Ultrafast Polaron Dynamics in Multiferroic LuFe$_{2}$O$_{4}$*

The multiferroic material LuFe$_{2}$O$_{4}$ has attracted much recent attention due to its strong magnetoelectric coupling. We used ultrafast optical spectroscopy to examine LuFe$_{2}$O$_{4}$ by employing a 1.55 eV probe pulse to examine the Fe$^{2+}->$Fe$^{3+}$ polaronic excitation as a function of temperature and pump photon energy. After 1.55 eV excitation, the photoinduced reflectivity change $\Delta $R/R decreases within $\sim $1 picosecond (ps), after which a $\sim $30 ps acoustic phonon oscillation is observed. The initial fast drop in $\Delta $R/R can be explained by Fe$^{2+}->$Fe$^{3+}$ polaron hopping, and the subsequent rapid recovery is due to polaron redressing; this is observed at all temperatures. Pumping the Fe$^{2+}$ on-site excitation at 3.1 eV revealed different dynamics. Notably, coupling between the on-site and charge transfer excitations was strongly suppressed above the antiferromagnetic ordering temperature, demonstrating the strong influence of charge and spin order on polaron dynamics.