Practical Electron Holography

Abstract

The underlying basis and motivation for electron holography is that the relative phase change of the electron beam that has passed through the specimen region of interest can be determined by comparison with a coherent reference wave. Thus, unlike the conventional electron microscope image, which represents the intensity of the electron wave without any direct phase information, electron holography enables the phase and amplitude at any point of the coherent wavefront to be obtained. This capability has many useful applications. For example, as will be evident from work described in later chapters, the reconstructed phase image provides the possibility for direct imaging on the nanometer scale of the electric and magnetic potentials within the sample, thereby enabling truly unique information to be obtained.⁴⁵¹ In addition, once the phase image is obtained, an inverse phase plate can be applied so that phase changes imposed by the transfer function of the objective lens, such as spherical aberration and two-fold astigmatism, can be corrected. It then becomes possible to exceed the conventional resolution limit of the electron microscope for bright-field, axial illumination imaging.²⁶¹ Finally, it seems appropriate to mention here that the reconstructed amplitude image, which was overlooked by the electron holographic community for many years, can be used to obtain valuable inelastic scattering data,²⁹⁷ Further details and other applications of the amplitude image can be found in the later chapter by Gajdardziska-Josifovska and Carim, pages 267 ff.