Optical Transitions of Trivalent Neodymium and Chromium Centres in LiNbO₃ Crystal Host Material

Abstract

LiNbO₃ (LN) is a crystal which combines excellent non-linear optical properties with the possibility of trivalent optical ion doping as rare earth ions or transition metal ions. A key problem in these trivalent optical ions-doped LN has been to determine the lattice location of the activator. This is of importance because some of their attractive features depend on the crystal field surrounding the active ion. Laser spectroscopy (LS) and channelled Rutherford backscattering (RBS) spectrometry have been applied to investigate trivalent rare earth ions as active ions, whereas Cr³⁺ ions can only be studied by LS. As an example, we shall focus our attention more precisely on Nd³⁺ centres in LiNbO₃:Nd³⁺ and LiNbO₃:MgO:Nd³⁺. The Nd³⁺ ions are in Li⁺ octahedral sites, off-centred by three different amounts along the c-axis, giving place to three non-equivalent optical centres. Co-doping with MgO produces an additional optical centre (Nd-Mg), in which Nd³⁺ ions are not located in octahedral lattice sites. Measurements performed on Cr³⁺-doped LN single 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 in the same way as rare earth dopants: three chromium centres (γ, α, β) are sufficient to explain the main optical features. The main centre γ occupies the Li⁺ regular octahedral site whereas 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. Stoichiometry effects on the centre distribution and possible models to explain the origin of the different centres is discussed. In addition, a nephelauxetic broadening of the ²→⁴A₂ transition in Cr³⁺ caused by disorder which originates from a variation in the values of the electron-electron Racah parameters. This article has been devised as the actual state of the art of the trivalent rare earth ions and the Cr³⁺ transition metal ion used as probe dopants in the LiNbO₃ crystal host.