SHG properties of pure and doped incipient ferroelectrics KTaO3 and SrTiO3 under applied electric fields

Abstract

The investigation of the incipient ferroelectric systems KTO and STO at low temperature with optical second harmonic generation (SHG) using a Nd:YAG-laser shows a similar behaviour for both crystal systems. The SHG intensity S increases drastically upon reduction and conversely decreases upon oxidation. Oxygen defect related inversion-symmetrybreaking-centers interacting with dipolar centers introduced by doping (f.i. in Li+ doped KTO) result in strong changes of the dipolar properties. The static and dynamic behaviour of the SHG intensity under applied electric field confirms the dipolar nature of the centers and allows us to estimate the activation energy for reorientation. The SHG signal S of nominally pure, but containing oxygen defect-related-dipole-centers, in KTO first is approximately proportional to E. For Li-doped KTO the proportionality-coefficient b increases strongly with increasing Lidoping. At moderately higher E-fields the SHG intensity S appears to be saturated in agreement with recent theoretical model calculations.