Theory of polarization rotation and conversion in vertically coupled microresonators

Two orthogonally polarized waveguide modes can exchange power through an intermediate level, if the polarization state of that level is tilted with respect to the polarization states of the waveguide. Microrings with modest sidewall angles support such tilted modes. Ultracompact, wavelength selective polarization rotators might be achieved. Simple analytic expressions are derived for polarization rotation by vertically-coupled resonators in terms of geometrical factors.