discovered_01_2016

WWW.HZDR.DE discovered 01.16 TITLE A catch-22 that researchers can only solve by employing a trick: Using a particularly small probe they move up to within a few nanometers of the sample – into the near field. In near-field microscopy, as it is called, only a tiny point directly under the tip of the probe is examined. Thus in order to investigate associated structures, the entire surface of the sample has to be scanned in a pre-defined grid. The free-electron lasers at HZDR’s Center for High-Power Radiation Sources, ELBE, deliver precisely the right amount of illumination, Winnerl explains: "We produce an intense infrared beam with an adjustable wavelength that can be directed at the tip." The backscattered light is measured with a detector. It contains optical information about the area of the sample underneath the tip which is not limited by the wavelength. A lab of their own at ELBE In order to incorporate all the many pieces of information from the individual points into an image of the sample, the computer puts them together like a mosaic – a principle that has been in use in scanning-force microscopy for some time: Here, too, a tiny needle just a few nanometers long travels across the sample in a fine linear grid. However, only the atomic forces between the tip of the needle and the surface of the sample are evaluated – the tip itself is still blind. "Only when the infrared radiation is measured with near- field microscopy is it also possible to examine the optical properties of the sample using the same linear scanning principle," explains Susanne Kehr, physicist in the Institute of Applied Physics (IAP) at TU Dresden. Kehr heads a junior research group which focuses on microscopy methods in the department chaired by Lukas Eng; the group are permanent guests at the ELBE center. "In the early years, we still used to pack a small car full of equipment at the university and drive over to Rossendorf. It was a bit like playing Tetris with sensitive microscopy equipment," says Kehr, smiling. Today, the group operates its own optics lab at ELBE where the researchers have permanently installed two near-field microscopes. The special thing about the lab is that the free-electron lasers can be operated in so-called continuous wave mode: "The lasers deliver radiation pulses with a high repetition rate and controllable intensity. That’s important because we rely on a continuous signal, and many samples are over-excited by individual strong pulses and can even be destroyed." Such excellent cooperation between the IAP Group and the scientists at HZDR has already produced striking results. Former HZDR doctoral student Markus Fehrenbacher, for example, developed a superlens in the context of this collaboration which facilitates imaging below the diffraction limit and strengthens optical signals in the sample. "In order to be able to examine lenses like this in detail, you need microscopy technology that is not limited by wavelength," says Winnerl. By using near-field microscopy, researchers have been able to show for the first time how the semiconductor gallium arsenide can be changed into a superlens by additional electrons from intentionally incorporated foreign atoms. By adjusting the concentration of such impurity atoms the lens can even be matched to a desired wavelength. COLLEAGUES: Stephan Winnerl of HZDR and Susanne C. Kehr of TU Dresden have been working together successfully for years. Photo: Oliver Killig

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