Site-selective measurement of relaxation properties at 980 nm in Er ^3+ -doped congruent and stoichiometric lithium niobate crystals

pump-probe-type saturation spectroscopic experiment has been performed at 980 nm to measure the homogeneous linewidth of the ^4 I _11/2 – ^4 I _15/2 transition and the lifetime of the ^4 I _11/2 state of erbium embedded into lithium niobate single crystals. There are several non-equivalent Er ^3+_Li^+ –V _Li^+ sites with differently oriented defect structure for charge compensation. These non-equivalent centers differ in their transition energies. Hence, our measurements have been done at two nearby frequencies, addressing groups of erbium ions located at non-equivalent sites. The results are compared with lifetime calculations and measurements found in the literature and with the predictions of our simple model calculation.