Optical and Photoluminescence Properties of Erbium-Doped Chalcogenide Glasses (GeGaS:Er)

We have examined the optical and photoluminescence (PL) properties of $\hbox{Er}^{3+}\!$-doped GeGaS glasses of near-stoichiometric composition $\hbox{Ge}_{28}\hbox{Ga}_{6.2}\hbox{S}_{65.3}{:}\hbox{Er}_{0.5}$. We have also used powdered samples of various mean sizes $\langle L\rangle$ to examine the dependence of the 1.54-$\mu$m PL emission spectrum and the PL decay time on the average sample size. Optical absorption spectra of $\hbox{Er}^{3+}$ ions arising from transitions between different energy manifolds, such as ${}^{4}\hbox{I}_{15/2}\hbox{--}{}^{4}\hbox{I}_{13/2}$, $^{4}\hbox{I}_{15/2}\hbox{--}{}^{4}\hbox{I}_{11/2}$, etc., have been used to extract $\Omega_{2}, \Omega _{4},\, \hbox{and}\ \Omega _{6}$ values using the Judd–Ofelt analysis and a Judd–Ofelt radiative lifetime $\tau _{{\rm JO}} = \hbox{2.6}$ ms for the ${}^{4}\hbox{I}_{13/2}\hbox{--}{}^{4}\hbox{I}_{15/2}$ transition. The PL emission spectra and the decay time have been found to depend on the mean sample size. The spectra are broader and the decay times are longer for larger sample sizes, due to photon trapping occurring in the sample. The extrapolated decay time to zero particle size yields a decay time that matches the Judd–Ofelt radiative lifetime almost perfectly, and confirms the argument that the true PL lifetime needs to be measured in fine powders to avoid reabsorption effects. We have estimated the maximum emission cross section as $\hbox{15.5} \times \hbox{10}^{-21}\ \hbox{cm}^{2}$.