Optical Linewidth and Line-Shape Studies of Energy Transfer Mechanisms between Rare-Earth Impurity Ions

Abstract

We present optical linewidth and line-shape studies of transitions of $4f$ impurity ions in doubly doped systems and show that interactions which involve excitation transfer between the impurity systems produce homogeneous broadening and line-shape changes in the optical transitions. In particular, we have studied the behavior of the 5985-\AA{} line of ${\mathrm{Pr}}^{3+}$ in doubly doped La${\mathrm{F}}_{3}$: ${\mathrm{Pr}}^{3+}$, ${\mathrm{Nd}}^{3+}$ as a function of ${\mathrm{Nd}}^{3+}$ concentration. The 5985-\AA{} ${\mathrm{Pr}}^{3+}$ transition at 4.2\ifmmode^\circ\else\textdegree\fi{}K involves two long-lived electronic states of the ${\mathrm{Pr}}^{3+}$ ion; it follows that in the singly doped La${\mathrm{F}}_{3}$: ${\mathrm{Pr}}^{3+}$ the transition shows a Gaussian shape which is characteristic of inhomogeneous strain and imperfection broadening. As ${\mathrm{Nd}}^{3+}$ is added to the system, a broader Lorentzian component begins to appear in the 5985-\AA{} ${\mathrm{Pr}}^{3+}$ transition and eventually (at ${\mathrm{Nd}}^{3+}$ concentrations of \ensuremath{\sim}1% or above) completely dominates the width and shape of the transition. The Lorentzian component of the transition line shape was successfully separated using a Fourier-transform technique and the width of this component was studied as a function of the ${\mathrm{Nd}}^{3+}$ concentration. The width is found to increase linearly with increasing ${\mathrm{Nd}}^{3+}$ concentration up to concentrations of 2%; at higher concentrations, the linewidth saturates and approaches an asymptotic value of \ensuremath{\sim}2 ${\mathrm{cm}}^{\ensuremath{-}1}$. We construe this homogeneous component to be evidence of a rapid short-range energy transfer mechanism between the ${\mathrm{Pr}}^{3+}$ and ${\mathrm{Nd}}^{3+}$ systems at the terminating ${(^{3}H_{6})}_{1}$ level of the 5985-\AA{} ${\mathrm{Pr}}^{3+}$ transition. Various mechanisms are investigated as causes of the energy transfer process; it is concluded that the spin-exchange interaction is the most probable cause for the transfer. Results of other spectroscopic measurements on the doubly doped La${\mathrm{F}}_{3}$: ${\mathrm{Pr}}^{3+}$, ${\mathrm{Nd}}^{3+}$ system are also presented.