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Thin metasurfaces instead of thick lenses

11.08.2023 - Producing metastructures on large surfaces with electron beam lithography.

Metasurfaces can help to make optical systems thinner in the future, while at the same time increasing their func­tionality. The problem: Until now, conventional manu­facturing processes have often only been able to realize small metasurfaces, often smaller than one square millimeter. German researchers at Fraunhofer IOF have now succeeded for the first time in producing a metasurface with a diameter of almost 30 centimeters using electron beam lithography – a world record. 

“After 500 years of lenses and mirrors, it is time to think ahead,” explains Falk Eilenberger, head of the Department of Micro- and Nano­structured Optics at the Fraunhofer Institute for Applied Optics and Precision Engineering IOF. Meta­surfaces can be an alternative here. These are components that concentrate their complete optical function in a surface and achieve this function in the surface through nano­structures. Eilenberger explains the difference to the classic lens as follows: “In lenses, the function is defined by the macro­scopic geometry. That's why the lens is thick and curved. Now we have a meta­surface instead. It's thin and on scales smaller than the wavelength of 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 to become a real alternative to the classic lens in the future. The researchers have therefore 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 inventors of meta­surfaces,” Eilenberger holds. “But we are the only ones who can demonstrate it on such a large scale.”

“To produce our meta­surface, we used a special writing strategy of electron beam litho­graphy – character projection,” explains Uwe Zeitner, a researcher at Fraunhofer IOF. Character projection is a method in which a pattern is divided into smaller units. An electron beam is then used to create each of these small patterns in turn on a surface. This enables the fabri­cation of complex structures with high precision and effi­ciency. “Using character projection, very high-resolution structures can be exposed in parallel at comparatively high speeds. This is unusual for electron beam litho­graphy,” Zeitner continues.

Together with his colleagues Michael Banasch and Marcus Trost, Zeitner has show that conventional litho­graphic techniques often reach their limits for fabricating larger structures. “Due to the small structure dimensions below the wavelength, high-resolution electron beam lithography is in principle very well suited for the fabrication of meta­structures,” the researcher said. “However, this technology is relatively slow. So far essentially only elements with relatively small areas have been realized with it – primarily on the order of a few square millimeters. For larger areas, the exposure time very quickly reaches unrealisti­cally large values.” By using character projection, the scientists were now able to address both the high resolution of electron beam lithography and the large element area without the exposure time exploding, according to Uwe Zeitner.

The new manu­facturing technology can help build optical systems signi­ficantly thinner in the future. “This technology can revolutionize imaging optical systems,” says Falk Eilen­berger, for example. “Because it will make it possible to reduce the size of systems while simultaneously increasing their optical functionality.” Uwe Zeitner adds concrete appli­cation examples: “Such large metasurfaces are parti­cularly advan­tageous for compact optics in which large deflection angles are required in a small space. This is the case, for example, in virtual/augmented reality glasses. Advan­tageous designs can also be realized with such approaches for very small optics in smartphones.” Other potential appli­cations include high-resolution spectro­scopy or computer-generated holograms. (Source: Fh.-IOF)

Reference: U. D. Zeitner et al.: Potential of E-beam lithography for micro- and nano-optics fabrication on large areas, J. Micro/Nano. Mat. Metro. 22, 041405 (2023); DOI: 10.1117/1.JMM.22.4.041405

Link: Micro- and Nanostructured Optics, Fraunhofer Institute for Applied Optics and Precision Engineering IOF, Jena, Germany

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