Optical second-harmonic generation at interfaces of ferroelectric nanoregions in SrSiO3:Ca.

Abstract

Optical second-harmonic generation (SHG) is observed in ${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{TiO}}_{3}$ with x=0.007 within the ferroelectric domain state below ${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}18 K. Surprisingly, its dependence on the polarization angle is incompatible with global ${\mathit{C}}_{2\mathit{V}}$ symmetry. The appearance of maximum intensity for ${\mathit{E}}_{\mathrm{\ensuremath{\omega}}}$\ensuremath{\parallel}${\mathit{E}}_{2\mathrm{\ensuremath{\omega}}}$ parallel to the nonpolar c axis suggests that SHG is primarily due to symmetry breaking at the interfaces between ferroelectric nanoregions and the paraelectric host material ${\mathrm{SrTiO}}_{3}$. The observed angular dependences are satisfactorily modeled by assuming the dominant component of the nonlinear surface polarizability to lie parallel to the surface normal of ellipsoidally shaped nanoregions. The unusual electric-field dependence of the SHG intensity recorded at various temperatures and its temporal relaxation after steplike changing the field are described in terms of nanoregion nucleation and condensation in the presence of dipolar random fields. \textcopyright{} 1996 The American Physical Society.