The compound eyes found in insects and some sea creatures are marvels of evolution. There, thousands of lenses work together to provide sophisticated information without the need for a sophisticated brain. New research from Penn engineers and physicists shows how liquid crystals can be employed to create compound lenses similar to those found in nature.
The study was led by Francesca Serra and Mohamed Amine Gharbi, postdoctoral researchers in the Department of Physics & Astronomy in the School of Arts & Sciences, along with Kathleen Stebe, deputy dean for research and a professor in the Department of Chemical and Biomolecular Engineering in the School of Engineering and Applied Science; Randall Kamien, a professor in Physics and Astronomy; and Shu Yang, a professor in Penn Engineering's departments of Materials Science and Engineering and Chemical and Biomolecular Engineering.
Previous work by the group had shown how smectic liquid crystal--a transparent, soap-like class of the material--naturally self-assembled into flower-like structures when placed around a central silica bead. Each "petal" of these flowers is a "focal conic domain," a structure that other researchers had shown could be used as a simple lens.
"Given the liquid crystal flower's outward similarity to a compound lens, we were curious about its optical properties," Gharbi says.
"Our first question," Serra says, "was what kind of lens is this? Is it an array of individual microlenses, or does it essentially act as one big lens? Both types exist in nature."
To make the lenses, the researchers spread the liquid crystal on a sheet of micropillars. Finding a suitable compound lens under a microscope, the researchers then put a test image--a glass slide with the letter "P" drawn on in marker--between it and the microscope's light source. Starting with the post in focus, they moved the microscope's objective up and down until they could see an image form.
Because the researchers saw multiple small images, instead of one large one, it was clear that each "petal" works like an independent lens.
As the microlenses get smaller as they get further from the pillar, their focal lengths change as well. That allows for 3-D image reconstruction, as comparing a subject's image in different lenses shows how far away it is.
A similar experiment showed how the lenses were sensitive to the polarization of light, a quality bees are thought to use to better identify flowers by seeing how light waves align as they bounce off their petals.
Replacing the "P" with a smiley face, vertically polarized light showed the images on the left and right of the pillar, whereas as horizontally polarized light showed images just on the top and bottom.
That these compound lenses self-assemble make them an attractive alternative to other artificial compound lenses, which require painstaking manufacturing techniques.
"If we ever wanted to mass-produce these lenses," Stebe says, "we can use the same technique on arbitrarily large surfaces. We know how to put the pillars in any given position and size, how to cast out thin films of smectic liquid crystal and exactly where and how the lenses form based on this geometric seed."