LUMINICENCE AND OPTICAL 2ND-HARMONIC GENERATION BY DIPOLAR MICROREGIONS IN KTAO3

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