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Physical review. B, Condensed matter

Trapping of optical excitation in Nd-doped Pr F3

John Hegarty, David L. Huber, William M. Yen

Published Jun 15, 1981• 52 citations
Publisher

Abstract

The dynamics of trapping of optical excitation in a disordered system of ions in a solid is studied using time-resolved fluorescence measurements. In the system Pr${\mathrm{F}}_{3}$ doped with Nd, the randomly distributed Nd ions act as sinks for the Pr excitation. The transfer to the Nd traps can occur in conjunction with transfer among the Pr ions. In this system the Pr-Pr transfer is strongly temperature dependent while the Pr-Nd transfer is temperature independent at low temperatures. By changing the temperature we can span the range from no Pr-Pr transfer to very (infinitely) fast Pr-Pr transfer in one single system. We have identified these limits. In addition we have placed boundaries on the applicability of the diffusion and hopping models in describing trapping in the intermediate temperature regime where the Pr-Pr and Pr-Nd transfer rates are comparable.

DOI
10.1103/physrevb.23.6271

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Physical Review Letters•Jul 14, 1975•2 citations
The dynamics of trapping of optical excitation in a disordered system of ions in a solid is studied using time-resolved fluorescence measurements. In the system PrF3{\mathrm{F}}_{3}F3​ doped with Nd, the randomly distributed Nd ions act as sinks for the Pr excitation. The transfer to the Nd traps can occur in conjunction with transfer among the Pr ions. In this system the Pr-Pr transfer is strongly temperature dependent while the Pr-Nd transfer is temperature independent at low temperatures. By changing the temperature we can span the range from no Pr-Pr transfer to very (infinitely) fast Pr-Pr transfer in one single system. We have identified these limits. In addition we have placed boundaries on the applicability of the diffusion and hopping models in describing trapping in the intermediate temperature regime where the Pr-Pr and Pr-Nd transfer rates are comparable.
We have investigated the process of quenching of fluorescence from the 3P0^{3}P_{0}3P0​ state of Pr3+{\mathrm{Pr}}^{3+}Pr3+ in La0.72{\mathrm{La}}_{0.72}La0.72​Pr0.25{\mathrm{Pr}}_{0.25}Pr0.25​Nd0.03{\mathrm{Nd}}_{0.03}Nd0.03​F3{\mathrm{F}}_{3}F3​ due to cross relaxation to Pr and Nd ions. The fluorescence decay of the ensemble of Pr ions after pulsed excitation is measured as a function of temperature in the (2-70)-K region. Cross relaxation occurs either directly or through a donor-donor transfer within the inhom-geneous line. Knowledge of the dynamics occurring within the inhomogeneous line derived from previous fluorescence line-narrowing experiments is used to analyze the cross-relaxation process from a microscopic viewpoint according to models for fluorescence decay in the presence of traps. In general the decay is consistent with an electric dipole-dipole transfer mechanism. At 2 K the decay can be well described by a direct transfer to Pr and Nd traps without donor-donor transfer. The temperature dependence of the decay at long times can be described for 35<T<7035<T<7035<T<70 K by the hopping model. The agreement with theory is much better if the cross relaxation to Nd ions is assumed to occur mainly via the lowest Stark level of the 4I92^{4}I_{\frac{9}{2}}4I29​​ manifold.
We have investigated the process of quenching of fluorescence from the 3P0^{3}P_{0}3P0​ excited state of Pr3+{\mathrm{Pr}}^{3+}Pr3+ in LaF3{\mathrm{F}}_{3}F3​ due to cross relaxation among the Pr ions. The fluorescence decay of the ensemble of Pr ions after pulsed excitation is measured as a function of temperature for Pr concentration of 20 mol%. About 12% of the ions are in nonquenching sites and contribute an exponential component to the decay. The remainder of the decay originates from the bulk of the ions in quenching sites. Cross relaxation occurs directly in steps after transfer within the inhomogeneous line. Knowledge of the dynamics occurring within the inhomogeneous line derived from fluorescence line-narrowing experiments is used to analyze the cross-relaxation process from a microscopic viewpoint