Salt sandwiches boost opto-electronics.

Although physicists figured out that some materials could respond to light in peculiar ways, it remained for chemists to create substances with these so-called nonlinear optical (NLO) properties, says Seth R. Marder, a chemist at the California Institute of Technology in Pasadena. Marder and his colleagues at NASA's Jet Propulsion Laboratory in Pasadena have made organic materials that, in electric fields, develop the uneven distribution of charges needed for NLO effects.

In 1989, they first produced the organic salt DAST, which they have now shown can work 20 times better than the NLO material lithium niobate as an electro-optic switch. It requires 1/350th the electric charge that lithium niobate needs to change the index of refraction and polarize the light passing through it, says Marder. Also, the material is stable, forms large crystals easily, and should cost about 10] a gram, he adds.

The group's X-ray diffraction studies have revealed that DAST's asymmetric polarity and its NLO properties arise because the salt's positive organic ions align parallel to one another in sheets, with sheets of negatively charged ions sandwiched in between the positive ones. "They find an orientation with respect to one another that makes them happy," says Marder. This configuration guarantees that like-charged ions do not face in opposite directions, which would cancel out the polarity.

In other work, Marder and his colleagues have synthesized organic molecules designed to optimize NLO properties. In these improved molecules, electrons readily shift between rings of carbon at each end of a carbon chain without disturbing the energy balance of the whole chain.