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 only provides the correct distribution of delay for a single color of light but also for a continuous spectrum of light,” says Nanfang Yu, associate professor of applied physics, who led the project. “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. 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.”
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, say the researchers, it can form “colorful” images in the near-infrared band because all of the colors are in focus at the same time – essential for color photography. 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.
The meta-lenses was fabricated using standard 2D planar fabrication techniques similar to those used for making computer chips. The process of mass manufacturing the meta-lenses, say the researchers, should be much simpler than producing computer chips, as they need to define just one layer of nanostructures compared to the numerous layers required by modern computer chips.
The advantage of the flat meta-lenses is that – unlike conventional lenses – they do not need to go through 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,” says Sajan Shrestha, a doctoral student in Yu’s group and co-lead author of a study on the project. “We can therefore send our lens designs to semiconductor foundries for mass production and benefit from economies of scale inherent in the industry.”
Looking ahead, the researchers are focused on improving the lenses’ efficiency, which is not yet optimal. Meanwhile, they are in talks with industry on further developing and licensing the technology, and anticipate that the meta-lenses could ultimately replace bulky lens systems, comparable to the way flat-screen TVs have replaced cathode-ray-tube TVs.
For more, see “Broadband achromatic optical metasurface devices.”
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