Free ride for electrons improves soft X-rays generation

The quest for bright attosecond pulses

Physicists at ETH Zurich discovered something interesting about generating soft X-ray pulses with electrons: when electrons are hindered in their movement – for example, by collisions with other atoms – the quality of the pulses suffers significantly.

These X-rays have similar properties to a laser beam but can form even shorter pulses. To study the fastest dynamics in matter, the pulses need to be as bright and as short as possible. If the electrons' motion is impeded, the light loses its laser-like characteristics and its bundling into attosecond pulses. This renders the light from such a source useless for studying ultrafast processes.

Laser-like soft X-rays

In an extremely nonlinear process – called high-harmonic generation, a phenomenon of high-intensity laser physics – attosecond pulses can be produced in the extreme ultraviolet or in the soft X-rays. While conventional sources in these spectral regions emit light in all directions, resulting in low-brightness X-rays, high-harmonic generation produces a laser-like beam by the interaction of intense pulses with gas atoms.

One atom, one electron, and that's it!

The process of high-harmonic generation was first explained by a model about 30 years ago [2]. This highly successful theory describes the generation of attosecond pulses with only one atom and one electron and implicitly assumes that the motion of the laser-accelerated electron at the origin of the X-ray beam is not hindered by other particles in the gas. However, as has now been found in a new study published in the journal “Optica”, this assumption does not hold anymore for the conditions typically encountered with today's lasers when generating soft X-rays high harmonics.

A crowd of atoms

Pierre-Alexis Chevreuil, a former PhD student at the Institute of Quantum Electronics at ETH Zurich, and his co-authors demonstrate experimentally in their study that this “single collision condition” no longer holds for soft X-ray generation. The electrons at the origin of the X-ray beam are disturbed by nearby atoms in the gas. This leads to a decrease in the brightness of the X-ray beam and a loss of the laser-like properties. The authors also expect that in this case the X-ray radiation is no longer emitted in the form of attosecond pulses, as would otherwise be the case in the high-harmonic generation process.

At the same time, the researchers detail the conditions under which the desired characteristics of the radiation can be recovered. With this insight, they open the way to significantly improve the quality of the light pulses that enable fundamental research on very fast dynamics in matter.