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Brittle Star Features Calcite Lenses

Photonics Spectra
Nov 2001
Daniel S. Burgess

It has five arms and thousands of eyes, wears an armored plate and has been around for 500 million years. No, it's not a monster from a 1950s grade-Z sci-fi movie, but the brittle star Ophiocoma wendtii. And it may lead researchers to develop new and better microlens arrays for photonics applications.

Researchers have discovered a brittle star that has adapted its armored plates into a visual system. Alexei Tkachenko and Joanna Aizenberg of Bell Labs display specimens of the marine organism. Photos courtesy of Lucent Technologies Inc.

A research team from Lucent Technologies Inc.'s Bell Labs in Murray Hill, N.J., the Weizmann Institute of Science in Rehovot, Israel, and the Natural History Museum of Los Angeles County in Los Angeles have reported the presence of single-crystal calcite microlenses on the dorsal arm plates of O. wendtii. They suggest that the lenses focus incoming light onto photosensitive nerve bundles in the organism that work in concert as somewhat of a compound eye.

A scanning electron micrograph of one of the brittle star's dorsal arm plates reveals an array of calcite microlenses. Each 40- to 50-µm-diameter lens corrects for spherical aberration and for birefringence.

Joanna Aizenberg, a researcher at Bell Labs and the leader of this project, explained that the brittle star -- an echinoderm along with true starfish, sea cucumbers, sea urchins and others -- was on a list of potentially interesting animals for investigations into unique biomaterials. Although they are invertebrates, the brittle stars, which first appeared in the Ordovician period, feature an internal structure of calcitic plates, as well as protective outer plates.

O. wendtii, in particular, is intriguing because the animal reacts to visual stimuli, such as those indicating the presence of a predator or of a refuge on the seafloor. When the researchers examined O. wendtii and a related but fairly photoinsensitive species, Ophiocoma pumila, under a scanning electron microscope, they noted a clear difference in the structure of the dorsal arm plates. O. wendtii's feature an array of 40- to 50-µm-diameter spherical calcite crystals, which they characterized in lithography experiments, using the lenses as a mask.

"Calcite, of course, is a really well-known optical material to elementary-school students for its pronounced birefringence," Aizenberg said. Only one orientation, she explained, would enable the animal to take advantage of the optical properties of the material while avoiding the effects of birefringence. "The brittle star uses this orientation in the single-crystal lenses." Specifically, the researchers determined that the lenses are oriented along the crystallographic c-axis.

Moreover, the lenses perform better than theory would predict. Based on the focal length and on the size of a spot at the focal plane, such a lens comprising two spherical surfaces should display a maximum operational diameter of approximately 6 µm and a light-enhancement factor of three to four. In fact, the organism's lenses have a maximum operational diameter of 20 µm and a light-enhancement factor of 50, indicating that they compensate for spherical aberration.

Evolutionary power

While the researchers hope to tease ideas for practical microlens design from the work, it also exemplifies the power of evolution to make the best of the materials at hand.

"We can find very interesting, very complex designs to optical problems in biological systems," Aizenberg said. "The microlenses are almost perfect optical elements, on a micron scale, which is beyond our current technology."

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