Comparative analysis of the Cr~(3+) centre spectroscopic properties in LiNbO_3 crystals from congruent to nearly stoichiometric compositions

摘要

The properties of Cr~(3+) in LiNbO_3 single crystals have been investigated by means of several experiments including the recordings of absorption and high resolution laser spectroscopy spectra with dye-laser excitation spectra, emission spectra and fluorescence decays under laser selective pumping. Measurements performed on crystals with composition varying between congruent and close to stoichiometry enabled new features to be revealed which lead us to re-interpret the optical spectra of chromium-doped LN. Three chromium centres are sufficient to explain the main optical features. The main chromium centre labelled γ, giving rise to the broad absorption and emission bands, corresponds to the low-crystal field scheme where the ~4T_2 level is lower than ~2E. The two other chromium centres α and β yield sharp lines concentration and the composition were studied suing emission spectroscopy at low temperature. The main centre γ seems slightly affected by chromium concentration or composition change. The centres α and β are shown to be strongly connected to the presence in their surrounding of intrinsic defects related to the crystal non-stoichiometry. From their behaviour versus Cr concentration, the origin of these centres is discussed in terms of perturbed substitution.