Study of antiparallel pyroelectric twins in KLiSO4 by second-harmonic scattering

Two-dimensional CCD images of far-field second-harmonic scattering (SHS) fr om the hexagonal phase of as-grown lithium potassium sulfate (KLiSO4) cryst als are reported. Quasi-phase matching is shown to be produced by a modulat ion of the nonlinear optical susceptibility chi((2)) originating from a reg ular array of antiparallel pyroelectric twins stacked along the principal c rystallographic axis normal to c. The mean width of the thin antipolar lame llae is found to be approximately 1.5 mu m Comparison with the optically ac tive counterpart of the modulated structure revealed by polarization micros copy provides us with the image of a pyroelectric substructure included in a chiral pattern. Such regular structure is shown to be clearly associated to the mechanism of crystal growth. Twin boundaries are then proposed to be a 'zig-zag' Like alternation of quasi distortion free domain walls develop ing parallel to the growth directions of the crystal. (C) 2000 Elsevier Sci ence B.V. All rights reserved.