Thin metasurfaces, not thick lenses
(Nanowerk News) “After 500 years of lenses and mirrors, it’s time to think ahead,” explains Dr. Falk Eilenberger, head of the Department of Micro and Nanostructured Optics at the Fraunhofer Institute for Applied Optics and IOF Precision Engineering. So-called metasurfaces can be an alternative here. It is a component that concentrates its complete optical function on the surface and achieves this function on the surface through nanostructures.
Eilenberger explains the differences in classic lenses as follows: “In a lens, its function is determined by the macroscopic geometry. That’s why lenses are thick and curved. Now we have a metasurface instead. The surface is thin and scales less than the wavelength of the light.”
Metasurfaces have been used in science and research for some time. However, the components here are often only a few square millimeters in size. This is sufficient for academic research, but not for many industrial applications, and certainly not a real alternative to classic lenses in the future.
Therefore, researchers at Fraunhofer IOF in Jena, Germany have devoted themselves to the question of how innovative metasurfaces can be realized on a larger scale. As a result, they are now presenting a metasurface with a diameter of 30 centimeters for the first time.
“We are not the discoverers of metasurfaces,” says Eilenberger. “But we’re the only ones who can demonstrate it on a large scale.”
High-resolution structure with high precision and efficiency
But how did the researchers reach this milestone? The answer: with the help of electron beam lithography.
“To generate our metasurface, we use a special writing strategy of electron beam lithography called character projection,” explained Prof. Dr. Uwe Zeitner, a researcher at Fraunhofer IOF and a member of the institute’s scientific directorate.
Character projection is a method in which patterns are divided into smaller units. The electron beam is then used to create each of these tiny patterns in turn on the surface. This enables the fabrication of complex structures with high precision and efficiency.
“By using character projections, very high-resolution structures can be exposed in parallel at relatively high speeds. This is unusual for electron beam lithography,” continues Zeitner.
Together with his Fraunhofer colleague Dr. Michael Banasch and Dr. Marcus Trost, Prof. Zeitner has described the potential of electron beam lithography for the fabrication of wide-area micro- and nano-optics in a paper that has now been published in Journal of Micro/Nanopatterning, Materials, and Metrology (“Potential of E-beam lithography for micro- and nano-optical fabrication over large areas”).
With their paper, the authors show that conventional lithography techniques often reach their limits to create larger structures.
“Due to the small dimensions of the structure below the wavelength, high-resolution electron beam lithography is in principle very suitable for fabricating meta structures,” the researchers said. “However, this technology is relatively slow. So far basically only elements with relatively small areas have been realized with it – mainly on the order of a few square millimeters. For larger areas, exposure times very quickly reach unrealistically large values. ”
By using character projections, scientists can now tackle both high-resolution electron beam lithography and large areas of elements without “blowout” exposure times, according to Uwe Zeitner. Therefore, the authors of the paper demonstrate that electron beam lithography can be a technique for fabricating micro- and nano-optical structures over large areas.
Size reduction with simultaneous increase in functionality
New manufacturing technologies could help build much thinner optical systems in the future.
“This technology could revolutionize optical imaging systems,” says Falk Eilenberger, for example. “Because it will make it possible to reduce the size of the system while increasing its optical functionality.”
Uwe Zeitner adds a concrete application example: “Such a large metasurface is especially advantageous for compact optics where large angles of deflection are required in small spaces. This occurs, for example, in virtual/augmented reality glasses. Favorable designs can also be realized with such an approach for very small optics in smartphones.”
Other potential applications include high-resolution spectroscopy or computer-generated holograms.
