XUV-Coherence-Tomography for Optometrists

Silvio Fuchs in a lab of the Friedrich Schiller University Jena developed the first XUV coherence tomography at laboratory scale. (Source: J.-P. Kasper, FSU Jena)

A visit to the opto­metrist often involves optical coherence tomo­graphy. This imaging process uses infrared radia­tion to pene­trate the layers of the retina and examine it more closely in three dimensions, without having to touch the eye at all. This allows eye specia­lists to diagnose diseases such as glaucoma without any physical inter­vention. However, this method would have even greater potential for science if a shorter radiation wave­length were used, thus allowing a higher reso­lution of the image. Physicists at Friedrich Schiller Uni­versity Jena, Germany, have now achieved just that.

For the first time, the Uni­versity physi­cists used extreme ultraviolet radiation (XUV) for this process, which was generated in their own laboratory, and they were thus able to perform the first XUV coherence tomo­graphy at labora­tory scale. This radia­tion has a wave­length of between 20 and 40 nano­metres. “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 Institute of Optics and Quantum Electronics of the Jena Uni­versity. “This makes such a research method very complex and costly, and only available to a few researchers.” The physi­cists from Jena have already demons­trated this method at large 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,” explains 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 trans­formed 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.

The advan­tage of XUV coherence tomo­graphy is that, in addition to the very high reso­lution, the radia­tion interacts strongly with the sample, because differrent substances react dif­ferently to light. Some absorb more light and others less. This produces strong contrasts in the images, which provide the researchers with important infor­mation, for example regarding the material compo­sition of the object being examined. “For example, we have created three-dimen­sional images of silicon chips, in a non-destructive way, on which we can distin­guish the substrate clearly from structures consisting of other materials,” adds Silvio Fuchs. “If this procedure were applied in biology – for inves­tigating 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 methods. Elements such as carbon, oxygen and nitrogen would themselves provide the contrast.”

Before that is possible, however, the physicists of the University of Jena still have some work to do. “With the light sources we have at the moment, we can achieve a depth reso­lution down to 24 nano­metres. Although this is sufficient for producing images of small structures, for example in semi­conductors, the structure sizes of current chips are in some cases already smaller. However, with new, even more powerful lasers, it should be possible in future to achieve a depth reso­lution of as little as three nano­metres with this method,“ notes Fuchs. “We have shown in prin­ciple that it is possible to use this method at labora­tory scale.” The long-term aim could ulti­mately be to develop a cost-ef­fective and user-friendly device combining the laser with the micro­scope, which would enable the semi­conductor industry or bio­logical labora­tories to use this imaging technique with ease. (Source: FSU Jena)

Reference: Silvio Fuchs et al.: Optical coherence tomography with nanoscale axial resolution using a laser-driven high-harmonic source, Optica 4, 903 (2017); DOI: 10.1364/OPTICA.4.000903

Link: Inst. of Optics and Quantum Electronics, Friedrich Schiller University Jena, Jena, Germany

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