Macroscopic length changes due to the alignment of elastic dipoles in KCl

The creation of defects in a crystal leads to distortion of the lattice. Using the model of an elastic dipole for the defect the mechanical interaction of defects with the lattice can be described by double force tensors. Anisotropic defects as $H$ and $V_k$ centers in alkali halides lead to an anisotropic distortion of the crystal. These defects are created in different (equivalent) orientations inducing an isotropic (macroscopic) length change. This equivalent distribution of defects can be disturbed by irradiation with polarized light, thus turning defects from one orientation to another. This results in a change of the macroscopic dimensions of the crystal and the length change in different directions of the crystal is a measure for the anisotropy of the distortion field induced by the defect. If a sufficient amount of anisotropic defects can be turned from one orientation into another, the corresponding macroscopic length change can be measured in different directions. In addition, the concentration of the turned defects is measured via optical-absorption measurements and the double force tensor for the $H$ and the $V_k$ center in KCl is determined.