A phase modulation waveguide structure includes one of a semiconductor and a semiconductor-on-insulator substrate, a doped semiconductor layer formed over the one of a semiconductor and a semiconductor-on-insulator substrate, the doped semiconductor portion including a waveguide rib protruding from a surface thereof not in contact with the one of a semiconductor and a semiconductor-on-insulator substrate, and an electrical contact on top of the waveguide rib. The electrical contact is formed of a material with an optical refractive index close to that of a surrounding oxide layer that surrounds the waveguide rib and the electrical contact and lower than the optical refractive index of the doped semiconductor layer. During propagation of an optical mode within the waveguide structure, the electrical contact isolates the optical mode between the doped semiconductor layer and a metal electrode contact on top of the electrical contact.