Emission and absorption cross sections at 810 and 860 nm bands of Er ³⁺ in LiNbO ₃ crystal

We have measured the unpolarized and polarized emission spectra of ⁴I_9/2→⁴I_15/2 (810 nm) and ⁴S_3/2→⁴I_13/2 (860 nm) electronic transitions of Er³⁺ in LiNbO₃ crystal under different incident directions and polarization states of excitation beam. From the measured emission spectra, the emission and absorption cross-section spectra were calculated based upon McCumber theory. It is found that Er³⁺ electronic transition shows interesting excitation beam direction effect in polarization dependence, spectral shape, and cross-section value. Both transitions are highly π-polarized as the excitation beam was aligned perpendicular to the optical axis of crystal while being highly σ-polarized as the excitation beam was oriented parallel to the optical axis of crystal. The spectral shape in the case of the perpendicular excitation is very different from that in the case of parallel excitation. The cross-section value in the perpendicular excitation case is at least 1.5 times larger than that in the parallel excitation case. These excitation direction effects are independent of the polarization state of excitation light, and are attributed to the selective Er³⁺ site excitation. In addition, the Er³⁺ 860 nm emission lifetime was measured to be 27 ± 5 μs and the quantum efficiency of the emission is 2.5%.