Peterborough, N.H. - If two theorists turn out to be correct, making optical components with a negative index of refraction could become as cheap and easy as the processes used in conventional optics.

While conventional optical components, like glass, naturally have a positive index of refraction, in the 1960s theorists predicted that it would be possible to do much more with optical components that had a negative refractive index, even though such materials do not exist in nature. And although progress has been made recently in building artificial materials with these properties, the industry is still far from having a practical negative-index-of-refraction material to work with.

That might change with a new study of the problem by Tom MacKay at the University of Edinburgh (Scotland) and Akhlesh Lakhtakia at Pennsylvania State University. They predict that it is possible to mix two positively refracting materials homogeneously to create a composite with a negative index of refraction.

About five years ago, researchers working in the microwave range were able to demonstrate that negative refraction could take place physically, but only if they built complex geometric configurations of conductors and insulators. Since microwaves have longer wavelengths than light, it was easier to build these structures.

"The interest in negatively refracting materials stems from the fact that these can be used to make near-perfect lenses capable of extremely high-resolution imaging," said MacKay. "The negatively refracting metamaterials that have been produced so far have highly complex architectures. Typically, they are made up of arrays of complex-shaped inclusions, such as double rings and oriented ring-post couples."

But building similar structures at visible wavelengths has proved very difficult.

"The two positively refracting materials used as ingredients in our approach belong to the general category of dielectric-magnetic materials. This category contains an enormous range of naturally occurring materials," said MacKay. "In our paper, we show that it is theoretically possible to combine two such positively refracting materials to produce a negatively refracting composite material. We also present one specific, representative, example. But the example we present is not the only possible example.

"The parameter space of dielectric-magnetic materials now needs to be explored to identify the full range of possible materials that can be combined, in the way we describe in our paper, to give a negatively refracting composite material," MacKay explained.

The latest findings are the fallout from a long-term study of composite optical materials. "A general theme of our work has been how to combine two or more relatively simple materials to produce a complex composite material. The basic approach was to take a known property of a material and find some way to extend its properties by finding complementary materials with which it could be blended. In the process, the theorists began to realize that it would be possible to create materials that had fundamentally new properties not shared by either material.

"The combination of two positively refracting materials to produce a negatively refracting composite material is a classic example of this," said MacKay.

To achieve negative refraction from a combination of two positively refracting materials, the individual granules of each component must be at least 10 times smaller than the wavelength of light. In that case, the light waves experience a uniformly transparent medium that has the right composite properties for negative refraction. "The positively refracting ingredients can be crystalline or amorphous-it doesn't matter which, because the structure of the ingredient granules is not 'seen' in the negatively refracting composite," he said.

Once the properties of such composites are proved out in experiments, it will be a simple matter to produce them in bulk using existing manufacturing processes, MacKay said.