École polytechnique: Second harmonic generation on an extreme ultraviolet source

An international collaboration has succeeded in doubling the frequency of extreme ultraviolet light, thus producing radiation with a wavelength of about ten nanometres. The experiment was carried out at the Applied Optics Laboratory.

Photons that make up light are usually rather solitary and do not interact with each other. But under special conditions, two photons with the same frequency can somehow join together to form one photon with twice the frequency. This frequency doubling, also known as second harmonic generation, is one of many so-called ‘non-linear’ phenomena and was observed as early as 1961 with visible laser light. This phenomenon has since been widely used, including in other areas of the electromagnetic spectrum such as infrared.

In this work published in Science Advances by scientists from different institutions such as the Universities of Jena in Germany, Berkeley in the United States and the Laboratory for Applied Optics (*LOA), the novelty lies in the wavelength range, the extreme ultraviolet or XUV range. This extends between the ultraviolet and X-ray ranges, i.e. wavelengths from 80 nanometres (nm) to a few nanometres. “Doing experiments in XUV is difficult stresses Fabien Tissandier, a researcher at LOA, in particular because this radiation is very easily absorbed by all types of materials. The entire experiment is therefore conducted in a vacuum.”

The first step is to create the initial radiation with a sufficiently high intensity to then generate a non-linear effect. One solution would be to use a free-electron laser, a source that is both intense and frequency-tunable, which has already been used to demonstrate the same phenomenon in X-rays. However, this equipment requires large facilities. At the laboratory level, the LOA has been developing and improving an XUV laser source for nearly 20 years, of which there are only a few in the world. Its general principle: an infrared laser is focused into a gas of Krypton atoms, from which electrons are then torn off. This forms a plasma of electrons and ions where multiple collisions bring many ions into an excited energy state. The ions release this energy by emitting identical photons in a cascade, resulting in a laser beam with a 32,8 nm wavelength.

This beam is then focused on a thin titanium foil with an intensity of around 10^11 Watts per square centimetre. “At these wavelengths, it is impossible to use non-linear crystals as in the visible or ultraviolet range, because the radiation would be completely absorbed,” continues the researcher. The second harmonic is therefore generated at the interface between the vacuum and the titanium foil, giving rise to a beam with a 16,4 nm wavelength. A doubling of the frequency is indeed equivalent to a twofold division of the wavelength. A diffraction grating and a CCD camera are used to characterise this radiation.

The conversion rate between the initial beam and the second harmonic is around 2%. “The main objective was to show that it was possible,” explains Fabien Tissandier. Although this radiation could eventually be used to analyse complex materials, it is the XUV source itself that is of interest to the LOA team. Increasing its energy, better determining its characteristics and refining the understanding of the physical mechanisms within the plasma are among the goals being pursued.

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