Study of antiparallel pyroelectric twins in KLiSO sub(4) by second-harmonic scattering

Two-dimensional CCD images of far-field second-harmonic scattering (SHS) from the hexagonal phase of as-grown lithium potassium sulfate (KLiSO sub(4)) crystals are reported. Quasi-phase matching is shown to be produced by a modulation of the nonlinear optical susceptibility Chi super((2)) originating from a regular array of antiparallel pyroelectric twins stacked along the principal crystallographic axis normal to c. The mean width of the thin antipolar lamellae is found to be approximately 1.5 mu m. Comparison with the optically active counterpart of the modulated structure revealed by polarization microscopy 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 developing parallel to the growth directions of the crystal.