The present invention provides silicon based thin-film structures that can be used to form high frequency optical modulators. Devices of the invention are formed as layered structures that have a thin-film dielectric layer, such as silicon dioxide, sandwiched between silicon layers. The silicon layers have high free carrier mobility. In one aspect of the invention a high mobility silicon layer can be provided by crystallizing an amorphous silicon layer. In another aspect of the invention, a high mobility silicon layer can be provided by using selective epitaxial growth and extended lateral overgrowth thereof.