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IR Lasers Produce X-Ray Optics

Photonics Spectra
Jan 2000
Jörg Schwartz

X-rays go through almost any material without interaction, making them useful for analysis but difficult to control. X-ray scattering and diffraction are well-known techniques for investigating materials such as plastics, metals, semiconductors and geological samples. Unfortunately, classical optical methods for beam-shaping and -guiding cannot be applied easily.

Hermann Mai and co-workers at Fraunhofer Institut für Werkstoff- und Strahltechnik have developed a laser-based method of producing advanced x-ray optics that overcome this problem.

Multilayer coating systems for short-wavelength radiation have been around for some time and have found their way into a number of extreme-ultraviolet and x-ray applications. Compared with dielectric mirrors for visible light, these coating systems generally consist of a much higher number of very thin layers -- as many as 250 as thin as 1 nm -- and must be produced with even higher surface precision.

The Fraunhofer process uses the stable and easily controllable energy of Nd:YAG laser pulses to evaporate the coating materials, improving control considerably. Computer-controlled scanning over the target (typically titanium, carbon, nickel or wolfram) generates a well-defined plasma beam that results in accurate layers with thickness variation less than 1 percent. According to Mai, pulsed laser deposition allows production control down to hundredths of a nanometer.

Not only is it possible to accurately adjust the average layer thickness, but well-defined lateral layer thickness gradients are achievable. This enables reproducible manufacturing of so-called Göbel mirrors, which have a parabolically bent layer structure. This feature makes a divergent beam strike at different locations and angles, producing a diffracted parallel beam. These optical systems generate a highly brilliant beam, increasing the speed, efficiency and sensitivity of material investigations.

The underlying pulsed laser deposition technology has been improved, patented and developed to industrial standard. For this achievement Mai and his group recently received the Fraunhofer award.

Göbel mirror systems produced in Dresden are integrated into microdiffraction systems manufactured by Bruker AXS Inc. of Karlsruhe, Germany, and Madison, Wis. The test equipment maker says this component provides increased x-ray flux and eliminates the need for a rotating anode x-ray source.

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