Compact polarization rotator for silicon-based slot waveguide structures.

A compact polarization rotator (PR) for silicon-based slot waveguides is proposed, where the slot region including the upper claddings is filled with liquid crystals (LCs). With the anisotropic features of the LCs, the transverse field components of eigenmodes have almost identical amplitudes, leading to a high modal hybridness. As a result, the TE (TM) polarization can be rotated efficiently to the TM (TE) polarization within a short length. The numerical results show that a PR 11.3 μm in length at an operating wavelength of 1.55 μm is achieved with an extinction ratio (ER) (insertion loss) of 12.6 (0.22) dB for TE-to-TM and 11.5 (0.30) dB for TM-to-TE. Moreover, the optical bandwidth for TE-to-TM (TM-to-TE) mode must be ∼64(∼29)  nm to keep the ER over 12 (10) dB. In addition, fabrication tolerances to the structural parameters are investigated, and field evolution along the propagation distance through the PR is also demonstrated.