Ultrafast Nonlinear Spectroscopy In (111) Oriented Bismuth Ferrite Oxide

We fabricated a (111) oriented epitaxial thin film of ferroelectric iron oxide, BiFeO3 and investigated the nonlinear optical properties and its femtosecond dynamics. The angle dependence of second harmonic (SH) intensity indicates that the point group of the film fabricated is trigonal 3m and one of the components d(22) in the second order nonlinear susceptibility tensor gives birth to the SH signal. By photoexcitation of CT band in BiFeO3, the value of d(22) shows sudden decrease by approximate to 15% within approximate to 150 fs and recovers with a time constant of about 220 fs. One of the explanations for the ultrafast suppression of d(22) or the polarity of the crystal is due to reduction of the Madelung energy of the crystal by the CT excitation and resultant temporal change to a symmetric perovskite as a photoexcited state.