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Attosecond X-ray light pulses

German researchers have showed that a 'flash of light can be shorter than the time it takes the wave carrying the flash to perform a full oscillation.' They were able to generate flashes of laser light so intense that the atoms exposed to it emit an attosecond X-ray pulse 'whose wave components, if oscillating more slowly, would represent nearly all colors of visible light, all the way from blue through green and yellow to red.' Even if this discovery made the cover of the August 10 issue of Science, don't expect an immediate impact on us, but it sure will accelerate basic research on material properties.
Written by Roland Piquepaille, Inactive on

Before going further, do you know what is an attosecond? It's 10-18 second or just a billionth of a billionth of a second. And German researchers have showed that a 'flash of light can be shorter than the time it takes the wave carrying the flash to perform a full oscillation.' They were able to generate flashes of laser light so intense that the atoms exposed to it emit an attosecond X-ray pulse 'whose wave components, if oscillating more slowly, would represent nearly all colors of visible light, all the way from blue through green and yellow to red.' Even if this discovery made the cover of the August 10 issue of Science, don't expect an immediate impact on us, but it sure will accelerate basic research on material properties.

Setup for attosecond-pulse generation

You can see above a "schematic of an experimental setup for attosecond-pulse generation and attosecond metrology as well as spectroscopy: the AS-1 attosecond beamline at MPQ. The intense, waveform-controlled few-cycle NIR laser pulse generates a subfemtosecond XUV pulse in the first interaction medium (jet of noble gas). The collinear XUV and NIR beams then propagate into a second vacuum chamber, where they are focused by a two-component XUV multilayer mirror into a gas target. The inner and outer part of the two-component mirror reflects and focuses the XUV and the (more divergent) NIR beam, respectively. By positioning the internal mirror with a nanometer-precision piezotranslator, the XUV pulse can be delayed with respect to the NIR pulse with attosecond accuracy." (Credit: Max-Planck-Institute of Quantum Optics)

This project has been performed at the Max-Planck-Institute of Quantum Optics. The research effort has been led by Prof. Dr. Ferenc Krausz, director of the MPQ-LMU Laboratory for Attosecond & High-Field Physics (LAP). I've already wrote about his research projects in "Here Comes the Attosecond" (June 11, 2003).

Before going further, you should visit this LAP image gallery. In particular, check this animated image of the field oscillations in a few-cycle pulse of red laser light sampled with 250-attosecond x-ray pulses. (Credit: Dr. Eleftherios Goulielmakis, LAP)

Now, let's see how these scientists have produced these "intense flashes of visible laser light containing more than 50% of their energy within a single oscillation cycle. This single, large amplitude, field oscillation is used to exert a well-controlled ultrastrong force on charged particles, such as electrons, allowing unprecedented precise control of their motion in and around atoms. On the crest of this ultra-intense wave cycle, the force is strong enough to pull an electron away from an atom with almost 100% probability. The freed electron is first pulled away from the atom with a speed of several thousand km/s. Even at this high velocity, the electron can travel only several nanometers before it is turned around and directed back toward its parent atom by the next half wave cycle, which exerts its force in the opposite direction. As a result, only some two thousand attoseconds after being freed, the electron is recombined with its parent atom, emitting an X-ray pulse during the recombination.

Obviously, the researchers are already working on the next step. "By using the entire band of available X-ray frequencies, which is more than twice as broad as the spectrum used to produce the 170-as pulse, X-ray pulses substantially shorter than 100 attoseconds can be generated. A mirror capable of reflecting and focusing all these X-ray waves is currently under development. Once available, it is likely to lead to the production of the world’s first light source producing powerful laser-like X-ray flashes of duration shorter than 100 attoseconds -- the first source producing sub-100-as light. In the near future these X-ray pulses will allow researchers to take 'freeze-frame' snapshots of electrons moving in molecules, allowing reconstruction of the motions that control information transfer on molecular scales as well as structural changes of both small and large biomolecules."

This research work has been published in a special section on attosecond spectroscopy by Science under the name "Attosecond Control and Measurement: Lightwave Electronics" (Volume 317, Number 5839, Pages 769-775, August 10, 2007). Here is the beginning of the abstract. "Electrons emit light, carry electric current, and bind atoms together to form molecules. Insight into and control of their atomic-scale motion are the key to understanding the functioning of biological systems, developing efficient sources of x-ray light, and speeding up electronics. Capturing and steering this electron motion require attosecond resolution and control, respectively (1 attosecond = 10–18 seconds). A recent revolution in technology has afforded these capabilities: Controlled light waves can steer electrons inside and around atoms, marking the birth of lightwave electronics."

And if you're really interested in this subject, here is a link to the full paper (PDF format, 8 pages, 646 KB). The illustration above was extracted from this document.

Finally, I want to thank Joseph Antony, who works at the Australian National University in the ANU Supercomputer Facility and who informed me about this research discovery.

Sources: Joint Press Release of the Max Planck Institute of Quantum Optics and the Munich Centre for Advanced Photonics , August 10, 2007; and various websites

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