Clear, Panoramic View from Miniature LensesEngineering 360 News Desk | November 11, 2015
University of Wisconsin-Madison (UW-Madison) engineers have created miniature lenses with a broad range of vision. The development could allow everything from surgical scopes to security cameras that can capture a broader perspective at a fraction of the size required by conventional lenses.
Researchers, led by Hongrui Jiang, professor of electrical and computer engineering at UW-Madison, have designed lenses no larger than the head of a pin and embedded them within flexible plastic. An array of the lenses rolled into a cylinder can capture a panoramic image covering a 170-degree field of view.
Insert image here: "We got the idea from compound eyes," says Jiang. "We know that multiple lenses on a domed structure give a large field of view."
The team can freely reconfigure the shape of the lens array because they use Fresnel zone plates rather than rely on conventional optics for focusing.
Conventional lenses use refraction—the way light changes direction while passing through different substances (typically stiff, translucent ones like glass)—to focus it on a single point. The zone plates focus by diffraction—bending light as it passes the edge of a barrier.
Each of Jiang's half-millimeter-diameter lenses resembles a series of ripples on water. Each concentric ring alternates between bright and dark. The distance between the rings determines the optical properties of the lens. The researchers can tune those properties in a single lens by stretching and flexing it.
"The dark areas must be very dark," says Jiang, whose work is funded by the National Institutes of Health. "[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 structure, making the material extremely 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. First, they patterned aluminum rings on top of solid silicon wafers and etched silicon nanowires in the areas between aluminum rings. Then they 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 a supple plastic.
This approach gave their lenses crisp focusing capabilities as well as the flexibility that enables them to capture a large field of view. Previous attempts at creating Fresnel zone plate lenses have suffered from fuzzy vision.
Jiang and his team are now exploring ways to integrate the lenses into existing optical detectors and incorporate silicon electronic components directly into the lenses themselves.