${\mathrm{Ce}}^{3+}$:${\mathrm{Na}}^{+}$ pairs in ${\mathrm{CaF}}_2$ and ${\mathrm{SrF}}_2$: Absorption and laser-excitation spectroscopy, and the observation of hole burning

Optical evidence of multiple ${\mathrm{Ce}}^{3+}$:${\mathrm{Na}}^{+}$ charge-compensation sites in codoped ${\mathrm{Ce}}^{3+}$:${\mathrm{Na}}^{+}$:${\mathrm{CaF}}_2$ and ${\mathrm{Ce}}^{3+}$:${\mathrm{Na}}^{+}$:${\mathrm{SrF}}_2$ crystals is reported. High-resolution absorption and laser-excitation spectroscopy reveal the presence of numerous lines in the 32 200–32 400-${\mathrm{cm}}^{\mathrm{-}1}$ region indicating a quasirandom distribution of pair sites. The absorption spectra of annealed ${\mathrm{Ce}}^{3+}$:${\mathrm{CaF}}_2$ and ${\mathrm{Ce}}^{3+}$:${\mathrm{Na}}^{+}$:${\mathrm{CaF}}_2$ crystals reveal isolated $O_h$ lines. This fact has aided the interpretation of the ${\mathrm{Ce}}^{3+}$:${\mathrm{Na}}^{+}$ pair spectra. Evidence for photoionization of charge-compensated ${\mathrm{Ce}}^{3+}$ sites followed by trapping of the electron at remotely compensated ($O_h$) ${\mathrm{Ce}}^{3+}$ sites is presented. Acceptor-type hole burning, the disappearance of the cubic ${\mathrm{Ce}}^{3+}$ site’s absorption line while exciting the ${\mathrm{Ce}}^{3+}$:${\mathrm{Na}}^{+}$ pair centers, is observed in both ${\mathrm{CaF}}_2$ and ${\mathrm{SrF}}_2$ hosts. The holes persist for weeks after their creation and survive thermal cycling to room temperature.