Optic-axis and voltage-controlled polarization splitting and tunability in BaTiO ₃ defect-based 1D photonic crystals for filtering applications

In the present work, we investigate the coupling and decoupling between the co-polarized and cross-polarized TE/TM modes under the combined influence of applied voltage and optic axis orientation in anisotropic uniaxial one-dimensional photonic crystals. The proposed structure consists of silicon dioxide and titanium dioxide as the periodic layers and barium titanate as the defect layer. Unlike the previous work, where either a single input polarization is considered or an inconsistent permittivity tensor has been employed to account for the simultaneous effects of the external electric field and optic axis rotation, here a physically consistent formulation is presented where the electro-optic effect and optic axis rotation are applied in different coordinate frames. This approach reveals that a polarized TE (or TM) input light splits into TE and TM output lights at specific optic axis angles. At the optic axis angle of 45°, the splitting modes have symmetry in their transmission. The voltage applied across the defect compensates for the effect of the optic axis angle and enables voltage-controlled suppression of polarization coupling. Additionally, it introduces tunability in the TM modes, whereas the incident angle provides tunability for all the modes. When the voltage is 40 V or greater, the coupling effect diminishes. These results provide an accurate physical understanding of tunable polarization control in one-dimensional photonic crystals and support the design of voltage-tunable optical filters and polarization-selective photonic devices, with the further possibility of advancing into the theoretical modeling of higher-dimensional photonic crystals.