First lens for extreme ultraviolet light

Helium gas beam bundles or scatters XUV light like a refractive lens

A beam of helium atoms acts like a lens - allowing it to focus the high-energy XUV radiation. © Max Born Institute Berlin
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Optical breakthrough: Researchers have for the first time developed a lens that can focus or diffuse extreme UV light. Because glass or other solids completely absorb this radiation, a beam of helium atoms serves as the lens. The gas atoms break the XUV radiation like a refractive lens and thus enable future completely new applications of this high-energy UV radiation, as the researchers report in the journal "Nature".

Refractive lenses are indispensable in everyday life: they are part of the eye, serve as eyeglasses, contact lenses or as a camera lens. In laser technology, lenses are used to focus and focus the coherent light beams. For many applications in technology and science, lenses have also been developed that can also focus or scatter electromagnetic radiation outside the visible wavelength range.

No lenses for the XUV

But there is a gap: Up to now, suitable lenses have been missing for extreme UV light (XUV). This short-wave, high-energy radiation covers the wavelength range from ten to 121 nanometers - the zone between UV and X-rays. Among other things, the XUV is used to apply circuits to semiconductors by means of lithography, but also to generate ultrashort laser pulses.

The problem: Because the XUV radiation is strongly absorbed by solid and liquid materials, all conventional lenses or prisms swallow them almost completely. In order to focus them even without a lens, research and industry have hitherto supported complex arrangements of special mirrors.

Spectral splitting of the XUV radiation by the gas lens: Radiation with colors close to the helium resonances are deflected either upwards or downwards. MBI Berlin

Helium gas instead of glass lens

A solution to this problem has now been researched by Lorenz Drescher of the Max Born Institute in Berlin. They developed a lens for XUV radiation that consists of helium gas instead of glass. For this purpose, the researchers generated a beam of helium atoms whose density increases toward the center. If XUV radiation now passes this gas jet transverse to its direction, it is refracted and focused or scattered depending on the gas density gradient. display

The whole thing works because the XUV light excites the gas atoms and then emit photons of the same wavelength and energy again. The Cloud: These re-emitted XUV photons are out of phase with the originals. "Because of the density gradient of the gas, this causes the upper part of the XUV pulse to undergo a different phase shift than the lower part, " explain the researchers. "And that's how the XUV wavefront tilts."

Optionally either lens or prism

In simpler terms: depending on the gas density, the energy-rich XUV radiation can either be bundled or scattered like in a prism. "The degree of distraction can be controlled by varying the gas density, " said Drescher and his colleagues. Even stronger and finer refractions could be achieved by combining several such gas lenses.

The big advantage: Compared with curved mirrors, which are otherwise used to focus XUV radiation, the gas lens barely loses XUV radiation. "This is the most important improvement because the generation of XUV beams is complex and often very expensive, " explains Drescher's colleague Bernd Sch tte. In addition, the gas lens is not so sensitive to damage: Because the helium atoms constantly nachstr men, they can not break.

"The XUV lens now makes it possible to transfer concepts based on refractive optics, such as microscopy, nanofocusing or the compression of ultrashort pulses, to the XUV range, " the researchers note. For example, the new gas lenses could be used to develop an XUV microscope, but XUV lithography could also be simplified. (Nature, 2018; doi: 10.1038 / s41586-018-0737-3)

(Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI), 29.11.2018 - NPO)