Ultrathin lens gives perfect image

Posted by Emma Woollacott

A new, ultra-thin flat lens has a focusing power at the very edge of what's theoretically possible, promising a distortion-free image.

Applied physicists at the Harvard School of Engineering and Applied Sciences (SEAS)  say that, at just 60 nanometers thick, the lens is essentially two-dimensional, yet has a focusing power close to the ultimate physical limit set by the laws of diffraction.

Operating at the wavelengths commonly used in fiber-optic communications, the device is completely scalable, from near-infrared to terahertz wavelengths, and simple to manufacture, they say.

"Our flat lens opens up a new type of technology,” says principal investigator Federico Capasso.

"We’re presenting a new way of making lenses. Instead of creating phase delays as light propagates through the thickness of the material, you can create an instantaneous phase shift right at the surface of the lens. It’s extremely exciting."

The team created the flat lens by plating a very thin wafer of silicon with a nanometer-thin layer of gold. Next, they stripped away parts of the gold layer to leave behind an array of V-shaped structures, evenly spaced in rows across the surface.

When a laser is shone onto the flat lens, these structures act as nanoantennas that capture the incoming light and hold onto it briefly before releasing it again. The delays  are precisely tuned across the surface of the lens, and change the direction of the light in the same way that a thick glass lens would - with one important difference.

The flat lens eliminates optical aberrations such as the fish-eye effect that results from conventional wide-angle lenses. Nor does it lead to astigmatism and coma aberrations, so that the resulting imageis completely accurate and doesn't need corrective techniques.

The array of nanoantennas can be tuned for specific wavelengths of light by simply changing the size, angle, and spacing of the antennas.

"In the future we can potentially replace all the bulk components in the majority of optical systems with just flat surfaces," says visiting graduate student Francesco Aieta.