The dielectric response of single-crystal UO$_{2}$ probed by terahertz time-domain spectroscopy

We measured the complex dielectric function, \textit{$\varepsilon =\varepsilon $}$_{r}$\textit{+i$\varepsilon $}$_{i}$, of single-crystal UO$_{2}$ in the temperature (T) range of 5-500 K by terahertz (THz) time-domain spectroscopy to study its dielectric response. A critical temperature point of 60 K is found for the measured temperature dependence of \textit{$\varepsilon $}$_{r}(T)$. The dispersion of \textit{$\varepsilon (\omega )$} in the THz range follows the trend of the damped resonant mode of the transverse optical phonon at \textit{$\omega $}$_{TO}$=8.4 THz. Examining the Lyddane-Sachs-Teller relationship reveals mode-softening of \textit{$\omega $}$_{TO}$ in the temperature range of 60-120 K. We then performed optical-pump THz-probe measurements to study the dynamic dielectric response of UO$_{2}$ following photoexcitation. We observe a small pump-induced change in \textit{$\varepsilon $} that lasts for more than one millisecond, indicating heavy electron behavior of photoexcited 5f electrons resulting from strong electron-lattice interaction. The slow relaxation is attributed to the diffusion of optical phonons generated by photoexcited 5f electrons in the UO$_{2}$ crystal.