A spectroscopic investigation of an extensive series of ${\mathrm{Er}}^{3+}$-doped and ${\mathrm{Er}}^{3+}{,\mathrm{Y}\mathrm{b}}^{3+}$-codoped soda-lime-silicate (SL) and aluminosilicate (AS) glasses is presented. Compared to SL glasses, $4f$ transitions in AS glasses show higher oscillator strengths, larger inhomogeneous broadening, and smaller crystal-field splittings of the respective excited-state multiplets. The ${\mathrm{Er}}^{3+}$ excited-state relaxation dynamics is adequately described by a combination of the Judd-Ofelt model and the energy-gap law. With the exception of ${}^{4}{I}_{13/2},$ multiphonon relaxation is dominant for all excited states, making it possible to efficiently pump the 1.55 \ensuremath{\mu}m ${}^{4}{I}_{13/2}{\ensuremath{\rightarrow}}^{4}{I}_{15/2}$ emission by excitation of ${}^{4}{I}_{11/2}$ at around 980 nm. The absolute ${}^{4}{I}_{13/2}$ luminescence quantum yield, for low 980-nm excitation density $(\ensuremath{\sim}5{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}),$ \ensuremath{\eta}, is $\ensuremath{\sim}0.9$ at 0.4 mol % ${\mathrm{Er}}_{2}{\mathrm{O}}_{3}$ and drops to about 0.65 upon increasing ${\mathrm{Er}}_{2}{\mathrm{O}}_{3}$ to 1.2 mol %, indicating the onset of energy-transfer processes. Samples with high ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ impurity concentration suffer from significantly higher quenching of ${}^{4}{I}_{13/2}$ luminescence at higher ${\mathrm{Er}}^{3+}$ concentrations. Energy migration to the minority of ${\mathrm{Er}}^{3+}$ ions coordinated to ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$, followed by efficient multiphonon relaxation accounts for this effect. At low excitation densities, the strong near-infrared absorption of ${\mathrm{Yb}}^{3+}$ in combination with efficient $\mathrm{Yb}\ensuremath{\rightarrow}\mathrm{Er}$ energy transfer increases the ${}^{4}{I}_{13/2}$ population density in ${\mathrm{Yb}}^{3+},{\mathrm{Er}}^{3+}$-codoped samples by up to 2 orders of magnitude compared to equivalent samples without ${\mathrm{Yb}}^{3+}$. The dependence of \ensuremath{\eta} on ${\mathrm{Yb}}^{3+}$ codotation of 0.4 mol % ${\mathrm{Er}}_{2}{\mathrm{O}}_{3}$-doped samples predicts that a minimum of $\ensuremath{\sim}0.8\mathrm{mol}%$ ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}$ is required to achieve efficient sensitization of ${\mathrm{Er}}^{3+}$ by ${\mathrm{Yb}}^{3+}$. The relative intensities of upconversion luminescence from ${}^{4}{S}_{3/2}$ and ${}^{2}{H}_{11/2}$ are used to analyze internal sample heating in detail. Due to the high absorption cross section of ${\mathrm{Yb}}^{3+}$, increasing the ${\mathrm{Yb}}^{3+}$ concentration in ${\mathrm{Yb}}^{3+},{\mathrm{Er}}^{3+}$-codoped samples of given length increases the absorbed power and subsequently the total density of multiphonon emission, leading to internal temperatures of up to 572 K in 0.4 mol % ${\mathrm{Er}}_{2}{\mathrm{O}}_{3}$ samples codoped with 4 mol % ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}$ and excited with $51{\mathrm{k}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$. Multiphonon relaxation from ${}^{4}{I}_{13/2}$ is shown to be inefficient even at these high internal sample temperatures. From upconversion luminescence spectra of a series of glasses, the thermal conductivity is estimated to be between $3.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ and $7.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}{\mathrm{W}\mathrm{}\mathrm{m}}^{\mathrm{\ensuremath{-}}1}{\mathrm{}\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$, in good agreement with the known value of $4.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}{\mathrm{W}\mathrm{}\mathrm{m}}^{\mathrm{\ensuremath{-}}1}{\mathrm{}\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ for soda-lime-silicate glass.
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Mar 12, 1999
Jan Ern + 6
Open Access