Variations of the second harmonic intensity at the low temperature phase transition IN K_1-xLi_xTaO₃ under electric field

Abstract

Optical second harmonic generation (SHG) of Nd:YAG laser light in K_1-xLi_xTaO₃ with x = 0.016 and x = 0.022 has been studied under zero-field cooling (ZFC) and by cooling under electric field (FC). A variation of the phase transition temperature and of the width of the thermal hysteresis with field is observed. The strong increase of the SHG-intensity under field cooling and the accompanying small changes of birefringence¹ can be interpreted as an increase of the mean size of ordered regions from antiparallel to parallel polarization. A comparison with the behavior of crystals with Li-concentration above the critical limit x_c = 0.022 for two samples with x = 0.036 and x = 0.063 is presented. In addition SHG-data of nominally pure KTaO₃ under the same electric field and cooling procedures indicate the existence of microregions and the influence of a lattice polarisation under field. The components of the nonlinear susceptibility tensor reveal a polar tetragonal low temperature symmetry in the Li containing crystals. The value of the SHG-intensity ratio β = d ₃₃/d₃₁ ² = 6.25 ± 0.6 turns out to be independent of the Li-concentration. An activation energy of E_a = 78 meV ≙ 905 K is determined from the SHG dynamics of the alignment of polar regions.