based on a recent model for fluorescence in the presence of traps. At 2 K the decay is consistent with an electric dipole-dipole transfer mechanism with a nearest-neighbor quenching rate of 8.9 \ifmmode\times\else\texttimes\fi{} 104{10}^{4}104 ${\mathrm{s}}^{\ensuremath{-}1}$ which is independent of temperature for T<32T<32T<32 K and which successfully predicts the observed decay rate in PrF3{\mathrm{F}}_{3}F3​. The temperature dependence of the decay for $8\ensuremath{\le}T\ensuremath{\le}32$ K is well described by a hopping model for excitation trapping. For T>35T>35T>35 K an activation process becomes dominant consistent with the onset of cross relaxation involving the 1D2(5)^{1}D_{2}(5)1D2​(5) and 3H6(1)^{3}H_{6}(1)3H6​(1) states in which a 190-${\mathrm{cm}}^{\ensuremath{-}1}$ phonon is created.
Time-resolved fluorescence line narrowing is used to study spectral energy transfer in PrF3{\mathrm{F}}_{3}F3​ and PrCl3{\mathrm{Cl}}_{3}Cl3​. The excitation spectrum of PrF3{\mathrm{F}}_{3}F3​ in the region of the ${(^{3}H_{4})}_{1}\ensuremath{\rightarrow}^{3}P_{0}$ absorption indicates that several strong satellite lines accompany the main transition, their origin being ascribed to different Pr3+{\mathrm{Pr}}^{3+}Pr3+ ions lying in strongly perturbed lattice sites. The spectral transfer within the inhomogeneously broadened main transiton is measured, where the energy mismatch is about 2 ${\mathrm{cm}}^{\ensuremath{-}1}$. This process is characterized by a strong ($\ensuremath{\sim}{T}^{4.3}$) dependence of the transfer probability on sample temperature. The growth of the full inhomogeneous emission profile accompanies a uniform decay of the donor line shape. A transfer process between the lowest-energy satellite ions is also measured. Here the energy mismatch is 32 ${\mathrm{cm}}^{\ensuremath{-}1}$ resulting in a weak temperature dependence for the transfer rate. Both the intraline and intersatellite transfer are characterized by simple exponential rate equations. Recent calculations of phonon-assisted transfer probabilities are compared to our measurements, with the intersatellite transfer ascribed to a "one-phonon direct" process whereas the transfer within the main transition occurs by a "one-phonon second-order" mechanism. In PrCl3{\mathrm{Cl}}_{3}Cl3​ the transfer times are faster than the 15-nsec response time of our detection system indicating a much stronger ion-ion coupling.
We present optical linewidth and line-shape studies of transitions of 4f4f4f 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 Pr3+{\mathrm{Pr}}^{3+}Pr3+ in doubly doped LaF3{\mathrm{F}}_{3}F3​: Pr3+{\mathrm{Pr}}^{3+}Pr3+, Nd3+{\mathrm{Nd}}^{3+}Nd3+ as a function of Nd3+{\mathrm{Nd}}^{3+}Nd3+ concentration. The 5985-\AA{} Pr3+{\mathrm{Pr}}^{3+}Pr3+ transition at 4.2\ifmmode^\circ\else\textdegree\fi{}K involves two long-lived electronic states of the Pr3+{\mathrm{Pr}}^{3+}Pr3+ ion; it follows that in the singly doped LaF3{\mathrm{F}}_{3}F3​: Pr3+{\mathrm{Pr}}^{3+}Pr3+ the transition shows a Gaussian shape which is characteristic of inhomogeneous strain and imperfection broadening. As Nd3+{\mathrm{Nd}}^{3+}Nd3+ is added to the system, a broader Lorentzian component begins to appear in the 5985-\AA{} Pr3+{\mathrm{Pr}}^{3+}Pr3+ transition and eventually (at Nd3+{\mathrm{Nd}}^{3+}Nd3+ 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 Nd3+{\mathrm{Nd}}^{3+}Nd3+ concentration. The width is found to increase linearly with increasing Nd3+{\mathrm{Nd}}^{3+}Nd3+ 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 Pr3+{\mathrm{Pr}}^{3+}Pr3+ and Nd3+{\mathrm{Nd}}^{3+}Nd3+ systems at the terminating (3H6)1{(^{3}H_{6})}_{1}(3H6​)1​ level of the 5985-\AA{} Pr3+{\mathrm{Pr}}^{3+}Pr3+ 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 LaF3{\mathrm{F}}_{3}F3​: Pr3+{\mathrm{Pr}}^{3+}Pr3+, Nd3+{\mathrm{Nd}}^{3+}Nd3+ system are also presented.
Data are reported which show that the temperature dependence of fluorescence linewidths in PrF3{\mathrm{F}}_{3}F3​ are qualitatively like those in LaF3{\mathrm{F}}_{3}F3​: Pr3+{\mathrm{Pr}}^{3+}Pr3+. This result is interpreted as supporting the explanation of German and Kiel for the anomalous temperature dependence of fluorescence linewidths in PrCl3{\mathrm{Cl}}_{3}Cl3​.