Luminicence and optical second harmonic generation by dipolar microregions in KTaO3

The incipient ferroelectric KTaO3 being cubic down to lowest temperatures, is not expected to yield second harmonic intensity (SHG) due to the inversion symmetry of the lattice. Nevertheless a weak SHG intensity of Nd:YAG laser light is observed, which increases with decreasing temperature. This low temperature SHG is drastically enhanced in KTaO3 samples which have been reduced at 1000 degrees C in H-2-atmosphere. Simultaneously the luminescence intensity observed at low temperature (T = 80 K) with an emission at lambda(max) = 520 nm is increasing strongly upon reduction as well. This increase is not only due to a thin reduced surface layer, but rather uniform throughout the volume of the sample crystal. Oxidation at 1000 degrees C in pure O-2-atmosphere diminishes both, the low temperature SHG- and the luminescence intensity, far below the values of the pure, 'as-grown' crystal. Sharp luminescence features reported to be at lambda = 687 nm in the literature and assigned to Ta3+ near oxygen vacancies, have not been observed in our crystals, neither in the as-grown, nor in the reduced or oxidized samples. The interpretation is based upon dipolar microregion around a defect core due to oxygen vacancy complexes, locally breaking the inversion symmetry.