Peterborough, N.H. - Materials scientists appear to be homing in-albeit by degrees-on optical materials that could leverage a negative index of refraction to exhibit extraordinary properties. Such "left handed" materials have long been predicted theoretically. But only recently, with the aid of nanostructures, have physical systems been built that have shown the required response, though at levels too weak to yield practical optical elements.
A Purdue University group led by Vladimir Shalaev showed an array of specially designed gold nanopillars in April that exhibited a small negative index of refraction at the 1.5-micron wavelength used in telecommunication systems. Now, Alexander Grigorenko and colleagues at Britain's Manchester University, working with a group at Russia's Chernogolovka Institute of Microelectronics Technologies, have employed a similar strategy to fabricate a material that has a negative permeability at visible wavelengths. The team demonstrated a slight negative index of refraction for light at certain polarizations and angles of incidence.
The Manchester group also found that tuning the impedance of a dielectric medium to the same value as the artificial material enables light to penetrate the material without any reflection-highly unusual for a conductor. The internal modulation of the light wave, resulting from magnetic resonances in the nanostructured material, was another unexpected effect.
In the Manchester architecture, defined using electron-beam lithography, pairs of tapered gold nanopillars were arrayed on a two-dimensional glass surface. At certain polarizations and angles of incidence, light induced opposite currents in each nanopillar pair, generating a magnetic field.
"Our future challenge is to increase the magnetic response of the material by optimizing the geometry of the nanopillars, by improving the coupling of the antisymmetric magnetic mode to light, and simply by increasing the density of nanopillars," said Grigorenko.
Since a light wave consists of self-reinforcing magnetic and electric fields that oscillate, the magnetic and electric characteristics of a material, combined with the frequency of the light, determine whether the light is reflected or transmitted and how it bends as it passes through the material. Normally, at the frequencies of visible light, no magnetic response will occur in normal materials that transmit light. That absence of a magnetic response explains why natural materials always have a positive index of refraction.
Magnetic permeability is key to producing materials with a negative index of refraction. By starting with Maxwell's equations for electric and magnetic fields, it is possible to predict the properties of materials by inserting negative values for magnetic permeability and other parameters.
A detailed theory of what would happen when a material had both negative permeability and negative electric permittivity was published by Russian physicist Victor Veselago of the Lebedev Physics Institute in 1968. Nearly 30 years later, in 1996, John Pendry at London's Imperial College proposed building such materials from arrays of tiny wires. The dimensions of such an array would have to be on the same order of magnitude as the wavelength of light, which for visible light would mean nanoscale dimensions. Pendry also showed that a flat slab of such a material would focus light like a lens, but without any distortion. It is that property that could revolutionize optics, provided practical, high-quality materials that exhibit it can be built.
Pendry also pointed out that at microwave frequencies, micromachined arrays of conducting rings would exhibit a magentic response. Researchers at the University of California, San Diego realized that concept in 2000, in a structure consisting of conducting rings enclosed in a resonant cavity.
The current work with gold nanorods resembles the microwave approach in some ways. The progress from microwave to optical frequencies is following a familiar pattern, Grigorenko pointed out, citing the history of the laser, which was first demonstrated at microwave frequencies.
"We can ask the same question about the connection between masers and lasers: Are lasers a scaled-down version of a microwave resonator architecture? And the answer would be, Yes and no," Grigorenko said. "Yes, because lasers are based on exactly the same ideas as masers: a resonator, an active medium and a pumping technique. No, because the laser's resonators are different-lasers have open resonators, while masers mostly work with closed resonators.The laser's active medium is different, and laser pumping is completely different. The transition from microwaves to visible light is always tricky because of the nature of Maxwell equations."
In the same way, the pairs of nanopillars form open resonators that generate the negative permeability required for novel optical effects. In the microwave version, the resonators are closed split-ring structures that use induced eddy currents.
Thus far, the Manchester University metamaterial has not produced a large enough negative index to demonstrate Pendry's concept of a flat lens. Also, the structure so far is two-dimensional, and in order to get the focusing effect, a three-dimensional slab will be required.
"At the moment, we are working on multilayered electron-beam lithography. But we do realize that any other, cheaper method of making 3-D materials would be of great advantage," said Grigorenko.
The next step for the researchers is to fine-tune the array to enhance the magnetic response while reducing the energy losses that the resonant modes create. So it may be some time before practical, cost-effective materials with essentially new optical properties become available.
Also, at present "it is very difficult to envisage" where this new capability will find applications in industry, Grigorenko said, much as it was for lasers in the early days of that research.
"The most promising appear to be the perfect lens and nanolaser applications. However, some other applications might be much easier to achieve, like optoelectronic modulators, biosensors, Raman and optical tweezers, and substrates. One might see the interest from industry coming from these areas," he said.
As researchers work toward 3-D materials, the 2-D forms might produce some useful applications, he said. "Our samples can be used as selective optical filters, antireflection coatings and very high-frequency modulators. Another possible area is bio and chemical analysis," Grigorenko said.
"The plasmon modes are usually very susceptible to subtle changes in environment, and our structure is not an exception. We have found that the fabricated nanomaterial can be used to detect very small changes in the ambient index of refraction. This behavior can be used for developing biosensors."
The pillar structure has an added benefit beyond optical applications, since it can strongly concentrate electromagnetic fields. This could lead to nanoscale lasers and "optical tweezers" in which light fields are used to manipulate physical objects.