Chiral molecules stabilize optical properties in polymers

By Yvonne Carts-powell

Polymer scientists at the American Chemical Society`s annual meeting in August showed that polymers made with one-handed chiral molecules had optical properties four times as stable as those made using conventional precursors. The researchers believe that the enhanced stability of chiral polymers is a route to overcoming the lifetime problems associated with conventional nonlinear optical polymers. If lifetime problems can be overcome, these materials may be very attractive as fast electro-optic modulators for optical communications.

Researchers at Molecular OptoElectronics Corp. (MOEC--Watervliet, NY) are developing the polymers for use in high-speed fiber-optic modulators under a contract from the U.S. Department of Defense. Plastics for electro-optical applications are attractive for the same reasons that plastics are used elsewhere: They are relatively inexpensive, easily molded into different shapes, and their properties can be tailored to the needs of the specific application. Plastics also have a few other advantages, such as having the potential of higher bandwidths, allowing faster modulation than crystals. Telecommunications systems operating at OC-192 (10 Gbits/sec) use bandwidths of about 10 GHz, explains MOEC vice president Michael Shimazu, whereas nonlinear optical polymer modulators could operate at more than 100 GHz. The company hopes to intersect the telecommunications market when it requires bandwidths of 40 to 60 GHz.

Polymer primer

Nonlinear optical polymers are made from shorter molecules (monomers) mixed with a small fraction of dye molecules (chromophores) that have uneven distribution of electrical charge. A chemical reaction changes the material into longer, more complex polymers, with the chromophores either interspersed among or linked to the polymer chains.

The polymer shows nonlinear optical properties when the chromophores are aligned, or "poled," typically by placing the plastic in an electric field during polymerization, in much the same way lithium niobate nonlinear crystals are poled.

Unlike crystals, however, the alignment of the chromophores in polymers is not stable. Over time the chromophores lose their alignment and the material loses its attractive optical properties. The MOEC researchers altered the structure of the polymers in a way that helps the chromophores maintain their alignment but otherwise changes few of the material`s overall properties.

Chiral

The material was altered by adding a single-handed chiral molecule to the polymer`s backbone. Chiral molecules appear in two mirror-image versions--left-handed and right-handed types, also called an R and S version (see figure). Each enantiomer, or handed version, rotates plane-polarized light in equal but opposite directions. This property can be used to test the chirality (the "handedness") of polymers, and it also can be used in optical applications. In nature, explains Shimazu, most chiral polymers, including DNA and proteins, are found in only one or the other handed version. Man-made polymers, however, tend to be a 50/50 mix of both handed versions. The MOEC researchers discovered that by making polymers that include single-handed molecules (either all left-handed or all right-handed), the result is a more stable nonlinear optical device. Janet Gordon and other researchers at MOEC made and characterized R- and S-type polymers using a chiral molecule called IBP. They created IBP monomers, separated the two types, and made a polycarbonate material out of one-handed IBP. The one-handed polymer was then doped with chromophore.

Most of the material`s properties remained similar between the single-handed and mixed versions of the polymer. Both polymer types were doped with choromophores and poled to create films with electro-optic properties measurable by bouncing a polarized laser beam off the film.

The films were heated to 136C and then measured to see how long the organization of the dye would last. Measurements taken from the first day through several months showed that the single-handed R version held onto its orientation, retaining its poling, for nearly four times as long as the mixed version. This is significant because it indicates that at room temperature and normal operating temperature, the lifetimes will be even longer. Optimizing the system is likely to provide even more stable polymers.

For some reason, the single-handedness of the chiral monomers helps the chromophores stay aligned. This preliminary result could be the first step in making polymer poling much more stable, a necessary step in commercializing nonlinear optical polymer devices.q

Yvonne Carts-Powell writes on photonics from Belmont, MA.