Generated attosecond flashes of unprecedented intensity

Alternative method developed for the production of ultrashort light flashes

The rotating mechanism of the target allows attosecond pulses to be generated at a repetition rate of 10 Hz. Each laser pulse creates a small crater (seen in the picture) on the smooth target surface. © Thorsten Naeser / MPQ
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Physicists have developed an alternative method for obtaining ultrashort light flashes, so-called attosecond pulses. These have a much higher intensity than conventionally generated, the researchers report in the online edition of the journal "Nature Physics".

However, the new method has another advantage over conventional ones: The new method should be scalable, that is, the higher the laser intensity, the shorter and more energetic the attosecond pulses. Attosecond pulses with an intensity much higher than the current state of the art would allow a range of interesting experiments and even attack-probe experiments with attosecond resolution.

Flashes of light "freeze" movement of electrons

New developments in laser technology have paved the way to create flashes of attosecond duration - an attosecond corresponds to 10 high -18 sec - with which the movement of the electrons in atoms and molecules can be "frozen". However, the range of possible applications is limited by the low intensity of current sources.

Scientists led by George Tsakiris and Professor Ferenc Krausz of the Max Planck Institute of Quantum Optics have now demonstrated in a new experiment that high-density solid-state relativistic plasmas are suitable for efficient conversion of infrared laser light into harmonic XUV radiation.

An infrared laser pulse from a few wave trains (half oscillation period: 1.3 femtoseconds = 1, 300 attoseconds) is focused on a solid state target. The reflected pulse contains a plurality of harmonics, from which a single attosecond pulse is selected by suitable filtering. MPQ

The physicists were able to generate large amounts of light energy in a period of less than a femtosecond. Achieving temporal and spatial resolution at subatomic scales would have far-reaching implications for many areas of research, from physics to chemistry, biology and medicine, to information technologies. display

Pulses from the titanium sapphire laser

The usual method for obtaining ultrashort coherent light pulses in the XUV spectral range is based on the production of so-called "harmonics", which arise when laser light passes through a gas target. The laser light is converted into radiation whose frequency is an integer multiple of the fundamental frequency.

In contrast, in the experiment described here, physicists focus short laser pulses from the titanium sapphire laser ATLAS (IR, 800 nm) onto a solid-state target. Whose surface is thereby completely ionized, so that there forms a high-density plasma in which the electrons oscillate at near light speed in the laser field.

Plasma waves generated

According to the scientists, two processes lead to the generation of harmonics: On the one hand, the oscillating electrons reflect the incoming light. If they run counter to the beam, the light is shifted to higher frequencies due to the Doppler effect. On the other hand - and this is the dominant process in this experiment - the electrons injected into the surface produce plasma waves in their wake.

Under certain circumstances, these are converted into electromagnetic radiation at higher harmonics than the frequency of the driver laser. A suitable filter removes the remaining IR radiation and selects a range of harmonics.

Researchers work with tricks

"There is no way to directly determine the temporal structure of the sequence of transmitted attosecond pulses, " explains Tsakiris, the project leader. We must therefore use a trick: we let two copies of the pulse trains interact with the atoms in a helium gas jet. By varying the time shift between them and recording the corresponding number of helium ions, we can conclude on the time structure of XUV radiation.

And Rainer H rlein adds: "We have shown for the first time that the harmonics generated in a solid are actually emitted as a dense series of attosecond pulses."

(idw - Max Planck Institute for Quantum Optics, 18.12.2008 - DLO)