25 April 2012—Scientists in England and Valencia, Spain, have constructed what may be the first practical metamaterial that manipulates visible light. The researchers, who predict it could be used for subpicosecond optical switches and finely controlled laser pulses, reported the results at the March meeting of the American Physical Society. The layered structure of the Valencia team’s material, in contrast to the makeup of earlier visible-light devices, means it can conceivably be built up into a usable, full-size object. However, not everyone is convinced this is possible.
Metamaterials are engineered to interact with light in unnatural ways. Most metamaterials are constructed of repeating features that interact with electromagnetic waves to give the material both a negative electric permittivity and a negative magnetic permeability. Electric permittivity is a measure of how resistant a material is to electric fields forming within it; the higher the permittivity, the smaller the electric field that forms per unit of charge. Magnetic permeability is similar, but the higher a material’s permeability is, the larger the magnetic field it can support. Materials with both negative permittivity and permeability have a negative index of refraction, so they bend light the wrong way.
These metamaterials, for the most part, don’t let much light through in the visible spectrum. Visible light waves dwindle to nothing after passing through material a fraction of a wavelength thick. But a unique manufacturing technique lets a small piece of the spectrum pass through the new metamaterial.
The researchers, led by Carlos Garcia-Meca and based at the Valencia Nanophotonics Technology Center and King’s College London, laid down alternating 15- to 35-nanometer-thick layers of silver and hydrogen silsesquioxane (a type of glass). They then etched rectangular holes through the layers with a focused ion beam to make a structure that looks something like a fishnet. This “nanofishnet” structure has become a standard arrangement in metamaterials, with each hole acting as an artificial atom. But Garcia-Meca says his group’s nanofishnet has two unprecedented features: its multilayer composition and its use of second-order magnetic resonance to create negative magnetic permeability for red and near-infrared light. To understand the concept of resonance, think of a guitar string. Its pitch correlates to the fundamental vibration of the string—the note that exactly fits its wavelength on the length of that string. But a guitar string produces more than just that frequency. It makes a complex sound with multiple resonances, each a whole number of half wavelengths.
The fishnet metamaterial is more complex than a guitar string, because it is two-dimensional, but the same idea holds. The researchers tuned their material so that the second-order magnetic resonance, which vibrates along the diagonal between the holes in the nanofishnet, is much stronger than the first-order resonance for red and infrared light. That difference creates the negative magnetic permeability for wavelengths in those parts of the spectrum. Combined with silver, which naturally has a negative electric permittivity, the negative permeability gives rise to a low-loss negative index of refraction.
The group was able to adjust the material’s index of refraction simply by varying the size of the holes in the fishnet. One version they produced had a negative index (and therefore low loss) for wavelengths from 620 to 713 nm; a second version had a negative index at wavelengths from 694 to 806 nm.
Garcia-Meca’s group “got the negative index, with a good figure of merit,” says Costas Soukoulis, a physicist at Ames Laboratory at Iowa State University who was not involved in the research. In visible wavelengths, Soukoulis says, this is unique. “But some people don’t believe it,” he says, because of the indirect way the team measured the negative refractive index. “[The skeptics] want to see a wedge experiment at optical wavelengths,” says Soukoulis, referring to the classic experiment in which a wedge of material bends light. A wedge would give an easily observed negative index of refraction, and Soukoulis’s own team is now trying to produce just such a wedge of metamaterial.
The new material’s formulation also takes advantage of the fact that the more layers it contains, the better the resonance and the less lossy it gets. The thickest piece of silver-and-glass metamaterial Garcia-Meca’s team has made so far has eight layers. Garcia-Meca says their technique can make a material 15 to 20 layers thick, totaling about 450 nm. But each layer needs to be precisely engineered. New manufacturing techniques will be required to make bulkier versions
The group admits that, despite the promise the research shows, making a bulk metamaterial out of silver has its drawbacks. Silver is hard to work with, because it oxidizes quickly in air and has a minimum thickness of 15 nm. (Layers of silver thinner than that clump into discontinuous islands of metal.) Gold is somewhat easier to work with and has properties that are almost as good for visible wavelengths. But both gold and silver are prohibitively expensive. And perhaps most troublesome, neither noble metal is compatible with today’s semiconductor technology.
Silver and gold are “major no-gos in most semiconductor clean rooms,” says Alexandra Boltasseva, a metamaterials researcher at Purdue University, in West Lafayette, Ind., who says these metals “can kill the [desirable] properties of the semiconductor structures.” Boltasseva is on the hunt for substitutes with properties equal to or superior to the noble metals. In her presentation at the March meeting, she demonstrated that titanium nitride has properties almost as good as gold’s for the infrared frequencies around 1.5 micrometers, which are widely used in telecommunications. Oxides such as aluminum-doped zinc oxide are also promising for the infrared and terahertz frequencies.
But for bending visible light the wrong way, nothing beats a sliver of silver and glass—except, perhaps, a wedge of it.
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