Upconversion and site-selective spectroscopy in erbium-doped

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Abstract
Employing absorption, excitation and luminescence spectroscopy in a wide spectral range, the upconversion processes are investigated in at low temperature. From the detailed analysis of the spectroscopic data and the power dependence of the upconverted intensity, excited state absorption and energy transfer between excited ions are found to be the important mechanisms. Structure in the Stark substate absorption suggests the ions occupy several different but well defined lattice sites for which the local symmetry is slightly perturbed, presumably by charge compensating defects. Site-selective measurements allow one to determine the ground state splittings for two of these sites. The observed energy transfer suggests the existence of interacting pairs of ions. .