Electron holography and holographic diffraction for surface studies

Abstract

The use of a reference wave coming from a particular type of atom has been applied in diffraction experiments from the early days of X-ray diffraction as a means for defining the relative phases of diffracted beams. Modern diffraction techniques for surface study, including diffuse LEED and photoelectron diffraction, apply the same basic concept of what has been called holographic diffraction in a variety of ways. The concept of holography was introduced by Gabor as a basis for the improvement of resolution in electron microscope imaging. An essential aspect was the use of a reference wave to allow relative phases of the image wave function to be defined. The use of holographic imaging principles for the imaging of surfaces with high-energy electrons in the reflection mode has been developed by analogy with off-axis transmission holography. Other forms of reflection-mode electron holography appear feasible by analogy with the Gabor scheme and other in-line holographic transmission modes. The development of very sharp field emission tips has introduced the possibility of low-energy electron holography in transmission and reflection modes. A plea is made for the use of appropriate terminology to distinguish clearly between these various uses of the holographic principle.