Combined excitation-emission spectroscopy of Er3+ ions in stoichiometric LiNbO3:: The site selectivity of direct and up conversion excitation processes

We studied the optical transitions of Er3+ ions in LiNbO3 for samples with stoichiometric composition that were produced from congruent ones by the vapor transport equilibration technique. Using combined excitation-emission as well as absorption, emission, and Raman spectroscopy with high resolution, we found and spectrally characterized a large number (>11) of different Er3+ sites. Comparing stoichiometric and congruent samples we found equivalent sites, however, with an altered distribution of their abundances. For the microscopic origin of the sites we propose that for the main sites the Er3+ is located on or close to a Li+ lattice site with differing arrangements of the required local charge compensation. On the basis of these results we investigated in detail the upconversion process that occurs under 980-nm excitation and found several mechanisms including direct and phonon-assisted excited state absorption and energy transfer, which are all strongly dependent on site and on the excitation energy.