Reassignment of electronic transitions in the laser-activated spectrum of nanocrystalline Y2O3:Er3+

The laser-activated spectra of nanocrystalline Y2O3:Er3+ recorded at various temperatures and two excitation energies are described and analysed herein. Based on recently published modified values of the C2-type Stark splitting of Er3+ in the cubic Y2O3 lattice, the Er3+ multiplets in the observed spectra have been re-assigned. The upconversion routes of three 2H11/2 → 4I15/2 hot bands from 4S3/2 Kramers doublets have been traced, based on an Arrhenius-type analysis of these hot bands. We observed 5 peaks in the 2H11/2 → 4I15/2 hot band region with a different temperature behaviour. These bands belong to the 2P3/2 → 4I9/2 multiplet and their deviating temperature behaviour has been explained in terms of a cross relaxation mechanism. From the ratio between the laser-activated spectra that were recorded at different excitation energies we concluded that the two-photon absorption process becomes more efficient upon increasing the excitation energy. We have also put forward an explanation for the gradual broadening of the fluorescence bands of the 2H11/2 → 4I15/2 multiplet in going from 518 nm to 542 nm. The higher Stark splitting levels are due to a stronger electrostatic field of the host lattice and experience therefore a higher electron-phonon interaction, which explains the observed increase of FWHM at larger wavelengths.