Excitation Direction Effect on Polarization Property of Er $^{3+}$ Electronic Transition in LiNbO $_{3}$

We report that the polarization property of Er³⁺ transition in LiNbO₃ waveguide or bulk material is closely related to the incident direction of excitation beam relative to the optical axis of the crystal. It is found that the 1.5-μm transition is highly σ - (π -) polarized as the excitation direction is perpendicular (parallel) to the optical axis, and the 0.67-μm transition reveals a reversed effect. Incident perpendicularly (parallel), the transition is highly π - (σ -) polarized. The effect is found to be independent of the polarization state and wavelength of the excitation light. It is attributed to the selective Er site excitation.