  We fabricate silicon waveguides in silicon-on-insulator (SOI) wafers clad
with either silicon dioxide, silicon nitride, or aluminum oxide, and by
measuring the electro-optic behavior of ring resonators, we characterize the
cladding-dependent and capacitively-induced free-carrier effects in each of
these waveguides. By comparing our measured data with simulation results, we
confirm that the observed voltage dependencies of the transmission spectra are
due to changes in the concentrations of holes and electrons within the
semiconductor waveguide, and we show for the first time how strongly these
effects depend on the cladding material which comes into contact with the
silicon waveguide. Additionally, the waveguide loss is found to have a
particularly high sensitivity to the applied voltage, and may therefore find
use in a wide range of applications which require low- or high-loss
propagation. Collectively, these phenomena may be incorporated into more
complex waveguide designs in the future to create high-efficiency electro-optic
modulators.
