New materials that create a negative index of refraction are opening up previously unexplored areas of optics, researchers say, and could result in improvements such as better medical imaging and cheaper optical lithography.
All known materials have a positive index of refraction, which controls the angle at which light bends when it passes through them. In 1968, Russian physicist Victor Veselago suggested that some materials could have a negative index, in which both the electrical permittivity and the magnetic permeability of an electromagnetic wave are negative. Sheldon Schultz of the University of California (San Diego, CA) confirmed this prediction in 2001 by building such a "metamaterial" (see oemagazine, April 2002, p. 9).
Light waves coming in from the left pass through the periodic structure of the plano-concave lens and focus to a point on the right.
Recently, Srinivas Sridhar, director of the Electronic Materials Research Institute at Northeastern University (Boston, MA), reported that he and his colleagues had built a plano-concave lens with a negative index using photonic crystals. Focusing lenses have always had to be convex, but they have also been confined by a single optical axis, a limited aperture, and an inability to focus light smaller than about half of its wavelengththe diffraction limit.
Sridhar built a lens, with one side flat and the other concave, out of a photonic crystal, in which a series of periodic features affect the material's dielectric constant and divert the light waves as desired. The concept is similar to that of photonic bandgap crystals, but the actual spacing of features is different. "Instead of focusing on a region where it does not transmit, which is the bandgap, here we're focusing on the region where it does transmit," Sridhar says.
The resulting lens has less aberration than a traditional lens of the same size. Because the photonic crystal is not a solid block, it also weighs less, making it appealing for aerospace applications.
Sridhar had built a flat lens in a similar manner, but it only worked for near-field applications. The concave side of his new lens focuses light from more distant sources. He also worked with microwave frequencies of 9 GHz but says the concept should scale to optical wavelengths. Dealing with those shorter wavelengths presents an engineering challenge because it means the periodicity of the structure has to be controlled to a scale of about 10 nm. Superlens
In a very different approach, Xiang Zhang, director of the Nanoscale Science Engineering Center at the University of California (Berkeley, CA), created a superlens to work at below-diffraction limits at optical wavelengths. He coated a 35-nm thin film of silver on top of a dielectric. When the incident light, in this case 365-nm light, strikes the surface, the electrons move collectively, setting up what Zhang calls surface plasma resonance.
In near-field imaging, the lens reverses the diffraction of evanescent waves, which disperse too rapidly to be handled by a standard lens, and focuses them, essentially adding lost data to the image and making it less blurry. "This is the first time you can do optical imaging at 60 nm," Zhang says.
By breaking the diffraction limit, he predicts researchers will be able to image the proteins within cells and see how cancer cells differ from normal cells, for instance. A superlens might also be used to bring optical lithography below the diffraction limit, creating the smaller feature sizes semiconductor chips will require in a few years while still using current laser technology.
"That would be terrific," Zhang says, "if you could do 30-nm optical lithography."
He stresses that his paper only proves the theory, and that it could be 10 years before anyone builds a practical tool with the superlens.
Sridhar says breaking what had seemed a law of physics could move optics forward in exciting ways. "The concept of negative refraction has opened up new possibilities and new ideas," he says.