'Revolutionary' ultra-thin flat lens enables full-color imaging

October 03, 2018 // By Rich Pell
Researchers at Columbia University (New York City, NY) have created what they say is the first flat lens capable of correctly focusing a large range of colors of any polarization to the same focal spot without the need for any additional elements.

At only a micron thick, their "flat" lens is much thinner than a sheet of paper and offers performance comparable to top-of-the-line compound lens systems. The new lens, say the researchers, offers the potential to drastically reduce the size and weight of any optical instruments used for imaging, including cameras, microscopes, telescopes, and eyeglasses.

The lens addresses an issue of ordinary lenses, which cannot focus light of different colors to a single spot due to dispersion - i.e., different colors are never in focus at the same time, and so an image formed by such a simple lens is inevitably blurred. This problem is solved in traditional imaging systems through the use of stacked multiple lenses - at a cost of increased complexity and weight.

Conventional lenses work by routing all the light falling upon them through different paths so that the whole light wave arrives at the focal point at the same time. They are built to do so by adding an increasing amount of delay to the light as it goes from the edge to the center of the lens, resulting in such lenses being thicker at the center than at the edge.

With a goal of inventing a thinner, lighter, and cheaper lens, the researchers used their expertise in optical " metasurfaces" - engineered two-dimensional structures - to control light propagation in free space. They built flat lenses made of pixels - or "meta-atoms" - where each meta-atom has a size that is just a fraction of the wavelength of light and delays the light passing through it by a different amount.

By patterning a very thin flat layer of nanostructures on a substrate as thin as a human hair, say the researchers, they were able to achieve the same function as a much thicker and heavier conventional lens system.

"The beauty of our flat lens is that by using meta-atoms of complex shapes, it not

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