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  • Handheld rainbow could spawn multispectral tools

Photonics Spectra
Apr 2012
Ashley N. Paddock, ashley.paddock@photonics.com

BUFFALO, N.Y. – A new kind of polymer that is cheap and easy to make and reflects many different wavelengths of light when viewed from a single perspective could form the basis of handheld multispectral imaging devices.

“Such portable technology could have applications in a wide range of fields from home improvement, like matching paint colors, to biomedical imaging, including analyzing colors in medical images to detect disease,” said Alexander N. Cartwright, professor of electrical engineering at the University at Buffalo.

Engineers at the university developed a one-step method to fabricate the polymer, which will make it feasible to develop small devices that connect with cell phones to conduct multispectral imaging, said Qiaoqiang Gan, assistant professor of electrical engineering.

“Our method is pretty low-cost, and because of this and the potential cell phone applications, we feel there is a huge market for improving clinical imaging in developing countries,” Gan said.


A rainbow-colored grating about 25 mm wide is shown under sunlight. Enlarged microscope images show the graded surface, with the black bars indicating a length of 10 µm. Courtesy of the University at Buffalo.


Because the rainbow filter’s colors are produced by its surface geometry and not some kind of pigment, the colors won’t fade over time. The same principle applies to the colors of butterfly wings and peacock feathers. The polymer fabrication technique was described in Advanced Materials (doi: 10.1002/adma.201104628).

To create the rainbow material, students Ke Liu and Huina Xu sandwiched a photosensitive prepolymer syrup between two glass slides. Next, they directed a laser beam through a curved lens placed above the prepolymer solution. The lens divided and bent the laser beam into light of continuously varying wavelengths.

As this light hit the solution, monomers in the solution joined into polymers, forming a continuous pattern of ridgelike polymer structures. Larger ridges rose where the light struck with more intensity. The thin filter structure that resulted is rainbow-colored when viewed under white light. This is because the periodic polymer layers reflect a continuous spectrum of colors, from red on one end to indigo on the other.

The single-step fabrication method – shining a laser light through a curved lens – is affordable and relatively simple.

The filter is about 25 mm long, but the technique the researchers used is scalable: It’s possible to create filters of different sizes by shining the laser through lenses of different sizes.

Gan said the next step is to improve the quality of the rainbow filter. The team also is beginning to explore ideas for incorporating the technology into handheld devices.

The university has submitted a provisional patent application detailing the production process to the US Patent and Trademark Office.

For more on Qiaoqiang Gan’s research, see “Slow Light Slowed Even More,” www.photonics.com/a46696.


GLOSSARY
light filter
A homogeneous optical medium or coating that transmits only in particular regions of the spectrum. It is used to change or modify the total or relative energy distribution of a beam of light.
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