Almost a decade ago, Harvard engineers unveiled the world’s first visible-spectrum metasurfaces — ultra-thin, flat devices patterned with nanoscale structures that could precisely control the behavior of light. A powerful alternative to traditional, bulky optical components, metasurfaces today enable compact, lightweight, multifunctional applications ranging from imaging systems and augmented reality to spectroscopy and communications. Now, researchers in the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) are doubling down, in a literal sense, on metasurface technology by creating a bilayer metasurface, made of two stacked layers of titanium dioxide nanostructures. Under a microscope, the new device looks like a dense array of stepped skyscrapers. Scanning electron microscope images of a free-standing bilayer metasurface from several views, including top (a), tilted (b-e), and cross-sectional (f). Courtesy of Harvard SEAS. “This is a feat of nanotechnology at the highest level,” said senior author Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS. “It opens up a new way to structure light, in which we can engineer all its aspects such as wavelength, phase and polarization in an unprecedented manner…It signifies a new avenue for metasurfaces that so far has been just scratching the surface.” Unlike conventional lenses, metalenses can be fabricated with existing semiconductor manufacturing, making possible compact, integrated optical systems in devices like smartphones, cameras, and AR displays. But the single-layer nanostructure design Capasso’s team invented has been in some ways limiting. For example, previous metasurfaces put specific requirements on the manipulation of light’s polarization — that is, the orientation of the light waves — in order to control the light’s behavior. “Many people had investigated the theoretical possibility of a bilayer metasurface, but the real bottleneck was the fabrication,” said Alfonso Palmieri, graduate student and co-lead author of the study. With this breakthrough, Palmieri explained, one could imagine new kinds of multifunctional optical devices — for example, a system that projects one image from one side and a completely different image from the other. Fabrication process for free-standing bilayer metasurfaces. Courtesy of Harvard SEAS. Using the facilities of the Center for Nanoscale Systems at Harvard, the team came up with a fabrication process for freestanding, sturdy structures of two metasurfaces that hold strongly together but do not affect each other chemically. While such multi-level patterning has been common in the silicon semiconductor world, it had not been as well explored in optics and meta-optics. To demonstrate the power of their device, the team devised an experiment in which they used their bilayer meta-lens to act on polarized light in the same way that a complicated system of waveplates and mirrors does. In future experiments, the team could expand into even more layers to exert control over other aspects of light, such as extreme broadband operation with high efficiency across the entire VIS-IR and NIR spectrum, opening the door to even more sophisticated light-based functionalities. The research was published in Nature Communications (www.doi.org/10.1038/s41467-025-58205-7).