Polymer Bragg Gratings Filled to Order

Hank Hogan

Highly reflective Bragg gratings using a photopolymer have been developed by a group of researchers at State University of New York (SUNY) at Buffalo, and at Science Applications International Corp. in Dayton and the US Air Force Research Laboratory at Wright-Patterson Air Force Base, both in Ohio. The technique could produce holographic reflectors, filters and other display enhancers, and it may enable the fabrication of biochemical sensors.

Backfilling the voids in the photopolymer-based Bragg gratings with materials of different refractive indices produces different performance characteristics.

Although existing photopolymer technologies display the former functionality, they do not offer the latter. Another important aspect of the new method is that the material involved may be produced very inexpensively, said SUNY electrical engineering professor Alexander N. Cartwright.

He explained that the technique arose from efforts to improve and simplify a photopolymer process that is used to create switchable Bragg grating structures. The process begins with syrup comprising a monomer, a photoinitiator, a coinitiator and liquid crystals. The syrup is exposed to two coincident, coherent laser beams, which interfere to produce alternating light and dark bands. The monomer polymerizes in the bright regions, and the liquid crystal ends up in the dark areas. The presence and location of the liquid crystals make the device electrically switchable.

Cartwright recalled that graduate student Vincent K.S. Hsiao wanted to create a one-step process. That required the addition of a solvent, so the scientists decided to try acetone. The result was a film in which the photoinitiator and coinitiator optically defined the polymerized regions, as expected. But they found that the acetone also separated from the other ingredients, evaporated and left behind holes: They had produced Bragg gratings with gaps.

By backfilling the voids with various materials of different refractive indices, they could select different performance characteristics. For instance, using air, which has a refractive index of 1, leads to a near-total transmissivity dip at about 525 nm. Using chloroform, which has a refractive index of 1.45, shifts the notch toward the red by 150 nm.

The researchers suggest that the technique has the potential to create high-performance reflection gratings at virtually any wavelength, diffraction efficiency or bandwidth. By backfilling with the right material, it may be possible to create a biochemical sensor.

Cartwright noted that significant research still must be performed before such applications become possible, but he thinks it will be worth the effort.

"We believe that the ability to accurately produce porous polymer structures can open up areas of research that are similar to [those involving] the use of porous silicon structures," he said.