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First flat lens puts focus on light polarisation

Columbia Engineering researchers have created the first flat lens to focus a large range of colours of any polarisation to the same focal point without the need for additional lenses.

The one micron thick ‘flat’ lens is claimed to offer performance comparable to high-end compound lens systems. The findings of the team, led by Nanfang Yu, associate professor of applied physics at Columbia University, New York, are detailed in Light: Science & Applications.

Conventional lenses route light through different paths so that the whole light wave arrives at the focal point simultaneously. Such lenses are manufactured to do so by adding an increasing amount of delay to the light as it goes from the edge to the centre of the lens.

With the goal of inventing a thinner, lighter, and cheaper lens, Yu’s team took a different approach. Using their expertise in optical metasurfaces – engineered two-dimensional structures – to control light propagation in free space, the researchers built flat lenses made of pixels, or so-called ‘meta-atoms’

Each meta-atom is said to have a size that is 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 very thin substrate, the researchers achieved the same function as a much thicker and heavier conventional lens system.

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“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 colour of light but also for a continuous spectrum of light,” Yu said. “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 miniaturised tools for medical imaging applications.”

Yu’s team fabricated the meta-lenses – which do not need to go through a grinding and polishing processes – using standard 2D planar fabrication techniques similar to those used for fabricating computer chips.

“The production of our flat lenses can be massively parallelised, 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 to 1.7 microns in the near-infrared to the same focal point, it can form “colourful” images in the near-infrared band because all of the colours are in focus simultaneously, which is essential for colour photography.

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According to Columbia, the lens can focus light of any arbitrary polarisation state, so that it works not only in a lab setting, where the polarisation can be well controlled, but also in real-world applications, where ambient light has random polarisation. It also works for transmitted light, which is 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,” said Adam Overvig, the study’s other co-lead author and doctoral student with Yu. “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.”

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