Ideas, Inventions And Innovations

An array of 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 three-dimensional imaging. They are also sensitive to the polarization of light, one of the qualities that are thought to help bees navigate their environments. Credit:University of Pennsylvania

These lenses produce sets of images with different focal lengths, a property that could be used for three-dimensional imaging. They are also sensitive to the polarization of light, one of the qualities that are thought to help bees navigate their environments.

The study was led by Francesca Serra and Mohamed Amine Gharbi, postdoctoral researchers in the Department of Physics and Astronomy in Penn's School of Arts and Sciences, along with Kathleen Stebe, the School of Engineering and Applied Science's deputy dean for research and a professor in Chemical and Biomolecular Engineering; Randall Kamien, professor in Physics and Astronomy; and Shu Yang, professor in Engineering's departments of Materials Science and Engineering and Chemical and Biomolecular Engineering. Yimin Luo, Iris Liu and Nathan Bade, members of Stebe's lab, also contributed to the study.

It was published in Advanced Optical Materials.

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," said Gharbi.

"Our first question," Serra said, "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 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 posts, transmitting an elastic energy cue that causes the crystal's focal conic domains to line up in concentric circles around the posts.

With these liquid crystal lenses 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 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.

"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 P's, one in each of the lenses."

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Ideas, Inventions And Innovations