Revolutionary ultra-thin meta-lens enables color imaging

Light of different colors moves at different speeds in different materials and structures. This is why we see white light splitting into its constituent colors after refraction through a prism, a phenomenon called dispersion. An ordinary lens can not focus a light of different colors on a single point because of the dispersion. This means that different colors are never crisp at the same time, so that an image formed by such a simple lens is inevitably blurred. Conventional imaging systems solve this problem by stacking several objectives, but this solution results in increased complexity and weight.

Columbia Engineering researchers created the first flat lens that can correctly focus a wide range of colors of any polarization on the same focus without any additional elements. From a micron to a thickness, their revolutionary "flat" lens is much thinner than a sheet of paper and offers performance comparable to that of high-end compound lens systems. The findings of the team, led by Nanfang Yu, associate professor of applied physics, are described in a new study published today by Light: Science & Applications.

A conventional lens works by routing all the light that falls on it through different paths, so that the entire light wave arrives at the focal point at the same time. It is designed to do this by adding a longer and longer delay to the light that passes from the edge to the center of the lens. This is why a conventional lens is thicker in its center than in its edge.

In order to invent a leaner, lighter and cheaper goal, Yu's team has taken a different approach. Using their expertise in optical "metasurfaces" – two-dimensional structures designed by engineering – to control the propagation of light in a free space, researchers constructed flat lenses based on pixels, or "meta-atoms". Each meta-atom has a size that represents only a fraction of the wavelength of light and delays the light that passes through it in a different amount. By creating a very thin, flat layer of nanostructures on a substrate as thin as a human hair, researchers have been able to fulfill the same function as a conventional system of much thicker and heavier conventional lenses. Looking ahead, they predict that meta-lenses could replace bulky lens systems, which is comparable to the way flat-screen televisions have replaced CRT TVs.

"The beauty of our flat lens lies in the fact that by using meta-atoms of complex shapes, it not only provides the correct distribution of delay for a single color of light, but also for a continuous spectrum of light" , Yu says. "And, because of their small thickness, they can potentially significantly reduce the size and weight of any optical instrument used for imaging purposes, such as cameras, microscopes, telescopes and even our glasses Think of a pair of glasses with thinner than a sheet of paper, cameras for smartphones that do not overflow, thin blocks of imaging and detection systems for driverless cars and drones, and miniaturized tools for medical imaging applications. "

Yu's team manufactured the meta-lenses using standard two-dimensional manufacturing techniques similar to those used for computer chip manufacturing. They argue that the process of mass manufacturing of meta-lenses should be much simpler than producing computer chips, as they need to define a single layer of nanostructures – in comparison, modern computer chips require many layers, some of which can go up to 100. The advantage One of the flat meta-lenses is that, unlike conventional lenses, they do not require expensive and tedious grinding and polishing processes.

"The production of our flat lenses can be massively parallelized, which allows to obtain large amounts of high-performance and economic lenses," notes Sajan Shrestha, a PhD student from Yu's group, co-author of the study. "So we can send our lens designs to semiconductor foundries for mass production and benefit from the industry-wide economies of scale."

Because the flat lens can focus light with wavelengths ranging from 1.2 to 1.7 microns in the near infrared of the same focal point, it can form "colorful" images in the near infrared band, because all colors are on the same time plane – essential for color photography. The lens can focus light of any arbitrary polarization state, so that it works not only in the laboratory, where the polarization can be well controlled, but also in real-world applications, where the ambient light has a random polarization. It also works for transmitted light, convenient for integration into an optical system.

"Our design algorithm exhausts all degrees of freedom to sculpt an interface into a binary pattern, which allows our flat lenses to achieve near-theoretical performance that a nanostructured interface can possibly achieve." , said Adam Overvig. other main co-author of the study and also a PhD student with Yu, said. "In fact, we have demonstrated some flat lenses with the best combined features theoretically possible: for a given meta-lens diameter, we have obtained the narrowest focal point possible over the widest range of wavelengths. "

Nader Engheta, professor at the University of Pennsylvania, H. Nedwill Ramsey, a specialist in nanophotonics and metamaterials who did not participate in this study: "This is an elegant work of Professor Nanfang Yu's group and an exciting development in the field of flat optics, this achromatic meta-lens, at the cutting edge of metasurfaces engineering, can pave the way for new innovations in a wide range of applications involving imaging technology, detection and compact cameras. "

Now that the meta-lenses built by Yu and his colleagues are approaching the performance of high-quality imaging lens assemblies, with a much smaller weight and size, the team is faced with to another challenge: improve the effectiveness of lenses. Flat lenses are currently not optimal because a small fraction of the incident optical power is either reflected by the flat lens or scattered in undesired directions. The team is optimistic that the issue of efficiency is not fundamental and employs to invent new design strategies to solve the problem of efficiency. They are also in talks with the industry on the development and licensing of the technology.

The study is titled "Broadband Achromatic Metalenses Metalenses".

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More information:
"Broadband achromatic dielectric metals" Light: Science & Applications (2018). DOI: 10.1038 / s41377-018-0078-x