The Hungarian Dennis Gábor discovered the principle of holography in theoretical work in 1947, when he tried to improve the resolution of electron microscopes. However, holography first became established in the 1960s with the invention of the laser - it also works with light. Physicists from the Max Born Institute (MBI) in Berlin have now returned to their original beginnings by using holography with electrons. The special thing about their method: The electrons that record the object are previously shot out of it with a laser, so they come from the object itself. The scientists report about it in the online edition of Science.
Holography, as it is known to most, needs coherent light - that is, light waves that vibrate in complete unison. The light is divided into two beams, the reference wave and the object wave. The reference wave falls directly onto a two-dimensional detector, for example a photographic plate. The object wave illuminates an object and is scattered at this, then it also falls on the detector. At the same time, the two light waves are superimposed, creating an interference pattern that provides information about the three-dimensional shape of the object.
What Gábor could not do, constructing a source of coherent electron beams, is almost standard in physicists experimenting with strong laser fields. They use ultrashort ultrashort laser pulses to eject electrons from atoms and molecules, this is called ionization. Such electrons are coherent and therefore formed the basis for the new holography experiment with xenon atoms. Marc Vrakking from the MBI describes what basically happens during ionization: "The strong laser field tears the electrons away from the atom. Because the laser field swings, some of them snap back like a rubber band. So they are moving towards the atom and we have a perfect electron source."
The ejected electrons now have different possibilities: some reunite with the atom, producing extremely ultra-violet (XUV) light, which is the basis for today's attosecond physics, one of the new main topics at the MBI. However, most of the electrons fly past the atom and form the reference wave in the holography experiments. The electrons, which are scattered by the atom, form the object wave. The scientists captured the electrons with a detector and were able to observe a characteristic interference pattern, which represents the three-dimensional state of the xenon atom.
Certain conditions were necessary in the experiment: In order to obtain a clear holographic image, the reference wave could not be influenced by the positively charged object, that is, the xenon ion. The electron source should therefore be as far away as possible from the object. For this reason, the researchers carried out the experiments with the free-electron laser FELICE (Free Electron Laser for Intracavity Experiments), which emits long-wave light in the range of 4 to 40 microns. Such waves "carry" the electrons far away from the atom before bringing them back.
The electrons are produced during ionization with minimal delays, these are below a femtosecond. The researchers were thus able to show through theoretical calculations that they had received time-resolved holographic images. An exact three-dimensional image of the xenon atom, the scientists from the interference patterns may not yet construct, but Vrakking considers such a thing in the future quite possibly. "We have shown for the first time that holography is possible on an atomic scale and time-resolved with this method," he says. This opens up new possibilities for the time-resolved observation of molecules.
The work was carried out in collaboration with researchers from the FOM institutes AMOLF and Rijnhuizen, The Netherlands.
The following highlight (also from 16.12.2010) stands with this in the thematic context.