Persistent and transient spectral hole burning inPr3+- andEu3+-doped silicate glasses

Persistent and time-resolved transient holes were burnt into the ${}_{}{}^1{}_{}{}^{}\mathrm{D}_2^{}{}_{}{}^{}$ ${\leftarrow }^3$ ${\mathrm{H}}_4$ transition of ${\mathrm{Pr}}^{3+}$ and the ${}_{}{}^5{}_{}{}^{}\mathrm{D}_0^{}{}_{}{}^{}$ ${\leftarrow }^7$ ${\mathrm{F}}_0$ transition of ${\mathrm{Eu}}^{3+}$ in silicate glasses. The holewidths were studied as a function of burning-fluence density, wavelength, and temperature between 0.4 and 12 K. Transient holes arising from optical pumping of ground-state nuclear hyperfine levels were found for both ions at T<3 K. Their lifetime decreases with increasing temperature. They are accompanied by symmetrically placed antiholes. For ${\mathrm{Eu}}^{3+}$, two pairs of antiholes were found which are attributed to the two europium isotopes, ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ and ${}_{}{}^{153}{}_{}{}^{}\mathrm{Eu}$. For ${\mathrm{Pr}}^{3+}$, an additional mechanism leading to persistent hole burning dominates above 3 K. The holewidth follows a ${\mathit{T}}^{(1.3\mathrm{}\pm 0.1)}$ dependence for ${\mathrm{Pr}}^{3+}$ and a ${\mathit{T}}^{(1.0\mathrm{}\pm 0.1)}$ dependence for ${\mathrm{Eu}}^{3+}$. The temperature dependence is the same as that of the contribution of the two-level systems to the specific heat measured on the same samples and in the same temperature range. The quadrupole splittings of the ${}_{}{}^7{}_{}{}^{}\mathrm{F}_0^{}{}_{}{}^{}$ ground state and the crystal-field splittings of the ${}_{}{}^7{}_{}{}^{}\mathrm{F}_1^{}{}_{}{}^{}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{F}_2^{}{}_{}{}^{}$ levels of ${\mathrm{Eu}}^{3+}$ as a function of excitation wavelength indicate that the strength of the noncubic component of the crystal field varies monotonically for excitation across the ${}_{}{}^5{}_{}{}^{}\mathrm{D}_0^{}{}_{}{}^{}$