Domain rearrangement in ferroelectric $Bi4Ti3O12$ thin films studied by in situ optical second harmonic generation

Electric-field induced rearrangement of domain microstructure in an epitaxial thin film of $Bi4Ti3O12$ on a $SrTiO3$ (001) substrate is studied by optical second harmonic generation measurements. The input polarization dependence of the second harmonic signal exhibits spatial symmetries that reflect the presence of eight different domain variants present in the film. Changes in these symmetries with the application of electric field are experimentally studied at $23° C$ and $60 °C,$ and theoretically modeled to extract the hysteresis loops that reflect quantitative changes in the fraction of domain variants. A strong correlation is observed between the dc electrical conductance (as distinct from transient currents) and the ferroelectric domain state of the film, which is proposed to arise from the creation and destruction of charged domain walls within the film with applied field.