Optical detection of ${\mathrm{Eu}}^{3+}$ sites in ${\mathrm{LiNbO}}_3$:${\mathrm{Eu}}^{3+}$ and ${\mathrm{LiNbO}}_3$:MgO:${\mathrm{Eu}}^{3+}$

The fluorescence of ${\mathrm{Eu}}^{3+}$ in ${\mathrm{LiNbO}}_3$ has been investigated using laser-excited site-selection spectroscopy. Four different crystal-field sites, denoted as Eu(1), Eu(2), Eu(3), and Eu(4) have been detected and optically characterized through their corresponding ${}_{}{}^5{}_{}{}^{}$ ${\mathit{D}}_0$ ${\to }^7$ ${\mathit{F}}_1$ magnetic-dipole emission transitions. The local symmetry of the ${\mathrm{Eu}}^{3+}$ ions in these sites deviates slightly from the ${\mathit{C}}_3$ main symmetry as a consequence of distortions perpendicular to the c ferroelectric axis. The relative concentrations of these sites have been studied as a function of the [Li]/[Nb] concentration ratio. It is suggested that the formation of ${\mathrm{Eu}}^{3+}$(${\mathrm{Nb}}^{5+}$)-${\mathrm{Eu}}^{3+}$(${\mathrm{Li}}^{+}$) pairs is responsible for the major Eu(3) and Eu(4) sites, while the Eu(1) and Eu(2) sites correspond to ${\mathrm{Eu}}^{3+}$ ions in ${\mathrm{Nb}}^{5+}$ lattice positions but perturbed by different charge-compensating defects. The effect of codoping with ${\mathrm{Mg}}^{2+}$ ions has also been investigated; the same ${\mathrm{Eu}}^{3+}$ sites as in the singly doped system are present, although the Eu(1) site is strongly enhanced in relation to the others.