PHILADELPHIA — Liquid crystal can be controllably formed into compound lenses similar to those found in insects, a team from the University of Pennsylvania (Penn) has demonstrated.
These lenses produce sets of images with different focal lengths, a property that could be used for 3-D imaging. They are also sensitive to the polarization of light, a quality thought to help bees navigate their environments.
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 type of structure that other researchers have shown can be used as a simple lens.
An array of smectic liquid crystal microlenses self-assemble around a central pillar. These lenses produce sets of images with different focal lengths, a property that could be used for 3-D imaging. They are also sensitive to the polarization of light, one of the qualities that are thought to help bees navigate their environments. Courtesy of the University of Pennsylvania.
"Given the liquid-crystal flower's outward similarity to a compound lens, we were curious about its optical properties," said postdoctoral researcher Mohamed Amine Gharbi.
"Our first question was, what kind of lens is this?" said postdoctoral researcher Francesca Serra. "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 used photolithography to fashion a sheet of micropillars, then spread the liquid crystal on the sheet. At room temperature, the liquid crystal adheres to the top edges of the micropillars, transmitting an elastic energy cue that causes focal conic domains to line up in concentric circles around the posts.
Because these liquid crystal lenses are so easy to make, the experiment to test their properties was also relatively simple. Finding a suitable compound lens under a microscope, the researchers put a test image – a glass slide with the letter "P" drawn in marker – between it and the microscope's light source. Starting with a micropillar in focus, they moved the microscope's objective up and down until they could see an image form.
"If the array worked as a single lens," Serra said, "a single virtual image would appear below the sample. But because they work as separate microlenses, I saw multiple Ps, one in each of the lenses."
Because the focal conic domains vary in size, with the largest ones closest to the pillars and decreasing in size from there, the focal lengths for each ring of the microlenses is different, ranging from a few microns to a few tens of microns. As the researchers moved the microscope objective up, the images of the Ps came into focus in sequence, from the outside layers inward.
"That they focus on different planes is what allows for 3-D image reconstruction," said Shu Yang, a professor in the departments of materials science and engineering and chemical and biomolecular engineering. "You can use that information to see how far away the object you're seeing is."
A second experiment also showed this parallax effect. Replacing the P with two test images, a cross with a square suspended several inches above it, the researchers showed that the cross intersected the square at different points in different lenses. This phenomenon would allow the reconstruction of the square and the cross's spatial relationship.
A third experiment showed that the team's lenses were sensitive to light polarization – a trait that had not been demonstrated in liquid crystal lenses before. Bees are thought to use this information to better identify flowers by seeing how light waves align as they bounce off their petals. By putting another image – a smiley face – above the microscope's lamp and a polarizing filter on top of it, the researchers were able to block the images from forming in some lenses but not others.
"For example," Serra said, "the lenses on the right and left of the pillar will show images just for vertically polarized light. This sensitivity results from the peculiar geometrical arrangement of these liquid crystal defects, which other artificial compound eyes or microlens arrays lack."
Answering fundamental questions about how these microlenses work extends this area of research in the direction of practical applications. With an understanding of the geometric relationships between the size of the micropillars, the arrangement of the focal conic domains and the focal lengths of the microlenses they produce, the team has shown how to grow these compound lenses to order.
"Last time we had tiny flowers. Now they are ten times bigger," said Kathleen Stebe, deputy dean for research at the School of Engineering and Applied Science and a professor of chemical and biomolecular engineering.
"That's important because it shows that the system scales," Stebe said. "If we ever wanted to mass-produce these lenses, 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."
Funding came from the National Science Foundation through Penn's Materials Science Research and Engineering Center, the U. S. Department of Education, and the Simons Foundation.
The research was published in Advanced Optical Materials (doi: 10.1002/adom.201500153).
For more information, visit www.upenn.edu.