Researchers at the Ludwig Maximilian University of Munich (LMU), in collaboration with a team at the Technical University of Munich (TUM), developed ultrathin optical components that react strongly to weak light and capture light more effectively than previous materials. These components, made from atomically layered systems, could be used to build smaller, more efficient photonic applications. For the atomic-layer assembly of these components, the researchers integrated metasurfaces with van der Waals (vdW) heterostructures. Metasurfaces are a class of nanophotonic materials that have regular patterns that are typically smaller than wavelengths of light. These tiny structures, called photonic resonators, can alter the amplitude, phase, and polarization of incident electromagnetic waves, including light. It is therefore possible to use certain metasurfaces to control the storage and amplification of light. Layered 2D materials like vdW materials can be assembled into vertical heterostructures that allow control over the atomic composition of each layer. “The best known 2D material is graphene, but actually there are quite a number of other ones now available,” professor Andreas Tittl said. “You can buy these materials in crystal form, remove individual layers under the microscope, and stack them kind of like paper.” The researchers call the ultrathin optical components made by integrating metasurfaces into multilayered vdW materials van der Waals heterostructure metasurfaces, or vdW-HMs. The vdW-HMs comprise ultrathin, multilayered vdW material stacks shaped into resonant nanostructures. “Instead of placing 2D materials on separate, ready-made nanostructures or using bulky external optical resonators, we worked the resonance structure directly into the vdW stack,” Tittl said. To build the stack, the researchers positioned a single, semiconducting layer of tungsten disulfide (WS2) between several protective layers of hexagonal boron nitride (hBN) and used a lithographic technique to incorporate periodic structures into the material stack. The structural parameters were added to amplify light-matter interactions in the vdW stack, similar to light amplification in a metasurface. Professor Andreas Tittl (left) and researcher Luca Sortino, in the laboratory at the Ludwig Maximilian University of Munich Nano-Institute. Courtesy of LMU. Light interacts efficiently with the material stack. The electrons in the material are excited by the incident light and coupled to the light particles. These hybrid light-matter particles, known as exciton-polaritons, exhibit both the properties of matter and of light, and can condense similar to a Bose-Einstein condensate, an extreme state of matter in which most of the particles exist in the same quantum mechanical state. The researchers performed theoretical modeling and simulation to ensure the highest possible interaction between light and matter and control unwanted diffraction in the vdW-HMs. They observed that, once optimized, the vdW-HMs reacted to light intensities over 1000x lower than previously reported. Light-matter interactions can occur in the nm range that cannot occur at larger scales. As such, nanophotonic components, like the vdW-HMs, offer exceptional optical properties. “Essentially, we’ve developed ultrathin resonators that capture light very efficiently so that we can use it,” researcher Luca Sortino said. “We’ve now got a toolkit, so to speak, for combining the two materials science concepts and extending this model to many other 2D materials.” The researchers plan to explore how their approach could be used to develop various nanophotonic components with customized optoelectronic characteristics. They hope the vdW-HMs will facilitate the development of ultrathin, flat optical components with new functionalities. Applications could include, for example, fast optical switches, neuromorphic computing, polariton lasers that could potentially be built directly into chips, and new platforms for researching quantum phenomena. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-025-01675-4).
