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  • Using Electrons in Holography
Dec 2010
BERLIN, Dec. 21, 2010 — Physicists at the Max Born Institute (MBI) in Berlin are returning to the principle of using electrons in holography. A special element in their approach is that the electrons that image the object are made from the object itself using a strong laser.

This principle was first discovered in 1947 by the Hungarian scientist Dennis Gábor in connection with attempts to improve the resolution of electron microscopes. The realization of the concept of holography had to wait until the mid-1960s when holograms were made using the newly-discovered laser light sources, rather than with electrons.

Experimentally measured velocity map image for the ionization of metastable Xe atoms by 7-µm light from the FELICE laser. The image shows the velocity distribution of the ionized electrons along (horizontal) and perpendicular to (vertical) the polarization axis.

Holography, as it is encountered in everyday life, uses coherent light. This light wave is divided into two parts, a reference wave and an object wave. The reference wave directly falls onto a two-dimensional detector, for example a photographic plate. The object wave interacts with and scatters off the object, and is then also detected. The superposition of both waves on the detector creates interference patterns, in which the shape of the object is encoded.

What Gábor couldn´t do to construct a source of coherent electrons is commonplace in experiments with intense laser fields. With intense, ultrashort laser fields, coherent electrons can readily be extracted from atoms and molecules. These electrons are the basis for the new holography experiment, which was carried using Xe atoms.

“In our experiment, the strong laser field rips electrons from the Xe atoms and accelerates them, before turning them around,” said Dr. Marc Vrakking, professor at MBI. “It is then as if one takes a catapult and shoots an electron at the ion that was left behind. The laser creates the perfect electron source for a holographic experiment.”

Some of the electrons recombine with the ion, and produce extreme ultraviolet (XUV) light, thereby producing the attosecond pulses that are the basis for the new attosecond science program that is under development at MBI. Most electrons pass the ion and form the reference wave in the holographic experiment. Yet other electrons scatter off the ion, and form the object wave. On a two-dimensional detector the scientists could observe holographic interference patterns caused by the interaction of the object wave with the Coulomb potential of the ion.

In order to successfully carry out the experiments, certain conditions had to be met. In order to create the conditions for holography, the electron source had to be put as far away as possible from the ion, ensuring that the reference wave was only minimally influenced by the ion. The experiments were therefore carried out in the Netherlands, making use of the mid-infrared free electron laser FELICE, in a collaboration that encompassed — among others — the FOM Institutes AMOLF and Rijnhuizen. At FELICE, the Xe atoms where ionized using laser light with a 7-mm wavelength, creating ideal conditions for the observation of a hologram.

The ionization process produces the electrons over a finite time interval of a few femtoseconds. Theoretical calculations under the guidance of MBI junior group leader Olga Smirnova show, that the time dependence of the ionization process is encoded in the holograms, as well as possible changes in the ion between the time that the ionization occurs and the time that the object wave interacts with the ion. This suggests a big future promise for the new technique.

“So far, we have demonstrated that holograms can be produced in experiments with intense lasers,” said Vrakking. “In the future we have to learn how to extract all the information that is contained in the holograms. This may lead to novel methods to study attosecond time-scale electron dynamics, as well as novel methods to study time-dependent structural changes in molecules.”

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An interference pattern that is recorded on a high-resolution plate, the two interfering beams formed by a coherent beam from a laser and light scattered by an object. If after processing, the plate is viewed correctly by monochromatic light, a three-dimensional image of the object is seen.  
The optical recording of the object wave formed by the resulting interference pattern of two mutually coherent component light beams. In the holographic process, a coherent beam first is split into two component beams, one of which irradiates the object, the second of which irradiates a recording medium. The diffraction or scattering of the first wave by the object forms the object wave that proceeds to and interferes with the second coherent beam, or reference wave at the medium. The resulting...
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