The properties of Cr3+ in LiNbO3 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 4T2 level is lower than 2E. The two other chromium centres α
and β yield sharp lines in emission and correspond to the high-crystal field scheme. Their
dependencies on both the chromium concentration and the composition were studied using 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.