BSTRACTDynamic tuning of optical properties is technologically important for the development of future integrated electronic and photonic devices. Here

we demonstrate the electrically tunable permittivity in optical frequency in epitaxial BaTiO

3

(BTO) thin films using spectroscopic ellipsometer measurements under DC bias condition. The lattice tetragonal distortion and domain switching of biased BTO is evident based on the increased anisotropy in permittivity in-plane and out-of-plane. Specifically

out-of-plane permittivity

arising from the

c-

domains

shows a continuous decrease with the applied voltage

relative to the in-plane permittivity

arising from the

a-

domains. The dielectric constant can also be reversibly adjusted through electric-reset cycles.