Ce3+:Na+ pairs in CaF2 and SrF2: Absorption and laser-excitation spectroscopy, and the observation of hole burning.

Abstract

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-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ 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.