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Crystals Compensate for Birefringence in Polymer

Photonics Spectra
Oct 2003
Daniel S. Burgess

The processing of optical polymers in the production of liquid crystal displays (LCDs) and lenses tends to produce birefringence in the materials. Manufacturers may compensate for this phenomenon by using alternative fabrication techniques, such as solvent casting or long-cycle-time injection molding, but these carry added expense and slower production times that can lead to higher costs. Now researchers at Keio University in Yokohama, Japan, and at Japan Science and Technology Corp. in Kawasaki have developed an approach that introduces inorganic crystals with the opposite birefringence to compensate for that of the polymer.

Akihiro Tagaya, group leader of the Koike Photonics Polymer Project at Japan Science and Technology, explained that birefringence is a serious problem for LCDs and lens systems. LCDs use plane-polarized light, and the change to elliptical polarization due to birefringence degrades the contrast and other visual characteristics of the displays. In the pickup lens systems in CD and DVD players, birefringence increases the spot size of the laser beam, thus compromising the performance of these systems.

In their experiments, the researchers incorporated rod-shaped strontium carbonate crystals into poly(methylmethacrylate-co-benzylmethacrylate). The crystals properly oriented themselves to compensate for the polymer's birefringence as the polymer chains in the material oriented themselves in processing. They found that the addition of the crystals nearly eliminated orientational birefringence in the drawn polymer film, although it did slightly increase scattering loss, which they suggest may be reduced by employing crystals smaller than approximately 200 X 20 nm.

Tagaya said that the team hopes to partner with companies in the industry and to demonstrate the technique on a larger scale.


GLOSSARY
injection molding
A method of producing high-quality plastic optics in large volumes by injecting the heated, liquified plastic at high pressure into a tooled mold with polished chromium stainless steel inserts to define the optical surface. The plastic solidifies as it cools, and the mold is removed.
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