Detection of internal electric fields in BaTiO3 thin films by optical second harmonic generation

We have studied optical second harmonic generation (SHG) in some BaTiO 3 thin films where, in each film, the SHG is unrelated to the crystallographic texturing of the film. We compared the mean value of the SHG coefficient averaged through the depth of the film to the values in ∼ 15 nm thick regions at the film-air and film-substrate interfaces, and we studied the temperature dependence of the SHG. The SHG coefficients were strong functions of depth in the film. We suggest that the observed SHG is due to internal electric fields from asymmetric distributions of oxygen vacancies in the films, such as those observed in numerous recent studies of imprinting in ferroelectric oxide thin film capacitors. As such, SHG provides a new means of studying imprinting. The technique provides depth resolution and does not require electrodes.