Practical Electron Microscopy and Database

As shown in Figure 2557, in off-axis electron holography, the two parts of the electron beam originate equivalently from two sources, S₁ and S₂, induced by the biprism, as in the standard Young’s experiment.

The interference width in the hologram, W, can be given by [1],

           ----------------------- [2557a]

where,
           α -- The deflection angle of the ray due to the action of the biprism,
           a -- The distance of the back focal plane of the objective lens to the biprism,
           R -- The radius of the biprism wire.

On the other hand, the width can also be given by, [2]

           ----------------------- [2557b]
where,
           U_F -- The biprism voltage,
           f -- The focal length,
           γ₀U_F -- The deflection angle at the biprism filament. [3]

Practically, the width W of the hologram is limited by the coherent current of the electron beam and by the strength of excitation of the subsequent intermediate lens. [2]

[1] Missiroli, G.F., Pozzi, G., Valdre, U., 1981. Electron interferometry and interference electron microscopy. J. Phys. E 14, 649–671.
[2] Michael Lehmann, and Hannes Lichte, Tutorial on Off-Axis Electron Holography, Microsc. Microanal. 8, 447–466, 2002.
[3] Lichte, H. (1991). Electron image plane off-axis holography of atomic structures. Adv Optical Electron Microsc 12, 25–91.