The fisheye lens goes flat and wide

Fisheye lenses let photographers take panoramic views in a single shot but, because they are made from multiple pieces of curved glass, they tend to be bulky and expensive.

Engineers in the US may have taken an important step in addressing this by creating what they say is the first flat fisheye lens capable of producing clear 180-degree images.

It’s a type of metalens: a wafer-thin material patterned with microscopic features that work together to manipulate light in a specific way.

And while it currently only works in the infrared part of the spectrum, the researchers say it could be modified to capture images using visible light.

The work by a team from Massachusetts Institute of Technology (MIT) and the University of Massachusetts at Lowell is described in a paper in the journal Nano Letters.

“This design comes as somewhat of a surprise, because some have thought it would be impossible to make a metalens with an ultra-wide-field view,” says MIT’s Juejun Hu. “The fact that this can actually realise fisheye images is completely outside expectation.”

Metalenses are still largely at an experimental stage but are seen as having the potential to reshape the field of optics.

Hu and colleagues say designs have been created that can produce clear images of up to 60 degrees, but going further in a traditional way would require more optical components, and that would necessarily add bulk.

Their design does not. It is a single transparent piece made from calcium fluoride with a thin film of lead telluride deposited on one side. They then used lithographic techniques to carve a pattern of optical structures into the film.

Each structure, or “meta-atom”, is shaped into one of several nanoscale geometries, such as a rectangular or a bone-shaped configuration, that refracts light in a specific way.

In conventional fisheye lenses, the researchers say, the curvature of the glass naturally creates a distribution of phase delays that ultimately produces a panoramic image. They determined the corresponding pattern of meta-atoms and carved this pattern into the back side of the flat glass. On the front, they placed an optical aperture for light.

“When light comes in through this aperture, it will refract at the first surface of the glass, and then will get angularly dispersed,” says MIT’s Mikhail Shalaginov.

“The light will then hit different parts of the backside, from different and yet continuous angles. As long as you design the back side properly, you can be sure to achieve high-quality imaging across the entire panoramic view.”

To make a similar lens for visible light, Hu says, the optical features may have to be made smaller to better refract that particular range of wavelengths, and the lens material would have to change. But the general architecture would remain the same.

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