Curved lenses, like those in cameras or telescopes, are stacked in order to reduce distortions and resolve a clear image. A new fabrication method from Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) replaces the stacks with a single flat lens.

The SEAS researchers have demonstrated the first planar lens that works within the visible spectrum of light — covering the whole range of colors from red to blue. The lens resolves nanoscale features separated by distances smaller than the wavelength of light.

A metasurface, an ultrathin array of tiny waveguides, bends light as it passes through.

“This technology is potentially revolutionary because it works in the visible spectrum, which means it has the capacity to replace lenses in all kinds of devices, from microscopes to cameras, to displays and cell phones,” said Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering and senior author of the paper.

In order to focus red, blue and green light — light in the visible spectrum — the team needed a material that would not absorb or scatter light.

The team decided on titanium dioxide, a ubiquitous material found in everything from paint to sunscreen, to create the nanoscale array of smooth and high-aspect ratio nanostructures that form the heart of the metalens.

The array was designed to resolve a structure smaller than a wavelength of light, around 400 nanometers across. At these scales, the metalens could provide better focus than a state-of-the art commercial lens.

Effective imaging systems are often heavy because the thick lenses must be stacked on top of each other. The new technique recduces weight and enables new medical applications, including flexible contact lenses, wearable optics, and augmented reality.

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