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uni'wissen 01-2012_ENG

which is irradiated into the object from the side. All parts of the object outside of this level remain unlit and thus dark. In order to create the light sheet, it is possible to use a special cylinder lens alignment or to move a laser beam back and forth quickly at the level of focus in order to obtain an even thinner sheet. However, the light radiated from the side is scattered and deflected by ­numerous cells and boundary surfaces – in other words by the layers between the various materials. This is where Rohrbach’s idea comes in: He wants to reduce the scattering by using new self- reconstructing beams. Bessel Beams Penetrate Deeper In a series of experiments, Rohrbach and his team demonstrated that specially formed laser beams can approximately reconstruct their original profile when various obstacles, such as light- scattering biological cells, repeatedly destroy the profile of the beam. This self-reconstruction works because scattered photons, i.e. quanta of light, are continuously replaced in the center of the beam by new ones coming from the side. “It is an astounding phenomenon that almost all of the photons coming from the side meet at the center almost simultaneously to form a new beam profile despite massive delays caused by the scattering cells.” In order to create these special laser beams, the Freiburg researchers converted conventional laser beams to so-called Bessel beams. The most flexible way of doing this is with a computer- controlled hologram that changes the trajectory of the photons depending on their position over the cross-section of the beam. It is known that the profile of Bessel beams in scatter-free space remains largely stable, but until recently it was completely unclear whether and to what extent they also can also revert to their original form in inhomogeneous material, i.e., where there is a lot of scattering. Rohrbach first succeeded in predicting this on a theoretical level with com­ puter simulations and then verified it with experi­ ments. He was thus able to demonstrate that holographically formed self-reconstructing laser beams are especially well suited for microscopy since they are more robust against disruptive scattering. The Bessel beams can penetrate more deeply into the objects under observation, such as pieces of skin or cancer cell clusters. However, even the functioning of Bessel beams is not completely trouble-free, because only around 20 percent of the light particles are located in the central main beam; the rest are transported around the center in a ring system. Better contrast, higher resolution: The central main beam of the laser illuminates the object line by line. At the same time, a camera also captures the object as through a single-slit diffraction. This masks the light from the ring system. Sharper image: The new microscopes deliver a more detailed view of individual areas of cancer cell clusters. Structure of a Bessel Beam Central main beam (maximum intensity) ­remains stable Scattered light: in the ring system Ideal propagation volume for the microscopic images Ring system