Almost a decade ago, Harvard engineers unveiled the world’s first visible-spectrum metasurfaces — ultra-thin, flat devices patterned with nanoscale structures that would precisely control the behavior of sunshine. A robust alternative to traditional, bulky optical components, metasurfaces today enable compact, lightweight, multifunctional applications starting from imaging systems and augmented reality to spectroscopy and communications.
Now, researchers within the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) are doubling down, literally, on metasurface technology by making a bilayer metasurface, manufactured from not one, but two stacked layers of titanium dioxide nanostructures. Under a microscope, the brand new device looks like a dense array of stepped skyscrapers.
The research is published in Nature Communications.
“This can be a feat of nanotechnology at the best level,” said senior creator 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 brand new option to structure light, by which we will engineer all its points reminiscent of wavelength, phase and polarization in an unprecedented manner…It signifies a brand new avenue for metasurfaces that to date has been just scratching the surface.”
For hundreds of years, optical systems have relied on bulky, curved lenses manufactured from glass or plastic to bend and focus light. The SEAS-led metasurface revolution of the last decade has produced flat, ultra-thin structures patterned with tens of millions of tiny elements that may manipulate light with nanometer precision. A striking example technology is the metalens: Unlike conventional lenses, metalenses may be fabricated with existing semiconductor manufacturing, making possible compact, integrated optical systems in devices like smartphones, cameras, and augmented reality displays.
After Capasso’s team reported their first working metalens that may bend visible light, they worked with Harvard’s Office of Technology Development to license the technology and begin an organization, Metalenz. They’ve since demonstrated a number of potential applications, including an endoscope, a synthetic eye, and a telescope lens.
However the single-layer nanostructure design Capasso’s team invented has been in some ways limiting. For instance, previous metasurfaces put specific requirements on the manipulation of sunshine’s polarization — that’s, the orientation of the sunshine waves — to be able to control the sunshine’s behavior.
“Many individuals had investigated the theoretical possibility of a bilayer metasurface, but the true bottleneck was the fabrication,” said Alfonso Palmieri, graduate student and co-lead creator of the study. With this breakthrough, Palmieri explained, one could imagine latest sorts of multifunctional optical devices — for instance, a system that projects one image from one side and a very different image from the opposite.
Using the facilities of the Center for Nanoscale Systems at Harvard, the team that included former postdoctoral researchers Ahmed Dorrah and Joon-Suh Park got here up with a fabrication process for freestanding, sturdy structures of two metasurfaces that hold strongly together but don’t affect one another chemically. While such multi-level patterning has been common within the silicon semiconductor world, it had not been as well explored in optics and metaoptics.
To exhibit the ability of their device, the team devised an experiment by which they used their bilayer metalens to act on polarized light in the identical way that a sophisticated system of waveplates and mirrors does.
In future experiments, the team could expand into much more layers to exert control over other points of sunshine, reminiscent of extreme broadband operation with high efficiency across all the visible and near infrared spectrum, opening the door to much more sophisticated light-based functionalities.
The research was supported by several federal funding sources, including the Office of Naval Research under grant No. N00014-20-1-2450, and from the Air Force Office of Scientific Research under grant No.s FA9550-21-1-0312 and FA9550-22-1-0243. The devices were made on the Harvard University Center for Nanoscale Systems, a part of the National Nanotechnology Coordinated Infrastructure Network, which is supported by the National Science Foundation under NSF award No. ECCS-2025158.
Staff acknowledgments: Stephan Kraemer supported the focused ion beam process, and Mac Hathaway supported the atomic layer deposition process.
