The photograph shows the inside of a vacuum chamber, in which a high-intensity laser beam (blue) is focused into a very small spot (diameter 1/100 millimeter) by a regular lens. The strong electric field of the laser pulses extracts electrons from the metal surface and accelerates them to one third of the velocity of light. These fast electrons generate high energetic x-rays in the copper tape by means of the same mechanism which is used in conventional x-ray tubes.

The wavelength of x-rays (1.5 Angströms) is smaller than the distance between atoms in a solid, and 3000 times smaller than the wavelength of visible light. The duration of an x-ray flash is 0.1 picoseconds, as short as the laser pulse used to generate it (1 picosecond = 1 millionth of a millionth of a second). This is the exposure time of the x-ray camera.

Every laser shot triggers a microexplosion in the copper tape, spreading particles in all directions.For 1000 shots per seconds, the band has to be moved at a velocity similar to a regular tape recorder (golden spools). The focusing lens must be protected by a moving and highly transparent plastic film (black spools).

In the current issue of Science magazine (Vol. 306, Dec. 3, 2004), scientists from the Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Berlin, Germany, report the direct observation of atomic motions in a semiconductor nanostructure. They use a novel, laser-driven source for ultrashort x-ray pulses to take a movie of atomic motions in a semiconductor nanostructure. “We can observe changes on ultrashort timescales with our femtosecond x-ray diffraction setup”, says Matias Bargheer, who has conducted the work together with Michael Woerner, Nikolai Zhavoronkov and Thomas Elsaesser.

X-rays can be used to look inside objects - think of your security check at the airport or your last visit at the dentist. Checking materials for micro-cracks or the atomic analysis of paint in old masterpieces are additional areas where x-rays are in extensive use. In these every-day applications as well as in fundamental research, the x-ray images are usually static in nature. The state of an object at a certain time or averaged over a time-intervall is pictured. For many applications, however, there is great interest to monitor and analyse processes by recording a sequence of snap-shots. A number of groups worldwide work on the generation of ultrashort x-ray flashes in order to look at elementary processes in nature, such as atomic and molecular motion or the breaking of chemical bonds. Often, these processes occur on time-scales below one picosecond (1 ps), that is a millionth of a millionth of a second.

The nanostructure investigated consists – similar to modern optoelectronic devices – of very thin layers of gallium arsenide (GaAs) and aluminum arsenide (AlAs). An ultrashort laser pulse excites lattice vibrations, i.e. periodic motions of the atoms in the crystal, which are recorded by diffracting an x-ray pulse off the oscillating structure. The very short wavelength of hard x-rays allows for high precision measurements of the atomic positions. A sequence of snapshots is recorded by varying the time delay between excitation and x-ray pulse in steps of approx. 0.1 ps. Although the amplitude of the atoms is 1000 times less than the distance between nearest neighbors, the researchers were able to reconstruct the atomic motion from this movie. They can for the first time unambiguously attribute the generation of the lattice vibrations to the so-called “displacive excitation of coherent phonons”. In this mechanism, the lattice motion is triggered by excitation of electrons, which remain excited during the vibration.

The work in Berlin, which is sponsored by the German Science Foundation (Topical Research Program 1134), demonstrates an unprecedented combination of spatial and temporal resolution. A large number of investigations concerning solids and biomolecular crystals can now be tackled. In a next step the MBI-team wants to look at ultrafast processes in superconductors. “Maybe we can solve the chicken-and-egg problem of phase transitions” says Bargheer. In some cases, it is not known, if the electronic system changes first and then forces the nuclei to adapt to a new configuration, or if the transition proceeds vice versa. Femtosecond x-ray diffraction may also help to clarify many fundamental phenomena where electrons show correlations, for example super-conductivity, magnetism and piezo-electricity.

Swinging Atoms
a short movie about atoms in a nanostructure
here

The cartoon shows a small fraction of a semiconductor nanostructure. Gallium, arsenic and aluminum atoms form a crystal lattice composed of alternating layers of the semiconductors gallium arsenide (GaAs) and aluminum arsenide (AlAs). The layers are much too thin to be seen with a light microscope. In fact, a stack of 2000 layers has a thickness of only 0.03 millimeters.

In our experiment we hit this stack with a special laser pulse. Its energy is only absorbed in the GaAs layers. All of them promptly start to expand. The AlAs layers are squeezed from both sides, but swing back to their original size. We take snapshots of this motion back and forth with our x-ray camera. The cartoon exaggerates the motion. In reality we have observed amplitudes of the atoms which is 1000 times smaller than the interatomic distance. The atoms traverse this short distance in an ultrashort time. This is why our camera needs to be extremely quick

Contact at the Max-Born-Institute:

Dr. Matias Bargheer, Tel. 030 6392 1472, bargheer@mbi-berlin.de
Dr. Michael Wörner, Tel. 030 6392 1470, woerner@mbi-berlin.de
Prof. Dr. Thomas Elsässer, Tel. 030 6392 1400, elsasser@mbi-berlin.de