PORTLAND, Ore. The world's first communications-wavelength metamaterial was recently demonstrated by a Princeton University design fabricated at Alcatel-Lucent. The clever semiconductor architecture sidesteps the need to craft nanoscale mechanical structures, as has been the universal approach until now, by alternating layers of indium gallium arsenide with layers of aluminum indium arsenide.
"When we started out on this project, we thought we would need a complicated architecture like other research groups are pursuing. But as our work progressed, we found that we could achieve the same results with a much simpler and straightforward layered structure," said Claire Gmachl, a Princeton University electrical engineering professor.
More than a dozen groups worldwide are trying to craft optical metamaterials from nanoscale mechanical structures. For instance, earlier this year the Department of Energy's Ames (Iowa) Laboratory showed a "fishnet" design cast in two metal screens with 100-nanometer (nm) holes separated by an optically clear dielectric. Also this year, Purdue University announced a design that uses arrays of nano-needles, each measuring 10 nm in diameter.
All these nanoscale mechanical structures were aimed at mimicking the physical properties of the seminal metamaterials that use arrays of sub-wavelength resonators that respond not only electrically but also magnetically to incident microwavesachieved with arrays of split-ring resonators. For the much shorter optical wavelengths, most researchers have been seeking ways to shrink these mechanical structures down to sub-micron sizes. However, the extreme difficulty of fabricating such tiny mechanisms led the Princeton team to find a better solution.
"Our material's architecture is deceptively simplejust two alternating layers of semiconducting materialaluminum indium arsenide and highly doped indium gallium arsenide," said Gmachl. "We could have used two other semiconductors, but we chose to use those materials which are already well understood for optical communications."
The individual layers were 80-nm thickmuch thinner than the wavelength of communications lasers that range from as low as 9 to as high as 20 microns. By using 100 alternating layers, the total thickness of the metamaterial was 8 microns.
How's it work?
The mechanism by which metamaterials reverse the basic optical properties of natural materialsrefracting electromagnetic radiation by bending it toward the angle of incidence, instead of away from the angle of incidenceis by introducing a resonant grid whose pitch is smaller than the wavelength to be transmitted through it. As a result, a flat planar lens made from metamaterials can focus light the same way as a concave natural lens, but without the aberrations that make concave lenses imperfect. Theoretically, the negative index of refraction of such metamaterials should enable perfect lenses to be constructeda feat impossible for natural materials.