Marine Creatures Provide Optical Lessons

As mentioned in this space last month, lens designers will continue to face even tougher challenges as the industry moves ahead in its quest for finer features (see Semiconductor International, September 2001). They might do well to take another lesson from nature.

Researchers at Lucent Technologies' Bell Laboratories (Murray Hill, N.J.), the Weizmann Institute of Science (Rehovot, Israel) and the Natural History Museum of Los Angeles County (Los Angeles) have discovered some remarkable microlenses within the skeletons of a species of brittlestar. The marine invertebrates use calcitic crystals not only as protective armor, but as optical detectors, the team found. And those light-sensing arrays display significant sophistication in lens design.

"Altogether, I do believe nature is quite ahead of technology," said Joanna Aizenberg, a research scientist at Bell Labs. She makes note of nature's clever engineering design solutions to difficult technology problems. The optical properties appearing in the brittlestar's complex arrays of microlenses, she said, display at least two important criteria in lens design — correction for birefringence and correction for spherical aberrations.

A colorized SEM shows the peripheral layer of a brittlestar’s dorsal arm plate, with the enlarged lens structures. Overlayed is a picture of a brittlestar itself. (Source: Bell Labs)

The researchers set up a lithographic experiment to study the lensing effect of the arrays. They embedded a lens array in polydimethylsiloxane (PDMS), using that as a mask in photoresist exposures. The experiments confirmed the focusing ability of the dorsal peripheral layer of the brittlestar's dorsal arm plate, where the microlens arrays are found (Figure).

Because calcite displays different refractive indexes depending on the polarization of the light source, avoiding birefringence is a complex matter. The lenses of the brittlestars achieve this by being crystallographically oriented — growing as single crystals with the optical axis parallel to the lens's axis. Lens designers could draw on such lessons. "If we are to use crystalline materials instead of amorphous materials, this is one thing to take into account," Aizenberg said.

The lenses of the brittlestars also appear to have minimal spherical aberration, a feat to which lens designers aspire. But recognizing the achievement (the brittlestars have an unusual shape at the lower surface of the lens) and reproducing it in lens designs are two different matters. "We're not yet able to do this — not at this level of sophistication, especially with crystalline lenses," Aizenberg said. Nonetheless, designers ought to be able to use the knowledge to improve on existing lenses, she said.

Another remarkable attribute in the microlens arrays, Aizenberg noted, is the size of the lenses — each 20-40 µm, and each individually addressed. "This is something we really want to know how to do in arrays," she said.

All in all, there is much that designers could learn from the microlenses of the brittlestar, including the idea of multifunctionality — making the lenses mechanically strong as well as optically superior. But it's not as if engineers will be able to develop a lens overnight that mimics nature, Aizenberg said. "Instead, we'll try to incorporate one idea at a time, getting better and better performance."

Aizenberg and her team plan to study the brittlestars further, exploring its neurofunctions and studying the concept of growing rather than shaping crystal materials, for example. The group's latest research is detailed in the Aug. 23 issue of Nature.