Metalens Enables Full-Color Imaging

Illustration showing a comparison between two types of flat lenses. In the foreground, a new type of flat lens focuses all colors of light to the same spot. As a comparison, the flat lens in the background is not color-corrected. (Source: A. Overvig, Columbia Eng.)

An ordinary lens cannot focus light of different colors to a single spot due to dispersion. This means different colors are never in focus at the same time, and so an image formed by such a simple lens is inevi­tably blurred. Conventional imaging systems solve this problem by stacking multiple lenses, but this solution comes at the cost of increased complexity and weight. Columbia Engi­neering researchers have created 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.

Only a micron thick, their revo­lutionary flat lens is much thinner than a sheet of paper and offers performance comparable to top-of-the-line compound lens systems. With the goal of inventing a thinner, lighter, and cheaper lens, Nanfang Yu’s team took a different approach. Using their expertise in optical meta­surfaces to control light propa­gation in free space, the researchers built flat lenses made of pixels. This 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, the researchers were able to achieve the same function as a much thicker and heavier conven­tional lens system. Looking to the future, they anticipate that the meta-lenses could replace bulky lens systems, comparable to the way flat-screen TVs have replaced cathode-ray-tube TVs.

“The beauty of our flat lens is 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 they are so thin, they have the potential to dras­tically reduce the size and weight of any optical instrument or device used for imaging, such as cameras, micro­scopes, telescopes, and even our eye­glasses. Think of a pair of eyeglasses with a thickness thinner than a sheet of paper, smartphone cameras that do not bulge out, thin patches of imaging and sensing systems for driverless cars and drones, and miniaturized tools for medical imaging appli­cations.”

Yu’s team fabri­cated the meta-lenses using standard 2D planar fabri­cation techniques similar to those used for fabricating computer chips. They say the process of mass manu­facturing meta-lenses should be a good deal simpler than producing computer chips, as they need to define just one layer of nano­structures – in comparison, modern computer chips need numerous layers, some as many as 100. The advantage of the flat meta-lenses is that, unlike conven­tional lenses, they do not need to go through the costly and time-consuming grinding and polishing processes.

“The production of our flat lenses can be massively paralle­lized, yielding large quantities of high perfor­mance and cheap lenses,” notes Sajan Shrestha, a doctoral student in Yu’s group. “We can therefore send our lens designs to semi­conductor foundries for mass produc­tion and benefit from economies of scale inherent in the industry.” Because the flat lens can focus light with wave­lengths ranging from 1.2 to 1.7 microns in the near-infrared to the same focal spot, it can form colorful images in the near-infrared band because all of the colors are in focus at the same time. The lens can focus light of any arbi­trary polari­zation state, so that it works not only in a lab setting, where the polarization can be well controlled, but also in real world appli­cations, where ambient light has random polarization. It also works for trans­mitted light, convenient for integration into an optical system.

“Our design algorithm exhausts all degrees of freedom in sculpting an interface into a binary pattern, and, as a result, our flat lenses are able to reach perfor­mance approaching the theoretic limit that a single nano­structured interface can possibly achieve,” Adam Overvig, also a doctoral student with Yu, says. “In fact, we’ve demon­strated a few flat lenses with the best theo­retically possible combined traits: for a given meta-lens diameter, we have achieved the tightest focal spot over the largest wave­length range.”

Adds Uni­versity of Penn­sylvania H. Nedwill Ramsey Professor Nader Engheta, an expert in nano­photonics and meta­materials who was not involved with this study: “This is an elegant work from Nanfang Yu’s group and it is an exciting development in the field of flat optics. This achromatic meta-lens, which is the state-of-the-art in engineering of meta­surfaces, can open doors to new inno­vations in a diverse set of appli­cations involving imaging, sensing, and compact camera tech­nology.”

Now that the meta-lenses built by Yu and his colleagues are approaching the perfor­mance of high-quality imaging lens sets, with much smaller weight and size, the team has another challenge: improving the lenses’ efficiency. The flat lenses currently are not optimal because a small fraction of the incident optical power is either reflected by the flat lens, or scattered into unwanted direc­tions. The team is opti­mistic that the issue of efficiency is not funda­mental, and they are busy inventing new design strategies to address the effi­ciency problem. They are also in talks with industry on further deve­loping and licensing the tech­nology. (Source: CUSEAS)

Reference: S. Shrestha et al.: Broadband Achromatic Dielectric Metalenses, Light Sci. & App., online 3 October 2018; DOI: 10.1038/s41377-018-0078-x

Link: Nanfang Yu Group, Dept. of Electrical Engineering, Columbia University, New York, USA

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