A Tabletop OCT Approach Utilizes Extreme Ultraviolet Radiation

Three-dimensional noninvasive imaging with nanoscale axial resolution has been demonstrated using a high-harmonic extreme ultraviolet (XUV) radiation source. Specifically, XUV radiation was used to perform optical coherence tomography (OCT) at laboratory scale.

Silvio Fuchs in a laboratory of the Institute of Optics and Quantum Electronics of the Friedrich Schiller University Jena. Courtesy of Jan-Peter Kasper/FSU Jena.

Use of XUV to perform OCT allows for a shorter radiation wavelength to be used — 20-40 nm — leading to a higher-resolution image. From the 20-40 nm wavelength, it is just a small step to the x-ray range, according to the University of Jena research team.

The researchers have demonstrated OCT with XUV radiation at large research facilities already. They have now discovered a potential way for applying it at a smaller scale, using an ultrashort, intense IR laser, which is focused in a noble gas, such as argon or neon.

“The electrons in the gas are accelerated by means of an ionization process. They then emit the XUV radiation,” said researcher Silvio Fuchs.

According to the researcher, the inefficiency inherent in this approach can be offset by using very powerful lasers.

The broadband photon flux of the high harmonic generation (HHG) source for the laboratory-scale XUV was efficiently utilized. A depth resolution of 24 nm and good material contrast were achieved. Researchers used a novel three-step one-dimensional phase-retrieval algorithm to avoid excessively demanding optics for XUV radiation and to suppress artifacts due to the elementary geometry. The images were recorded in reflection geometry, facilitating the analysis of, for example, operating semiconductor samples.

“Large-scale equipment, that is to say particle accelerators such as the German Elektronen-Synchotron in Hamburg, are usually necessary for generating XUV radiation,” said Fuchs. “This makes such a research method very complex and costly, and only available to a few researchers."

One advantage of this method is that with XUV OCT, the radiation interacts strongly with the sample, producing high-contrast, well-defined images.

“For example, we have created three-dimensional images of silicon chips, in a nondestructive way, on which we can distinguish the substrate clearly from structures consisting of other materials,” said 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 color samples, as is normal practice in other high-resolution microscopy methods. Elements such as carbon, oxygen and nitrogen would themselves provide the contrast.”

The researchers believe that development of future laser sources will reduce the measurement time and could open the water window for extreme ultraviolet coherence tomography (XCT), enabling an axial resolution of 3 nm in biological materials.

“With the light sources we have at the moment, we can achieve a depth resolution down to 24 nm. 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 the future to achieve a depth resolution of as little as 3 nm with this method,” said Fuchs. “We have shown in principle that it is possible to use this method at laboratory scale.”

One potential long-term goal for the research team is the development of 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.

The research was published in Optica, a publication of The Optical Society (doi: 10.1364/OPTICA.4.000903).