Front row, from left: Purdue scientists Alexander Kildashev, Vladimir M. Shalaev and Vladimir Drachev. Back row, from left: students Uday Chettiar, Wenshan Cai and Hsiao-Kuan Yuan.
Science may be incremental, but in the case of metamaterials that appear “invisible” to certain wavelengths of light, the increments are coming in rapid succession. A field of investigation that began at the start of the decade and progressed through the microwave region of the spectrum is now tickling the edge of the visible band.
At Purdue University in Indiana, a research team has created a metamaterial that has negative values for both effective permeability and effective permittivity—a so-called “dual-band” or “double-negative” refractive index—for light with a wavelength of 813 nm. Furthermore, the group’s metamaterial showed a single-negative refractive index at 772 nm, which is clearly in the visible range (Opt. Lett., doc. ID 78578, posted April 17).
The researchers included Purdue scientists Vladimir M. Shalaev, Alexander Kilashev and Vladimir Drachev and students Shijun Xiao, Uday K. Chettiar, Wenshan Cai and Hsiao-Kuan Yuan.
The material is a sub-wavelength-sized cross-grating built from two perforated 33-nm-thick silver layers separated by a 38-nm-thick layer of alumina (a dielectric). To demonstrate the double-negative refractive index, the team illuminated the grid such that the incident magnetic field was polarized along the set of wider parallel strips.
Fishnet metamaterial developed by Vladimir Shalaev and colleagues at Purdue University. The holes are 120 nm across and separated by about 300 nm.
It’s harder to make double-negative metamaterials than the single-negative variety, Shalaev said. Thus, experiments with single-negative metamaterials tend to involve shorter wavelengths of radiation than double-negative substances. The 813-nm light is the shortest wavelength at which the dual-band negative index has been achieved, and the 772-nm result slightly edges out the 780-nm single-negative result reported by a team at the University of Karlsruhe in Germany (Opt. Lett. 32, 53).
The Purdue group is one of several engaged in a fast-moving race—both theoretical and experimental—to fabricate materials that will deflect light in exotic ways. At press time, three California Institute of Technology researchers reported negative-index refraction in the blue-green region of the spectrum (Science 316, 430), but their results applied only to surface plasmon polaritons in two-dimensional waveguides, Shalaev said.
Will scientists ever be able to create metamaterials that will be undetectable to visible wavelengths of light—which is the everyday meaning of “invisible,” after all? According to Shalaev, current metamaterials have been tested for single wavelengths, whereas a true “invisibility cloak” would have to perform flawlessly over a spectrum hundreds of nanometers wide.
Given that the refractive index of materials is wavelength-dependent, construction of such a cloak could be a real challenge. However, Shalaev and three of his Optics Letters co-authors recently proposed a way to create a cylindrical cloak that would work at optical frequencies (Nature Photon. 1, 224) in a manner similar to that demonstrated recently at microwave frequencies by John Pendry of Imperial College and David Schurig and David R. Smith at Duke University (Science 314, 977).
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