The miniaturization of electronics, in particular the electronic sensors on which digital images are captured, has seen digital cameras shrink to such a degree that they are now standard equipment on mobile phones. The main thing holding back further downsizing is the lens through which the light is focused onto said image sensor. A team of applied physicists from Harvard University has now overcome this roadblock by creating a lens that, at just 60 nanometers thick, is effectively two-dimensional. Not only that, the ultrathin lens focuses light without the distortions seen in conventional lenses.

Federico Capasso, Francesco Aieta and Patrice Genevet from the Harvard School of Engineering and Applied Sciences (SEAS) created the new lens by starting with an ultrathin wafer of silicon, which they plated with a nanometer-thin layer of gold. They then stripped away parts of this gold layer to leave an array of V-shaped structures in evenly spaced rows across the surface.

These structures act as nanoantennas that, when a laser is shined onto the lens, capture and hold the incoming light briefly before releasing it again. In the same way that a traditional glass lens would, these precisely tuned delays change the direction of the light, but without the optical aberrations seen in conventional lenses. In fact, its creators say the lens produces images that are completely accurate and don't require any complex techniques to correct the “fish-eye” effect and astigmatism and coma aberrations that can occur with traditional lenses.

The surface of the flat lens (left) is coated with concentric rings of gold optical nanoantennas (inset) that impart phase delays of varying magnitude (right) (Image: Francesco Aieta)

The new lens operates at near-infrared wavelengths, but is scalable to terahertz wavelengths with the nanoantennas able to be tuned for specific wavelengths of light by changing their size, angle and spacing. The researchers say their new lens is also easy to manufacture.

“Our flat lens opens up a new type of technology,” says principal investigator Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS. “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.”

“In the future we can potentially replace all the bulk components in the majority of optical systems with just flat surfaces,” adds lead author Aieta, a visiting graduate student from the Università Politecnica delle Marche in Italy.

The team's paper is published online in the journal Nano Letters.

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