Three-beam effects inPendellösungfringes

Abstract

The perturbations of standard two-beam Pendellösung fringes due to simultaneous excitation of an additional Bragg beam have been studied. For a qualitative understanding of the observed local changes in both fringe position and intensity, a plane-wave description has proved to be sufficient. Through three-beam calculations of Bloch-wave excitation coefficients it is found that the main intensity terms may be ascribed to the same four dispersion surface branches which are excited also in the corresponding two-beam case. The continuous variation observed in the fringe period and position is explained from the variation in the width of the dispersion surface gap. This gap varies with the sign and size of the deviation parameter of the simultaneously excited beam. For a given deviation parameter the gap width also depends on the sign of the product between the three structure factors involved. The resulting dependence of the fringe displacement on this sign may be utilized to determine the three-phase structure invariant experimentally.