While lens technology has come a long way, it is still difficult to make a compact and thin lens (rub a finger over the back of a cellphone and you’ll get a sense of how difficult). But what if you could replace those stacks with a single flat—or planar—lens?

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have demonstrated the first planar lens that works with high efficiency within the visible spectrum of light—covering the whole range of colors from red to blue. The lens can resolve nanoscale features separated by distances smaller than the wavelength of light. It uses an ultrathin array of tiny waveguides, known as a metasurface, which bends light as it passes through, similar to a curved lens.

The research is described in the journal Science.

“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 camera, 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 the near future, metalenses will be manufactured on a large scale at a small fraction of the cost of conventional lenses, using the foundries that mass produce microprocessors and memory chips.”

“Correcting for chromatic spread over the visible spectrum in an efficient way, with a single flat optical element, was until now out of reach,” said Bernard Kress, Partner Optical Architect at Microsoft, who was not part of the research. “The Capasso group’s metalens developments enable the integration of broadband imaging systems in a very compact form, allowing for next generations of optical sub-systems addressing effectively stringent weight, size, power and cost issues, such as the ones required for high performance AR/VR wearable displays.”

In order to focus red, blue and green light—light in the visible spectrum—the team needed a material that wouldn’t absorb or scatter light, said Rob Devlin, a graduate student in the Capasso lab and co-author of the paper.

“We needed a material that would strongly confine light with a high refractive index,” he said. “And in order for this technology to be scalable, we needed a material already used in industry.”

The team used 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.

“We wanted to design a single planar lens with a high numerical aperture, meaning it can focus light into a spot smaller than the wavelength,” said Mohammadreza Khorasaninejad, a postdoctoral fellow in the Capasso lab and first author of the paper. “The more tightly you can focus light, the smaller your focal spot can be, which potentially enhances the resolution of the image.”

Source: Phys.org