Moon-Shaped Metamaterial Broadens Manipulatable Bandwidths

A new engineered broadband material crafted from artificial atoms more than doubles the range of light wavelengths that can be manipulated by such metamaterials, a development that could lead to perfect microscope lenses or invisibility cloaks.

Metamaterials — man-made materials that exhibit properties not found in the natural world, such as a negative refractive index — have revolutionized optics in the past decade; so far, however, they have failed to reach their full potential because of their inability to function over broad bandwidths. Designing such a material that works across the entire visible spectrum remains a considerable challenge.

All natural materials have a positive index of refraction — the degree to which they refract light. The nanoscale artificial "atoms" that constitute the metamaterial prism shown here, however, were designed to exhibit a negative index of refraction, skewing the light to the left. Technology that manipulates light in such unnatural ways could one day lead to invisibility cloaks. Courtesy Stanford University.

Unlike a natural material, the optical properties of which depend on the chemistry of the constituent atoms, a metamaterial derives its optical properties from the geometry of its nanoscale unit cells, or "artificial atoms." By altering the geometry of these unit cells, one can tune the refractive index of the metamaterial to positive, near-zero or negative values. It's at those near-zero or negative values when really interesting physical phenomena can occur, because the higher a material's refractive index, the more it distorts light from its original path.

One hitch is that any such material needs to interact with both the electric and magnetic fields of light. Most natural materials are blind to the magnetic field of light at visible and infrared wavelengths. Previous efforts have created artificial atoms composed of two constituents - one that interacts with the electric field, and one with the magnetic. A drawback to this combination approach is that the individual constituents interact with different colors of light, and it is typically difficult to make them overlap over a broad range of wavelengths.

Engineers at Stanford University said their metamaterial, which significantly broadens the range, is an important step toward a future where a "perfect lens" could allow direct observation of an individual protein in a light microscope or where invisibility cloaks could completely hide objects from sight.

Assistant materials science and engineering professor Jennifer Dionne and her group set out to design a single metamaterial "atom" with characteristics that would allow it to efficiently interact with both the electric and magnetic components of light.

Starting with a two-dimensional, planar structure that possessed the desired optical properties, they ultimately created an object shaped like a crescent moon that exhibits a negative refractive index over a wavelength range of roughly 250 nm in multiple regions of the visible and near-infrared spectrum. They believe that, with a few tweaks to its structure, the material could become useful across the entire visible spectrum. "We could tune the geometry of the crescent, or shrink the atom's size, so that the metamaterial would cover the full visible light range, from 400 to 700 nm," graduate student Ashwin Atre said.

"Metamaterials will potentially allow us to do many new things with light, things we don't even know about yet. I can't even imagine what all the applications might be," said postdoctoral fellow and group member Aitzol Garcia. "This is a new tool kit to do things that have never been done before."

The work appears in Advanced Optical Materials(doi: 10.1002/adom.201370025)

For more information, visit: www.stanford.edu