Optical bands of ${\mathrm{C}}^{3+}$ induced by ${\mathrm{Mg}}^{2+}$ ions in ${\mathrm{LiNbO}}_3$:Cr,Mg

In this paper, the role of Mg ions in the optical properties of ${\mathrm{Cr}}^{3+}$ (absorption, luminescence, and life-time) in doubly doped ${\mathrm{LiNbO}}_3$ single crystals is analyzed. Additional R lines (denoted as ${\mathrm{R}}_1^{\mathrm{″}}$ and ${\mathrm{R}}_2^{\mathrm{″}}$), associated with ${\mathrm{Cr}}^{3+}$ ions perturbed by nearby ${\mathrm{Mg}}^{2+}$ ions, were observed. Divalent magnesium ions affect also the ${\mathrm{T}}_1^4$ and ${\mathrm{T}}_2^4$ excited-state levels of ${\mathrm{Cr}}^{3+}$ ions, producing a red shift in the ${\mathrm{A}}_2^4$→${\mathrm{T}}_1^4$ and ${\mathrm{A}}_2^4$→${\mathrm{T}}_2^4$ absorption bands. This red shift explains the change observed in the color of Mg-doped samples. The crystal-field parameters and Racah parameters of this ${\mathrm{Cr}}^{3+}$ defect site are reported and compared with those corresponding to singly doped crystals.