Lichtgedanken 03

S C HW E R P U N K T 44 A visit to the optometrist often invol- ves optical coherence tomography. This imaging process uses infrared radiation to penetrate the layers of the retina and examine it more closely in three dimen- sions, without having to touch the eye at all. This allows eye specialists to diagno- se diseases such as glaucoma without any physical intervention. However, this method would have even greater potential for science if shorter wavelengths were used, thus allowing a higher resolution of the image. Phy- sicists at Friedrich Schiller University Jena have achieved just that and they have reported their research findings in the specialist journal »Optica«. First XUV coherence tomography at laboratory scale For the first time, the University phy- sicists used extreme ultraviolet radia- tion (XUV) for this process, which was generated in their own laboratory, and they were thus able to perform the first XUV coherence tomography at lab- oratory scale. This radiation has a wa- velength of between 20 and 40 nanome- tres—from which it is therefore just a small step to the X-ray range. »Large-scale equipment, that is to say particle accelerators such as the German Elektronen-Synchotron in Hamburg, are usually necessary for generating XUV radiation,« says Silvio Fuchs of the In- stitute of Optics and Quantum Electro- nics of the Jena University. »This makes such a research method very complex and costly, and only available to a few researchers.« The physicists have alrea- dy demonstrated this method at large With ultrashort X-ray pulses, processes and structures can be resolved down to the nanometre range. X-ray pulses are usually generated in huge particle accelerators, such as the DESY in Hamburg. But the access to such facilities is limited and their operation is extremely expensive. Physicists in Jena are therefore developing »handy« laser systems, which enable ultrashort X-ray pulses at laboratory scale and thus allow a wide variety of applications in the lab. Imaging with »X-ray vision« BY SEBASTIAN HOLLSTEIN research facilities, but they have now found a possibility for applying it at a smaller scale. In this approach, they focus an ultrashort, very intense infrared laser in a noble gas, for example argon or neon. »The electrons in the gas are accelerated by means of an ionisation process,« ex- plains Fuchs. »They then emit the XUV radiation.« It is true that this method is very inefficient, as only a millionth part of the laser radiation is actually trans- formed from infrared into the extreme ultraviolet range, but this loss can be offset by the use of very powerful laser sources. »It’s a simple calculation: the more we put in, the more we get out,« adds Fuchs. Strong image contrasts are produced The advantage of XUV coherence tomo- graphy is that, in addition to the very high resolution, the radiation interacts strongly with the sample, because dif- ferrent substances react differently to light. Some absorb more light and others less. This produces strong con- trasts in the images, which provide the researchers with important information, for example regarding the material com- position of the object being examined. »For example, we have created three-di- mensional images of silicon chips, in a non-destructive way, on which we can distinguish the substrate clearly from structures consisting of other materi- als,« adds Silvio Fuchs. »If this procedu- re were applied in biology—for investi- gating cells, for example, which is one of our aims—it would not be necessary to colour samples, as is normal practice in other high-resolution microscopy me- The generation and application of ultraviolet radiation has become a key topic at a num- ber of research institutes in Jena. Physics doctoral candidate Robert Klas can be seen here behind such a test setup. He and his colleagues in the team led by Prof. Jens Limpert focus laser pulses in a birefringent crystal, which doubles the frequency of the original infrared light. As a result, laser pulses in the green wavelength range are generated. In a second step in what is called cascaded frequency conversion, these pulses are focused again, resulting in pulses of an even higher frequency in the extreme ultraviolet range (XUV). Limpert and his team are leaders in, among other things, the development of ultrashort pulse lasers with a very high medium-range performance, which can then be used to produce intensive XUV radiation. F E AT U R E