ABSTRACT Electro-optic (EO) effect in single crystalline thin films and superlattices made from relaxor ferroelectric lead titanate doped lead magnesium niobate (PMN-PT) and rare-earth doped PMN-PT materials have been investigated. Strong in-plane EO anisotropy has been found in both single crystalline thin films and superlattices. A large out-of-plane EO coefficient was observed in PMN-PT superlattices. Such a significant enhancement can be attributed to the spontaneous polarization coupling between the two crystalline phases in superlattices when the stacking period is below 90 nm. When doping with erbium, the EO effect in PMN-PT single crystalline film films decreases as the erbium concentration increases. This research has been motivated by the development of various promising integrated photonic devices such as waveguide EO modulators, waveguide lasers, and photonic bandgap devices.