First Lens for Extreme-Ultraviolet Light

Focusing of an XUV beam by a jet of atoms that is used as a lens. (Source: O. Kornilov, L. Drescher)

Scientists from the Max Born Insti­tute MBI in Berlin have developed the first refractive lens that focuses extreme ultra­violet beams. Instead of using a glass lens, which is non-trans­parent in the extreme-ultraviolet region, the researchers have demonstrated a lens that is formed by a jet of atoms. The results provide novel oppor­tunities for the imaging of bio­logical samples on the shortest timescales.

Refraction is the under­lying physical principle behind lenses which play an indis­pensable role in everyday life: They are a part of the human eye, they are used as glasses, contact lenses, as camera objectives and for controlling laser beams. Following the discovery of new regions of the electro­magnetic spectrum such as ultra­violet (UV) and X-ray radiation, refractive lenses were developed that are speci­fically adapted to these spectral regions. Electro­magnetic radiation in the extreme-ultra­violet (XUV) region is, however, somewhat special. It occupies the wavelength range between the UV and X-ray domains, but unlike the two latter types of radiation, it can only travel in vacuum or strongly rare­fied gases.

Nowadays XUV beams are widely used in semi­conductor litho­graphy as well as in fundamental research to understand and control the structure and dynamics of matter. They enable the gene­ration of the shortest human made light pulses with atto­second durations. However, in spite of the large number of XUV sources and appli­cations, no XUV lenses have existed up to now. The reason is that XUV radiation is strongly absorbed by any solid or liquid material and simply cannot pass through conven­tional lenses.

In order to focus XUV beams, MBI researchers have taken a different approach: They replaced a glass lens with that formed by a jet of atoms of a noble gas, helium. This lens benefits from the high trans­mission of helium in the XUV spectral range and at the same time can be precisely controlled by changing the density of the gas in the jet. This is important in order to tune the focal length and mini­mize the spot sizes of the focused XUV beams.

In comparison to curved mirrors that are often used to focus XUV radiation, these gaseous refrac­tive lenses have a number of advantages: A new lens is constantly gene­rated through the flow of atoms in the jet, meaning that problems with damages are avoided. Furthermore, a gas lens results in virtually no loss of XUV radiation compared to a typical mirror. “This is a major improvement, because the gene­ration of XUV beams is complex and often very expensive,” Bernd Schütte explains. The researchers have further demon­strated that an atomic jet can act as a prism breaking the XUV radiation into its consti­tuent spectral components.

The develop­ment of the gas-phase lenses and prisms in the XUV region makes it possible to transfer optical techniques that are based on refraction and that are widely used in the visible and infrared part of the electro­magnetic spectrum, to the XUV domain. Gas lenses could e.g. be exploited to develop an XUV micro­scope or to focus XUV beams to nanometer spot sizes. This may be applied in the future, for instance, to observe structural changes of biomo­lecules on the shortest timescales. (Source: MBI)

Reference: L. Drescher et al.: Extreme-ultraviolet refractive optics, Nature, online 28 November 2018; DOI: 10.1038/s41586-018-0737-3

Link: Time-resolved XUV-science, Max Born Institute, Berlin, Germany

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