An electro-optic element includes a first waveguide, which is a plasmonic waveguide, including a first core having a ferroelectric material and a cladding having a first cladding portion. The first cladding portion includes, at a first interface with the ferroelectric material, a first cladding material. The electro-optic element includes a first and a second electrode for producing an electric field in the ferroelectric material when a voltage is applied between the first and second electrodes, for modulating a real part of a refractive index of the ferroelectric material. The element includes, in addition, a crystalline substrate on which the ferroelectric material is epitaxially grown with zero or one or more intermediate layers present between the substrate and the ferroelectric material. The element may have a second waveguide, which is a photonic waveguide, including for enabling evanescent coupling between the first and second waveguides.