New ultra-thin lens to make cameras, telescopes lighter

Scientists have developed the first ultra-thin flat lens that works with high efficiency within the visible spectrum of light and could replace heavy, bulky lenses in cameras, telescopes and microscopes.
Curved lenses, like those in cameras or telescopes, are stacked in order to reduce distortions and resolve a clear image. That is why high-power microscopes are so big and telephoto lenses so long.
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 colours 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.
"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, Professor at Harvard.
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.

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

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.

"The more tightly you can focus light, the smaller your focal spot can be, which potentially enhances the resolution of the image," Khorasaninejad said.
The team designed the array to resolve a structure smaller than a wavelength of light, around 400 nanometres across. At these scales, the metalens could provide better focus than a state-of-the art commercial lens.
"Our lens can be produced in a single step - one layer of lithography and you have a high performance lens, with everything where you need it to be," Wei Ting Chen, coauthor and a postdoctoral fellow in the Capasso Lab.
"This technique reduces weight and volume and shrinks lenses thinner than a sheet of paper. Imagine the possibilities for wearable optics, flexible contact lenses or telescopes in space," said Khorasaninejad.

The research was published in the journal Science.