Introduction

In conventional electron microscopy, specimens are observed using the intensity of an electron beam. However, in electron holography [2.1], the phase as well as the intensity of the electron beam transmitted through a specimen is first recorded on film as an interference pattern, which is called a ‘hologram’. The illumination of a laser beam onto this hologram then produces an optical image of a specimen in three dimensions. To be more exact, the wavefronts of the scattered electron beam are reproduced as wavefronts of a laser beam. Although the optical wavelength is 105 times larger than the electron wavelength, the two wavefronts are otherwise alike.

Once the image is completely transferred from inside the electron microscope onto an optical bench, versatile optical techniques can be used for electron optics. For example, the effect of the spherical aberration in the objective lens of the electron microscope can be compensated for to improve the resolution which was the original objective for which Gabor invented holography [2.1]. This is done by optically adding aberration with an opposite sign. The phase distribution of the electron beam can also be drawn on an electron micrograph by using an optical interferometer in the optical reconstruction stage of electron holography [2.2]. An electron microscope with an electron biprism [2.3] can provide an interferogram, but not a contour map nor a phase-amplified interference micrograph.

These possibilities were opened up by the development of a ‘coherent’ field-emission electron beam [2.4] which is indispensable for forming high-quality electron holograms.