Photoinduced refractive-index changes in several ${\mathrm{Eu}}^{3+}$-, ${\mathrm{Pr}}^{3+}$-, and ${\mathrm{Er}}^{3+}$-doped oxide glasses

We have observed permanent and transient refractive-index grating formation following excitation of the ${}_{}{}^5{}_{}{}^{}$ ${\mathit{D}}_2$ state of ${\mathrm{Eu}}^{3+}$ in a silicate, a metaphosphate and a pentaphosphate glass at 300 K, using four-wave mixing (FWM). These refractive-index changes recently have been ascribed to a high-energy phonon-mediated structure rearrangement involving two-level systems. In order to test this model, these measurements were extended to several ${\mathrm{Er}}^{3+}$- and ${\mathrm{Pr}}^{3+}$-doped glasses, which both have transitions that create similar energy phonons. The ${}_{}{}^4{}_{}{}^{}$ ${\mathit{I}}_{11/2}$ state of an ${\mathrm{Er}}^{3+}$-doped silicate and a metaphosphate glass was resonantly excited at 980 nm. No permanent gratings were observed in these ${\mathrm{Er}}^{3+}$ glasses. A single, resonant-pump-beam experiment showed only transient thermal lensing in the ${\mathrm{Er}}^{3+}$-silicate glass. This is consistent with the FWM results of this glass. Similarly no permanent refractive-index gratings were observed following excitation of the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_0$ state in ${\mathrm{Pr}}^{3+}$-doped sodium silicate, metaphosphate, and lanthanum and aluminum borate glasses. The ${\mathrm{Er}}^{3+}$ and ${\mathrm{Pr}}^{3+}$ results are inconsistent with the two-level-system model and its analysis of the difference in polarizability between the two potential wells as applied to the ${\mathrm{Eu}}^{3+}$ glasses.