Have you ever watched the ripples of waves spreading on the water surface? When two waves meet, an interference pattern is formed. Holography uses this principle of interference. Light is divided into two paths, one penetrating the object (signal light) and the other not penetrating (reference light). When these two light waves meet and overlap, they form interference fringes, which are recorded as holograms containing three-dimensional information of the object. When the original light is applied to the hologram, the object is reconstructed as a three-dimensional image. In order to create a clear hologram, it is important to use highly coherent light (with uniform wave phases).
Holograms can be created using electrons as well, because electrons have wave characteristics. The process for creating a hologram is fundamentally the same as that of light. In order to create a clear electron hologram, we need an electron beam with good coherence just as with the light source. Dr. Akira Tonomura succeeded in developing the holography electron microscope in which holograms were formed.
In a holography electron microscope, electron waves are split in two paths, only one of which passes through the specimen. These two electron waves are then bent toward the center to interfere with each other and interference fringes are formed. The device used to bend the electron waves is called an electron biprism.
These interference fringes are recorded on films (holograms) and then images are reconstructed by illuminating a laser beam on the holograms. The images can also be reconstructed using a computer from holograms filmed by a TV camera.
1. Electrons have a dual characteristic of particle and wave. In an electron holography, the wave characteristic is used.
2. When coherent electron wave emitted from a field-emission electron gun
3. passes through a sample to be investigated, the wave front is modified by the magnetic field, electric field or three-dimensional configuration of the sample.
4. Let half of the electron wave pass through the sample; this wave is called the "object wave." The other half, which does not pass through the sample, is called the "reference wave."
5. Place an electron biprism composed of a fine filament bridged in the center and two plate-shaped electrodes that are both at ground potential. When a positive potential is applied to the filament, two electron beams opposite sides of the filament are attracted towards the filament and overlaps on the lower plane.
6. Since these two beams, the object wave and the reference wave, are coherent, an interference pattern is produced, creating interference fringes. This is the electron hologram. The hologram contains sample information, such as electric field, magnetic field and three-dimensional configuration. The hologram is either recorded on a film or taken into a CCD camera and then processed by a computer to reproduce the original image.