Constantly expanding the field of perception into dimensions that are initially hidden from the naked eye, drives science forward. Today, increa­singly powerful micro­scopes let us see into the cells and tissues of living organisms, into the world of micro­organisms as well as into inanimate nature. But even the best micro­scopes have their limits. “To be able to observe structures and processes down to the nanoscale level and below, we need new methods and techno­logies,” says Silvio Fuchs from the Institute of Optics and Quantum Electronics at the University of Jena. This applies in particular to techno­logical areas such as materials research or data processing.

“These days, electronic components, computer chips or circuits are becoming increa­singly small,” adds Fuchs. Together with colleagues, he has now developed a method that makes it possible to display and study such tiny, complex structures and even see inside them without destroying them. Now, the researchers present a new method – Coherence Tomography with Extreme Ultra­violet Light (XCT for short) – and show its potential in research and application.

The imaging procedure is based on optical coherence tomo­graphy (OCT), which has been established in ophthal­mology for a number of years, explains doctoral candidate Felix Wiesner. “These devices have been developed to examine the retina of the eye non-invasively, layer by layer, to create three-dimensional images.” At the ophthal­mologist, OCT uses infrared light to illuminate the retina. The radiation is selected in such a way that the tissue to be examined does not absorb it too strongly and it can be reflected by the inner structures. However, the physicists in Jena use extremely short-wave UV light instead of long-wave infrared light for their OCT. “This is due to the size of the structures we want to image,” says Felix Wiesner. In order to look into semi­conductor materials with structure sizes of only a few nanometers, light with a wavelength of only a few nanometers is needed.

Generating such extremely short-wave UV light (XUV) used to be a challenge and was almost only possible in large-scale research faci­lities. Jena physicists, however, generate broadband XUV in an ordinary laboratory and use high harmonics for this purpose. This is radiation that is produced by the inter­action of laser light with a medium and it has a frequency many times that of the original light. The higher the harmonic order, the shorter the resulting wave­length. “In this way, we generate light with a wave­length of between 10 and 80 nanometers using infrared lasers,” explains Gerhard Paulus, Professor of Nonlinear Optics at the University of Jena. “Like the irradiated laser light, the resulting broad­band XUV light is also coherent, which means that it has laser-like properties.”

The physicists exposed nano­scopic layer structures in silicon to the coherent XUV radiation and analysed the reflected light. The silicon samples contained thin layers of other metals, such as titanium or silver, at different depths. Because these materials have different reflective properties from the silicon, they can be detected in the reflected radiation. The method is so precise that not only can the deep structure of the tiny samples be displayed with nanometer accuracy, but – due to the differing reflective behaviour – the chemical compo­sition of the samples can also be deter­mined precisely and, above all, in a non-destructive manner. “This makes coherence tomo­graphy an interesting appli­cation for inspecting semi­conductors, solar cells or multilayer optical components,” says Paulus. It could be used for quality control in the manu­facturing process of such nano­materials, to detect internal defects or chemical impurities. (Source: U. Jena)

Reference: F. Wiesner et al.: Material-specific imaging of nanolayers using extreme ultraviolet coherence tomography, Optica 8, 230 (2021); DOI: 10.1364/OPTICA.412036

Link: Nonlinear Optics, Institute of Optics and Quantum Electronics, Friedrich Schiller University Jena, Jena, Germany