Femtosecond spectroscopy of multiferroic BiFeO$_{3}$ thin films*

BiFeO$_{3}$ is a multiferroic material characterized by a room temperature antiferromagnetic and ferroelectric phase and a 2.6eV bandgap. Terahertz emission from BiFeO$_{3}$ thin films following excitation above bandgap by UV femtosecond pulses has been observed [1]. The THz emission is related to ultrafast depolarization of the ferroelectric order in the film as observed from the hysteresis of the amplitude of the emitted THz signal as a function of a biasing electric field. Experiments probing the electronic and lattice dynamics with sub-picosecond resolution are an essential step towards identifying the intrinsic mechanism responsible for depolarization and THz emission in BiFeO$_{3}$. In this study we investigate the carrier dynamics of epitaxial BiFeO$_{3}$ thin films using degenerate pump-probe spectroscopy at 400 nm with sub-50 fs pulses. We have observed a 5 THz coherent optical phonon oscillation consistent with the A$_{1}$ phonon mode as observed in Raman spectroscopy [2]. In this talk, the relation of the observed dynamics to the THz emission will be discussed. [1] D.S. Rana, et al., Advanced Materials 21, 2881 (2009). [2] S. Kamba, et al, Phys. Rev. B 75, 024403 (2007).