Physical Principles of Electron Microscopy: An Introduction to TEM ...

186 Chapter 7
There are now many alternative holographic schemes, the most popular
being off-axis holography (Voelkl et al., 1999). In this method, an electron
biprism is used to combine the object wave, representing electrons that pass
through the specimen, with a reference wave formed from electrons that pass
beyond an edge of the specimen (Fig. 7-6). The biprism consists of a thin
conducting wire (< 1 �m diameter) maintained at a positive potential (up to a
few hundred volts), usually inserted at the selected-area-aperture plane of a
field-emission TEM. The electric field of the biprism bends the electron
trajectories (Fig.7-6) and produces a hologram containing sinusoidal fringes,
running parallel to the wire, that are modulated by information about the
specimen; see Fig. 7-7. Highly astigmatic illumination is used to maximize
the size of the coherence patch where the interfering wavefronts overlap.
W
hologram
specimen
objective
Figure 7-6. Principle of off-axis electron holography. Electrons that pass through the sample
are combined with those of an off-axis beam that is attracted toward a positively biased wire
W, whose axis runs perpendicular to the diagram. W is the electron-optical equivalent of a
glass biprism, as indicated by the dashed lines.