Title

Energy transfer from Gd$\sp{3+}$ to Cr$\sp{3+}$ in Cr-doped Gd$\sb3$Sc$\sb2$Ga$\sb3$O$\sb{12}$

Date of Completion

January 1996

Keywords

Physics, Condensed Matter|Engineering, Materials Science

Degree

Ph.D.

Abstract

A preliminary excitation experiment was performed in the near ultraviolet on Gd$\rm\sb3Sc\sb2Ga\sb3O\sb{12}{:}Cr\sp{3+}$. The resulting excitation spectra, obtained by monitoring the Cr$\sp{3+}$ fluorescence while exciting the Gd$\sp{3+}$ ions, revealed efficient gadolinium to chromium energy transfer. The intent of this thesis investigation was to elucidate the nature of this energy transfer process. In the results presented here, we have measured the dynamics of this energy transfer process by pumping the $\sp6$P$\sb{7/2}$ and $\sp6$P$\sb{5/2}$ levels of the Gd$\sp{3+}$ with a frequency-doubled pulsed dye laser at 312 nm and 307 nm respectively. The resulting Cr$\sp{3+}$ fluorescence was detected using photon counting instrumentation, which yields time-dependent measurements with three orders of magnitude dynamic range. The rise time and decay time of the Cr$\sp{3+}$ fluorescence is exponential in character and the data was fit to$$\rm N(t)=N\sb\max\ e\sp{-t/T\sb{d}}(1-e\sp{-t/T\sb{r}})$$where T$\sb{\rm d}$ is the Cr$\sp{3+}$ decay time and T$\sb{\rm r}$ is the rise time. The model is consistent with one where the Gd$\sp{3+}$ to Cr$\sp{3+}$ transfer is described by a single transfer rate indicating a single step transfer over a single fixed distance. The temperature dependence of the Gd$\sp{3+}$ to Cr$\sp{3+}$ transfer rate from 10 K to 300 K shows two distinct regions. At temperatures above 100 K the transfer rate increases linearly with temperature which is highly indicative of a one phonon-assisted transfer process. In the region below 100 K a much stronger temperature dependence is observed. The total energy transfer process is a rapid migration among the gadolinium ions followed by an energy transfer step to the chromium ions. The energy migration and transfer are both mediated by exchange interaction. In the temperature region above 100 K the Gd$\sp{3+}$ to Cr$\sp{3+}$ rate dominates the temperature dependence and in the region below 100 K the Gd$\sp{3+}$ to Gd$\sp{3+}$ transfer rate dominates.^ The temperature dependence of the Gd$\sp{3+}$ to Cr$\sp{3+}$ energy transfer following excitation of the higher lying gadolinium $\sp6$I manifolds was also measured. The transfer rate was temperature independent and significantly faster than the rates observed following excitation of the $\sp6$P manifolds. ^