the metallic strip will be correspondingly modulated.