Creating Advanced Metasurfaces with Titanium Dioxide Layers

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) are producing a bilayer metasurface composed of two stacked layers of titanium dioxide nanostructures.

Nearly ten years ago, engineers at Harvard introduced the world’s first visible-spectrum metasurfaces. These ultra-thin, flat devices were patterned with nanoscale structures capable of precisely controlling the behavior of light.

Metasurfaces offer a powerful alternative to traditional, bulky optical components, enabling compact, lightweight, and multifunctional applications in areas such as spectroscopy, communications, imaging systems, and augmented reality.

This is a feat of nanotechnology at the highest level. 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.

Federico Capasso, Study Senior Author, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow, John A. Paulson School of Engineering and Applied Sciences, Harvard University

For centuries, optical systems relied on large, curved lenses made of glass or plastic to bend and focus light. The metasurface revolution of the past decade, led by SEAS, has resulted in flat, ultra-thin surfaces designed with millions of small elements that can manipulate light with nanometer precision.

Metalenses are a notable example of this technology. Unlike traditional lenses, metalenses can be manufactured using existing semiconductor techniques, enabling the creation of compact, integrated optical systems for devices such as smartphones, cameras, and augmented reality displays.

After Capasso’s team developed the first working metalens capable of bending visible light, they partnered with Harvard’s Office of Technology Development to license the technology and establish Metalenz. They have since demonstrated a range of potential applications, including an endoscope, an artificial eye, and a telescope lens.

However, the single-layer nanostructure design developed by Capasso's team has some limitations. Previous metasurfaces, for instance, required precise manipulation of light polarization—the orientation of light waves—to control light behavior.

Many people had investigated the theoretical possibility of a bilayer metasurface, but the real bottleneck was the fabrication.

Alfonso Palmieri, Study Co-Lead Author and Graduate Student, John A. Paulson School of Engineering and Applied Sciences, Harvard University

The development enables the creation of multifunctional optical devices, such as a system that can project one image from one side and a completely different image from the other.

Using the facilities at Harvard's Center for Nanoscale Systems, the team—comprising former postdoctoral researchers Ahmed Dorrah and Joon-Suh Park—developed a fabrication process for freestanding, robust structures composed of two metasurfaces that remain strongly bonded without chemically interacting with each other.

Although multi-level patterning is common in the silicon semiconductor industry, it has not been widely explored in optics and metaoptics.

To demonstrate the device's capabilities, the scientists designed an experiment in which the bilayer metalens was used to manipulate polarized light, similar to how a system of waveplates and mirrors would work.

The team envisions further developments in controlling light with more layers, potentially enabling advanced light-based features such as efficient broadband operation across the visible and near-infrared spectrum.

The study received support from several federal funding sources, including the Air Force Office of Scientific Research (grant numbers FA9550-21-1-0312 and FA9550-22-1-0243) and the Office of Naval Research (grant number N00014-20-1-2450).

The devices were created at Harvard University’s Center for Nanoscale Systems, which is part of the National Nanotechnology Coordinated Infrastructure Network, funded by the National Science Foundation under NSF award number ECCS-2025158.

Journal Reference:

Dorrah, A. H. et. al. (2025) Free-standing bilayer metasurfaces in the visible. Nature Communications. doi.org/10.1038/s41467-025-58205-7