Scanning Microscopy


High resolution electron microscopy is a means for imaging not only the local structure at a defect or interface but also the displacement field in the surrounding lattice. However, in general it is difficult or tedious to analyse this field which can extend across the entire micrograph. The optical moiré technique, which is based on interference effects between the experimental lattice image and an artificial reference lattice, allows a rapid and accurate measurement of the displacements. Small violations of translation, rotation or mirror symmetries give rise to large changes in the periodicity or orientation of the moiré pattern.

The spacing and orientation of the reference lattice should be chosen in order to produce the appropriate optical moiré pattern. The different types of patterns: simple rotation, simple parallel and "mixed", are then best interpreted by reference to reciprocal space vectors of the component lattices. Displacements in the experimental image revealed by the moiré pattern represent displacements in the actual specimen under certain conditions. The technique therefore provides a means for detecting lattice defects and for measuring lattice rotations and rigid body shifts.

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