A new High-Z optical modulator has a waveguide and electrodes on a substrate, a buffer layer with a low dielectric constant between the waveguide and the substrate, and a substance between the waveguide and the substrate with a dielectric constant lower than a dielectric constant of the substrate to the side and below the plane of the waveguide, thereby improving electro-optic field overlap, increasing RF speed and increasing transmission line impedance. The material with a dielectric constant lower than the substrate extends between the waveguide and the electrodes to a depth below the waveguide equal to or greater than the lateral distance between the waveguide and electrodes. This material may be air and may be introduced by cutting away portions of the substrate around the waveguide with a precision dicing saw. The electrodes may be placed even with the waveguide or below the waveguide on the cut-away portion of the substrate.