Electron holography of ferroelectric materials

Abstract

Electron holography gives to the electron microscope the ability the visualize, and measure, electric and magnetic fields at high spatial resolution . The principle of the method is shown in figure (1). Some fraction of a coherent electron beam, typically produced from a cold field emission electron gun, is incident on the specimen while the rest passes through vacuum, or through some other region of the sample, to form a reference wave. An electrostatic bi-prism converges these two components to form an interferogram or ‘hologram’ at the viewing screen. The hologram consists of a fringe pattern, with a spacing determined by the potential on the bi-prism, modulated by the intensity variations of the transmitted bright-field signal. Variations in the phase of the transmitted signal are stored as shifts in the fringe pattern. The hologram can then be reconstructed, either optically or by using a computer and appropriate software, to yield two images, one - the 'Amplitude Image' - containing normal intensity contrast (bright field) information, and a second- the 'Phase Image'- which displays variations in the relative phase of the signal as changes in grey level. The phase of the transmitted beam will be shifted by an amount proportional to the mean inner potential of the specimen multiplied by the thickness, and also by the potential that the electron encounters as it travels through the sample.