Spectroscopy of upper energy levels in an Er³⁺-doped amorphous oxide

A spectroscopic study of the population mechanisms in erbium-doped amorphous aluminum oxide up to the 2H11/2/4S3/2 levels is performed. Via luminescence decay measurements, absorption and emission spectra, and a Judd-Ofelt analysis we determine luminescence lifetimes, radiative and non-radiative decay-rate constants, and branching ratios of the Er3+ inter-manifold transitions. With a continuous-wave pump-probe technique the excited-state absorption (ESA) spectrum is recorded between 900 and 1800 nm and the cross-sections of the ESA transitions 4I13/2 -> 4I9/2, 4I13/2 -> 4F9/2, and 4I11/2 -> 4F7/2 are determined. The microparameters and efficiencies of resonant and phonon-assisted energy-migration and energy-transfer-upconversion (ETU) processes among Er3+ ions occurring from the first and second excited states are evaluated. From the ratio of the 4S3/2 and 4F9/2 luminescence intensities as a function of Er3+ concentration we prove the existence and quantify the macroscopic ETU coefficient of the two-phonon-assisted ETU process (4I13/2, 4I11/2) -> (4I15/2, 4F9/2).