Laser-light diffraction studies on the electric-field response of the helical director configuration in smectic-$C*$ liquid crystals

We report results from laser light diffraction studies on the electric-field-induced dynamic modulation of the helical director configuration of $SmC*$ liquid crystals. In contrast to what is predicted by common theoretical models for bulk samples, we found no increase of the helix pitch with increasing electric fields in planar-aligning cells. This can be explained by the appearance of disclinations that are pinned at the cell surfaces but also the large dimension of the sample cell along the helix axis is expected to play a role. Even more surprisingly, we found a decreasing pitch with an increasing field for one of the compounds investigated and a field-controllable rotation of the diffraction pattern around the sample cell normal in two other materials.