Liquid crystal based compound lenses work like insect eyes

The lenses produce sets of images with different focal lengths, a property that could be used for three-dimensional imaging.

Taking advantage of the geometry in which the liquid crystals like to arrange themselves, the engineers and physicists at the University of Pennsylvania were able to grow compound lenses with controllable sizes, as the video below demonstrates.

The compound eyes found in insects and some sea creatures use thousands of lenses to provide information without the need for a sophisticated brain. Human artifice can only begin to approximate these naturally self-assembled structures.

Francesca Serra and Mohamed Amine Gharbi, postdoctoral researchers in the Department of Physics and Astronomy in Penn’s School of Arts and Sciences, led the study, which was published in Advanced Optical Materials.

Previous work had showed 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’ is a ‘focal conic domain’, a structure that can be used as a simple lens.

“Our first question 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,” Serra said.

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.

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 an image formed.

“If the array worked as a single lens, 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,” Serra said.

Because the focal conic domains vary in size, with the largest ones closest to the pillars and descending in size from there, the focal lengths for each ring of the microlenses was different. 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 3D image reconstruction,” said Shu Yang, a professor in Penn Engineering’s departments of Materials Science and Engineering and Chemical and Biomolecular Engineering.

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 the lenses were sensitive to light polarisation. Bees are thought to use this information to better identify flowers by seeing how light waves align as they bounce off petals.

Discovering how the microlenses work extends this area of research in the direction of practical applications.