Two-Photon Fluorescence and Second-Harmonic Generation at the Interface of SrTiO₃ Thin Film and NdGaO₃Substrate

Strontium titanate SrTiO₃ (STO) is a popular ferroelectric material exhibiting excellent chemical and thermal stabilities and promising electrical and physical properties for various technology applications. Studies on STO thin films revealed a dramatic difference in dielectric properties compared to the bulk. In addition, due to the mechanical coupling between STO thin film and its substrate, the cubic symmetry of the thin film breaks. We investigate this structural symmetry breaking of STO thin film by using the rotational anisotropy second harmonic generation (SHG) technique in the reflection geometry. To prepare the sample, we epitaxially grow STO (001) thin film on NdGaO₃ (NGO) (110) substrate by pulsed laser deposition (PLD) technique. In the SHG experiment, we employ a femtosecond pulsed laser as a source of excitation. The polarization of the fundamental and reflected SH radiation are either parallel or perpendicular to the incident plane. The reflected two-fold SHG intensity indicates that the predominant two-fold NGO symmetry at the interface lowers the 4-fold symmetry of the STO thin film. Moreover, we observe very intense two-photon fluorescence (TPF) signals emanating from the interface which exhibits polarization and angular dependency. We determine the TPF signals originate from the interface where the inversion symmetry is broken.