Ultra-thin meta-lens enables fully-focused all-color imaging

04 Oct 2018

Development at Columbia University overcomes problem of dispersion of different wavelengths; diverse applications.

Light of different wavelengths travels at different speeds in different materials and structures. But an ordinary lens cannot focus light of different wavelengths to a single spot due to dispersion effects. This, say a team of researchers at the University of Columbia’s Engineering Department, NY, means different colors are never in focus at the same time: “so an image formed by such a simple lens is inevitably blurred”.

Conventional imaging systems can solve this problem by stacking multiple lenses, but this approach has the disadvantages of increased device size, complexity and weight. This increases costs and reduces the potential for integrated dives for various imaging and display purposes, say the team.

The Columbia Engineering researchers have created the first flat lens capable of correctly focusing a large range of wavelengths of any polarization to the same focal point without the need for any additional elements. At just 1µm thick, their revolutionary "flat" lens offers performance comparable to top-of-the-line compound lens systems. The findings of the team, led by Nanfang Yu, associate professor of applied physics , are outlined in a new study, published in Light: Science & Applications.

With the goal of inventing a thinner, lighter, and cheaper lens, Yu's team deployed their expertise in optical metasurfaces – engineered two-dimensional structures that control light propagation in free space. The researchers built flat lenses made of pixels, or meta-atoms. Each meta-atom has a size that is just a fraction of the wavelength of light, which delays the light passing through it by a different amount of time.

By patterning a thin flat layer of nanostructures on a substrate of approximately 100 µm thickness, the researchers were able to achieve the same functionality as a much thicker and heavier conventional lens system. Looking to the future, Yu believes that such meta-lenses could replace bulky lens systems, comparable to the way flat-screen TVs have replaced cathode-ray-tube TVs.

Diverse applications

"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 wavelength of light but also for a continuous spectrum of light," commented Yu. "Because they are so thin, these lenses 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 spectacles."

Yu's team fabricated the meta-lenses using standard 2-D 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 production of our flat lenses can be massively parallelized, yielding large quantities of high performance and cheap lenses," said Sajan Shrestha, a doctoral student in Yu's group who was co-lead author of the study. "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 µm to 1.7µm 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 and place. 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. This arrangement also works for transmitted light, convenient for integration into an optical system.

University of Pennsylvania H. Nedwill Ramsey Professor Nader Engheta, an expert in nanophotonics and metamaterials, who was not involved with this study, commented, "This is an elegant work from Professor 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."

Future plans

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’s next challenge is to improve these 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.

The following video describes the metalens work at the University of Columbia’s Engineering Department: