A Study of Electrostatic Phase Plates using Electron Holography

Abstract

Contrast in transmission electron microscopy is normally provided by variations in the intensity of the transmitted electron beam, but weakly scattering objects such as biological tissue and polymers may not cause a sufficient change in amplitude to provide a useful image. The phase of the electron beam, however, is altered and if the phase change can be made to yield amplitude variation then the imaging can be carried out as desired. This can be accomplished by the use of some phase shifting device which alters the phase of the scattered wave by π/2 with respect to the unscattered wave. A number of such devices have been discussed in the literature but their design has typically been optimized empirically. One of the methods for performing Phase Contrast Microscopy in the TEM is by using an electrostatic phase plate in the back focal plane of the objective lens. This device consists of a charged fiber which causes small deviations in the ray paths of the scattered and the unscattered waves thus introducing a suitable phase shift between them.