Observation of strongly damped GHz phonon-polariton oscillations in ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$

We report on the first observation of strongly damped phonon-polariton beats and picosecond carrier relaxation dynamics of ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$ near the metal-insulator phase transition. The observed 60 GHz coherent oscillations show clear softening as the temperature approaches $T_C$ and persist well into the ferromagnetic-metallic state. The characteristic energy and the fast decay component of the transient reflectivity signal near $T_C$ is directly related to the Jahn-Teller distortions.