A metal-oxide semiconductor, MOS, capacitor optical modulator , comprising: a doped semiconductor layer having a waveguide structure ; a dielectric layer disposed over the waveguide structure of the doped semiconductor layer; a gate region disposed over the dielectric layer, wherein the gate region comprises a transparent electrically conductive material having a refractive index lower than that of silicon; and a first metal contact disposed over the gate region; a second metal contact disposed over the doped semiconductor layer, the second metal contact being proximate to a drain of the MOS capacitor optical modulator; a third metal contact disposed over the doped semiconductor layer, the third metal contact being proximate a source of the MOS capacitor optical modulator; characterized in that : the first metal contact, the gate region, and the waveguide structure of the doped semiconductor layer are vertically aligned with each other, wherein the respective sidewalls of the metal contact, gate region and waveguide structure are co-planar with each other; and the MOS capacitor optical modulator further comprises a cladding material formed around the waveguide, the first metal contact, the second metal contact and the third metal contact, wherein the cladding material has a refractive index lower than the refractive index of the gate region, such that the sidewalls of the gate region, and sidewalls of the first metal contact are covered by the cladding material. a doped semiconductor layer having a waveguide structure ; a dielectric layer disposed over the waveguide structure of the doped semiconductor layer; a gate region disposed over the dielectric layer, wherein the gate region comprises a transparent electrically conductive material having a refractive index lower than that of silicon; and a first metal contact disposed over the gate region; a second metal contact disposed over the doped semiconductor layer, the second metal contact being proximate to a drain of the MOS capacitor optical modulator; a third metal contact disposed over the doped semiconductor layer, the third metal contact being proximate a source of the MOS capacitor optical modulator; characterized in that : the first metal contact, the gate region, and the waveguide structure of the doped semiconductor layer are vertically aligned with each other, wherein the respective sidewalls of the metal contact, gate region and waveguide structure are co-planar with each other; and the MOS capacitor optical modulator further comprises a cladding material formed around the waveguide, the first metal contact, the second metal contact and the third metal contact, wherein the cladding material has a refractive index lower than the refractive index of the gate region, such that the sidewalls of the gate region, and sidewalls of the first metal contact are covered by the cladding material. the first metal contact, the gate region, and the waveguide structure of the doped semiconductor layer are vertically aligned with each other, wherein the respective sidewalls of the metal contact, gate region and waveguide structure are co-planar with each other; and the MOS capacitor optical modulator further comprises a cladding material formed around the waveguide, the first metal contact, the second metal contact and the third metal contact, wherein the cladding material has a refractive index lower than the refractive index of the gate region, such that the sidewalls of the gate region, and sidewalls of the first metal contact are covered by the cladding material. <EOS>