Scratching a polymer to guide light waves.

As a key component of digital wrist-watches, calculators, and portable computers, liquid-crystal displays have become a familiar sight. Consisting of a thin film of milky fluid sandwiched between a pair of polymer-coated glass plates, such devices respond to electrical signals. These control signals shift the orientation of liquid-crystal molecules in selected regions to alter the film's optical characteristics and create patterns of light and dark on the display.

"Now, researchers have developed a technique for altering the optical characteristics of liquid-crystal displays to enable them to channel light. They use the needlelike tip of a scanning force microscope to gouge out sets of tiny grooves on nylon-coated glass plates. These grooves, in turn, align liquid-crystal molecules in a specified direction between the plates.

Martin Ruetschi of the University of Basel in Switzerland and his coworkers describe their method in the July 22 Science.

"We expect that this technique of writing an orientation pattern with a tip of [a scanning force microscope] will trigger the development of new devices," the researchers say. Such devices may eventually play an important role in the development of optical computers and other equipment for processing optical signals.

Liquid crystals are generally made up of large, mobile, rodlike molecules that tend to organize themselves into a lattice. To orient these molecules in a particular direction, manufacturers of liquid-crystal displays typically rub the glass-mounted polymer layers that go into their devices with velvet rollers. The rubbing direction determines the initial alignment of the molecules.

The Basel group found that the tip of a scanning force microscope dragged across a polymer surface makes parallel grooves very similar to those created by rubbing. But the researchers could control the width, depth, and spacing of the grooves and the position and size of the resulting patterns far more precisely than they could just by rubbing the polymer layer.

This enabled the researchers to investigate systematically how the characteristics of the grooves influence the polymer layer's ability to orient liquid-crystal molecules and, hence, the film's optical properties. By selecting a suitable pattern of grooves, they could align liquid-crystal molecules in just the right way for the liquid-crystal layer to act as a waveguide for laser light. This prototype waveguide, 6 micrometers wide and 5 millimeters long, behaves much like an optical fiber and readily channels a laser beam.