Strong enhancement of Raman-induced nonreciprocity in silicon waveguides by alignment with the crystallographic axes

Raman gain in silicon photonic wires depends on the relative propagation directions of the pump and Stokes waves due to strong longitudinal mode-field components. Here we show that this Raman-induced nonreciprocity can be varied in a wide range by changing the orientation of the waveguide with respect to the crystallographic axes. In a ⟨001⟩ orientation, the nonreciprocity reaches its maximum which is larger by more than two orders of magnitude as compared to the traditional ⟨011⟩ orientation. The waveguide is then practically Raman-inactive for copropagation, while the contradirectional Stokes wave may experience significant Raman gain.