Sharp R-lines in absorption and emission of Cr3+ in stoichiometric (VTE) LiNbO3

Abstract

R-line absorption and luminescence spectra of Cr3+ doped LiNbO3 show a distinct dependence on crystal stoichiometry. In the Cr3+ R-line region around 730 nm, up to five transitions can be observed in absorption and emission. In stoichiometric LiNbO3:Cr crystals the line width (FWHM) in both absorption and emission decreases by about a factor of 2.5 in comparison with the same transitions in congruent samples. The emission line at lowest energy in the R-line spectrum (lambda = 734.5 nm) disappears completely in stoichiometric LiNbO3:Cr. This results, discussed together with models dealing with charge compensated Cr3+ pairs on Li and Nb sites and unpaired Cr3+ on Li site, is indicating a vanishing of the unpaired Cr-Li(3+) with increasing [Li]/[Nb] ratio. Luminescence lifetime measurements in stoichiometric LiNbO3:Cr reveal values similar to results for congruent material of tau = 295 mu s for the 731 nm line and tau = 268 mu s for the 735 nm line, underlining the R-line character of these transitions.