Luminescence of Cr3+ in lithium niobate:: influence of the chromium concentration and crystal composition

The luminescence spectrum of Cr3+ in LiNbO3 has been investigated, especially in the R-line range from 7200 up to 7500 Angstrom. For this, we have studied several chromium-doped LiNbO3 crystals with various Li/Nb ratios and different doping concentrations. In addition to the lines which were previously reported, several new peaks were detected in our investigations. In fact, the structure, the shape and the relative intensity of each line are shown to be strongly dependent on the content of doping and on the intrinsic defects related to the non-stoichiometry of the crystal. Among all peaks, we focus our attention on four main lines, which can be separated into two groups, according to their behaviours. The contribution of the first group to the whole spectrum diminishes with increasing Li/Nb ratio so that it vanishes for the stoichiometric composition. The second group of lines, occurring at higher wavelengths, is reported for the first time and is shown to be closely related to the Li/Nb ratio, especially when approaching the stoichiometric composition. These lines exhibit a similar behaviour with increasing Li/Nb ratio or Cr concentration. An explanation of the possible origins of these four peaks is given.