Compound eyes inspire miniature lenses with greater scope

Their new approach is claimed to have created the first-ever flexible Fresnel zone plate microlenses with a wide field of view, a development with applications ranging from surgical scopes to security cameras to capture a broader perspective at a fraction of the size required by conventional lenses.

Led by Hongrui Jiang, professor of electrical and computer engineering at UW-Madison, the researchers designed lenses no larger than the head of a pin and embedded them within flexible plastic. An array of the miniature lenses rolled into a cylinder can capture a panorama image covering a 170-degree field of view.

“We got the idea from compound eyes,” says Jiang, whose work has been published in Scientific Reports. “We know that multiple lenses on a domed structure give a large field of view.”

The researchers can also reconfigure the shape of the lens array through the use of Fresnel zone plates, which focus by diffraction.

According to UW-Madison, each of Jiang’s half-millimetre diameter lenses resembles a series of ripples on water emanating out from the splash of a stone. In bull’s-eye fashion, each concentric ring alternates between bright and dark. The distance between the rings determines the optical properties of the lens, and the researchers can tune those properties in a single lens by stretching and flexing it.

Previous attempts at creating Fresnel zone plate lenses are said to have suffered from fuzzy vision.

“The dark areas must be very dark,” said Jiang, whose work is funded by the US National Institutes of Health. “Essentially, it has to absorb the light completely. It’s hard to find a material that doesn’t reflect or transmit at all.”

His team overcame this obstacle by using black silicon to trap light inside the dark regions of their Fresnel zone plate lenses. Black silicon consists of clusters of microscopic vertical pillars, or nanowires. Incoming light bouncing between individual silicon nanowires cannot escape the complex structure, making the material darker than dark.

Rather than laying down layers of black silicon on top of a clear backdrop, Jiang and his team took a bottom-up approach to generate their lenses. They first patterned aluminium rings on top of solid silicon wafers, and etched silicon nanowires in the areas between aluminium rings. They then seeped a polymer between the silicon nanowire pillars. After the plastic support solidified, they etched away the silicon backing, leaving bull’s-eye patterned black silicon embedded in supple plastic.

This approach is said to have given their lenses unprecedented crisp focusing capabilities, plus the flexibility that enables them to capture a large field of view.

Jiang and his team - including postdoctoral scholar Mohammad J. Moghimi, graduate student Jayer Fernandes and recent graduate Aditi Kanhere - are exploring ways to integrate the lenses into existing optical detectors and directly incorporate silicon electronic components into the lenses themselves.