Researchers have created compound lenses inspired by the eyes of insects using liquid crystals that could be used in 3D imaging.
The researchers took advantage of the geometry in which the liquid crystals naturally arrange themselves to create compound lenses of controllable sizes. In further experimentation, the team from the University of Pennsylvania proved such lenses can capture images with different focal sizes and respond to light polarisation.
The current study, published in the Advanced Optical Materials journal, follows on an earlier finding that liquid crystals of soap like materials naturally self-assemble into flower-like structures when placed around a central silica bead. Each ‘petal’ of such flower structures then functions as a simple lens.
"Given the liquid crystal flower's outward similarity to a compound lens, we were curious about its optical properties," said Mohamed Amine Gharbi, a postdoctoral researcher in the Department of Physics and Astronomy in Penn's School of Arts and Sciences.
The researchers used photolithography to create the lenses using a sheet of micropillars to support the liquid crystal. At room temperature, the liquid crystal adheres to the top edges of the micripillars, transmitting an elastic energy cue that causes the crystal's focal conic domains to line up in concentric circles around the posts.
To study the properties of the lens, the researchers positioned a glass slide between the compound lens placed under a microscope 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, a single virtual image would appear below the sample,” explained Francesca Serra who led the team. “But because they work as separate microlenses, I saw multiple P's, one in each of the lenses."
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 is different. As the researchers moved the microscope objective up, the images of the P's came into focus in sequence, from the outside layers inward.
"That they focus on different planes is what allows for 3-D image reconstruction," explained Professor Shu Yang. "You can use that information to see how far away the object you're seeing is."
In a second experiment, the researchers replaced the P with two test images - a cross and a square suspended several inches above it. The researchers showed that for different lenses, the cross intersected the square at different points – a phenomenon that would allow the reconstruction of the square and the cross's spatial relationship.
A third experiment showed that the compound lens was sensitive to light polarization, a trait that had not been demonstrated in liquid crystal lenses before.
The ability to see light polarisation is believed to be extremely important for bees, enabling them to identify flowers based on 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, the researchers were able to block the images from forming in some lenses but not others.
The discovery is far from practical applications but the researchers have already achieved a ten-fold increase in the size of the lens. “If we ever wanted to mass-produce these lenses, we can use the same technique on arbitrarily large surfaces,” said Kathleen Stebe, professor in Chemical and Biomolecular Engineering.
The video below explains how the compound lens works: