A titanium dioxide metamaterial lens

A titanium dioxide metamaterial lens

John A. Paulson School of Engineering and Applied Sciences/Harvard University

It might be small, but it’s a big feat. A lens has been built that is thinner than the waves of light it focuses. Such lenses, made from light-warping metamaterials, might someday replace the heavier glass lenses used in everything from microscopes to phone cameras.

In a normal lens, a curved glass surface a few millimetres or even centimetres thick redirects light rays to a common focal point. To improve the image – say, to take out distortions, or make sure different wavelengths of light all get focused correctly – you have to keep adding glass layers.

As a result, cameras, microscopes and telescopes are limited in part by the size and heft of the lenses they require.

“Virtual reality has the same problem,” says Reza Khorasaninejad, who designed the new lens with a team led by Federico Capasso of Harvard University. “They want to have high-resolution imaging systems, but what they will end up having is heavy helmets.”


Metamaterials, by contrast, can bend light towards a common point using structures that are as small or smaller than the wavelengths of the light waves themselves. “Our lens is flat, but I call it virtual curvature,” Khorasaninejad says.

Tiny Stonehenge

Using a beam of electrons, the team carved “nanofins” – 600-nanometre-tall blocks that together resemble the world’s smallest Stonehenge – out of a block of titanium dioxide. They mounted that lens on a thin piece of glass that provides a rigid backing, without focusing any light itself. Across the titanium oxide lens, the nanofins are rotated at different angles to catch the polarised light, which lets them pull light rays together.

They tested three lenses, tuned to red, green, and violet light. Each could focus light more sharply than a 55-millimetre-thick Nikon microscope lens with similar optical properties – even though the 600-nanometre-thick metamaterial lens was 100,000 times thinner than the Nikon. And unlike previous metamaterial lenses that handle visible wavelengths, this design doesn’t lose much light in the process.

An image of onion shell

An image of onion shell, seen though the new lens

John A. Paulson School of Engineering and Applied Sciences/Harvard University

“This is an entirely novel way of constructing a lens,” says John Pendry of Imperial College London. It still can’t compete with a glass lens in handling different colours at once, he points out. “But many applications can work with monochromatic light,” he says.

The team has plans to expand the lens’s colour range. “The first step was to make a lens that at least can do a good job in the visible,” Khorasaninejad says. “The next step is to do colour correction.”

Journal reference: Science, DOI: 10.1126/science.aaf6644

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