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02/10/2020 - Flat Lens Gets Ahead of the Optical Curve

By Alan S. Brown

a microscope image of lots white ovals and circles on a black backgroundAided by a microscope, you can see the silicon pillars Faraon uses to direct light so that a flat surface can act as a focusing lens.

The word "lens" takes its name from the Latin word for lentil. Both are both hemispheric shapes bound together on their flat surface. So a "flat lens" sounds like a contradiction of terms. Yet that is exactly what Andrei Faraon is working on at California Institute of Technology's Nanoscale and Quantum Optics Lab.

Why flat lenses? They could be used in lots of places where conventional optical lenses are too bulky, Faraon, a member of the Kavli Nanoscience Institute at Caltech, explained. Built onto semiconductor sensors, surgeons could attach them to guide surgical tools and researchers could use them in microscopes designed to look at transparent objects inside cells.

There are several reasons conventional optical lenses fall short for these and other applications. Those lenses work by bending light, so it focuses on a single point in the center. Yet the curves used to gather light make conventional lenses expensive to manufacture. They also distort colors and produce less sharp images on their edges. To correct for those flaws, optical systems use additional lenses, which makes them bulky and more expensive. 

Finally, in today's digital world, engineers must couple those lenses with a separate image sensor, which makes them even bulkier and more costly.

Faraon thought he had a clear solution: Build a flat lens using the same techniques used to fabricate silicon semiconductor image sensors so that the lens and sensor form a small, single integrated system.

But first, he had to find a way to collect and focus light using a flat surface. 

That's what metasurfaces do. They are surfaces made of conventional materials--in this case, silicon--shaped into nanosized structures that alter how those materials respond to light. The most famous example of an optical metamaterial is a cloak developed at Duke University that bends light and makes objects appear invisible.

Faraon's metasurfaces are dotted with 600-nanometer-high silicon cylinders that alters the path and speed of light as it passes through them. (Light always moves the same speed in a vacuum, but slows down when it passes through a medium like water or silicon.)

"As the light propagates through the pillars, it works like a convex lens," he said. "The pillars in the center are fatter, so they delay light slightly longer than the thinner pillars on the edge."

Through careful calculations, Faraon's team designed an array of pillars that brought all light together on top of a flat image sensor like those used in smartphones and digital cameras. 

At first, the metasurfaces produced images that were blurry around the edges. So, Faraon's group borrowed a trick used by conventional lenses and put two metasurfaces on top of one another (with the nanopillars on the outside). 

This enabled him to create a lens that gathered light from a 70-degree viewing angle and focused it crisply on a single plane. Faraon has also made flat telephoto lenses that zoom in on an image. 

The lenses should also be easy to mass produce at reasonable cost, since they are made by the same silicon semiconductor technology used to make conventional image sensors. 

"If you know what to do, it's a simple to design compared with the design that goes into a computer processor," Faraon said. 

Yet flat lenses have one major shortcoming: they work for one wavelength of light only. If a camera with a flat lens took a picture of an outdoor scene, all that would show up on the image is a specific wavelength of red, blue, or another color.

Surprisingly, this is not a deal breaker. After all, lasers and LED lights also emit only a single wavelength of light. So, any application that uses a laser or LED to illuminate something could use a flat lens to gather light reflected from that object. 

That makes them ideal for surgical instruments like endoscopes. These are long, thin rods with cameras and LED lights at the end, which doctors insert into body cavities and organs to perform minimally invasive surgery. 

Coupled with infrared LEDs, it could be used as night vision cameras for security systems.

Another possible use lies in differential phase contrast imaging, a technique often used to look at transparent objects, like cells, organelles inside cells, and crystals. It uses very small differences in the speed at which light, usually from an LED, passes these objects to enhance the edge of the sample. 

Faraon is already working on several of these applications, proving that a flat lens is not really a contradiction in terms. 

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