11 Jan 2006
Materials that have a negative refractive index at optical wavelengths have now been fabricated by scientists in Europe and the US for the first time. Rob van den Berg explains why they could ultimately lead to new types of lens, antireflection coating and biosensor.
The concept of light rays that bend the wrong way on entering a material, an optical Doppler effect that operates in reverse or a perfect lens that can focus beyond the diffraction limit may sound crazy, but these are just some of the intriguing possibilities that have recently excited scientists following the demonstration of materials that have a negative refractive index in the visible and infrared. In fact, 2005 could be remembered as the year when scientists turned the fundamental laws of optics on their head.
Over the past 12 months, several research groups have succeeded in fabricating artificially engineered materials that have a negative refractive index, and they have also started to explore their optical properties. These metamaterials, also known as left-handed materials, promise to create entirely new prospects for controlling and manipulating light, with potential benefits in the fields of optical sensing, nanoscale imaging and photolithography.
The origins of the field date back to 1968, when Russian physicist Victor Veselago first predicted the existence of materials that have a negative refractive index. The refractive index of a material depends on its response to the electric and magnetic components of an electromagnetic wave, measured by its electric permittivity and magnetic permeability. Most materials have positive permittivities - place one in an electric field and the direction of the field induced inside the material will have the same orientation as that of the applied field. The majority also have positive permeabilities and react to magnetic fields in a very similar way.
Veselago speculated that, for materials in which both the permittivity and the permeability are less than zero, the refractive index would also be negative and thus light would refract in the opposite direction to that experienced with conventional materials. It was immediately clear that negative refraction does not occur in nature: whereas for certain frequencies most metals have a negative permittivity - which makes them opaque - they all have a positive permeability. Thus you had to rely on artificial materials (metamaterials) to realize these amazing properties. Unfortunately, no-one knew how to make them.
This all changed at the end of the 1990s when John Pendry, a theorist at Imperial College London, suggested that creating a material featuring an array of resonator-type structures might be the way forward. Using Pendry's ideas, David Smith from the University of California in San Diego succeeded in making a material with a negative refractive index at microwave frequencies.
Smith fabricated interlocking units of thin fibreglass sheets imprinted with copper rings and wires. Sure enough, microwaves incident on a sample of this metamaterial were bent in the opposite direction from normal as they passed through.
Interest in negative index materials then took off again in 2000 when Pendry theorized that slabs of such a material could create a "superlens" that would be able to overcome the diffraction limit and offer improved imaging performance.
He postulated that such lenses could potentially recover the evanescent light that is usually lost from an image as it passes through a lens. Conventional imaging optics cannot access these "evanescent waves", which decay rapidly with distance. However, materials that have a negative index of refraction should be able to restore this lost light and thereby provide imaging capabilities beyond the diffraction limit to which all "normal" lenses are subject.
A group from the University of California, Berkeley, managed to confirm this controversial prediction last year. Nicholas Fang and his colleagues made use of the fact that, at a scale much below the optical wavelength, it is not necessary for both the permittivity and the permeability to be negative to create a negative-index material.
They used a 35 nm thick layer of silver, which had a negative permittivity, to create an optical "superlens" that beats the diffraction limit. In experiments the lens was able to transfer the image of a lithographically written pattern, with a feature width of 40 nm, to a nearby layer of photoresist.
In the absence of the silver layer, the lines imaged onto the photoresist had a measured line width of more than 300 nm - roughly half the wavelength of the light used as the illumination source. With the silver layer present, however, the evanescent waves were recovered and a markedly better resolution was obtained with an observed line width of less than 90 nm.
It was not easy getting the properties of the thin silver film just right. Its surface had to be extremely smooth, because any imperfections would scatter the incident light and wash out the finer details. Also the thickness of the film had to be optimized to prevent absorption losses.
At the same time, several other groups were busy trying to make an optical material with a negative index by exploiting Pendry's idea of using resonator structures. However, to achieve this in the optical regime, the resonators need to be on the nanometre scale unlike the centimetre-sized structures used by Smith for microwaves.
Recently, several groups have succeeded in obtaining a negative permeability, which is a precursor for negative refraction, at optical wavelengths. Vladimir Shalaev and colleagues at Purdue University obtained their result - a refractive index of -0.3 - in a material consisting of closely spaced pairs of parallel gold nanorods, each measuring approximately 100 × 700 nm (Optics Letters 15 December 2005).
"The rods conduct a current because they are a metal, producing an effect we call optical inductance, while a material between the rods produces another effect called optical capacitance," said Shalaev. "The result is the formation of a very small electromagnetic circuit, which works at optical frequencies including the near infrared." The circuit acts as a tuning fork that interacts with incident light and has a well defined resonance. This behaviour can result in negative refraction at frequencies higher than the resonance frequency - an observation that agrees with previous calculations performed by the group.
The Purdue team says that the structure is relatively easy to fabricate and could indeed lead to optical superlenses. "Portable and versatile, the new lens would have the potential to revolutionize the market for most technology areas where light is used," said Shalaev's team member Alexander Kildishev. "These include the optical recording of enhanced DVDs, nanofabrication and optical lithography and enhanced sensing, such as in biomedical sensors and implants."
At the moment the current material is too "lossy" (i.e. too much of the light is absorbed) to exhibit this perfect behaviour, but Shalaev believes that this problem can be overcome. He is also confident that his laboratory will be able to extend its negative index results into the visible part of the spectrum. "We just need to change the size of our structures slightly," he commented.
Recently, another group of researchers managed to create a negative index material that operates in the visible regime using a very similar approach. Alexander Grigorenko and his colleagues from the University of Manchester, UK, used nanofabrication methods to make a patterned surface consisting of tapered gold pillars arranged periodically in pairs (Nature 17 November 2005).
Over a limited frequency range in the visible part of the spectrum, these pairs behave as small, high-frequency bar magnets, which cancel the magnetic component of the incident radiation. The effect is due to the excitation of plasmon resonances between the material's pillars and it leads to a gold structure with a negative permeability and negative refraction index.
The material exhibits some curious optical properties, including acting as a perfect antireflection coating. By matching the impedance (defined as the ratio of the permittivity and permeability) to that of an adjacent dielectric, Grigorenko was able to stop the gold film from reflecting green light. The effect is akin to impedence matching in the world of electronic engineering, which is used to stop signals reflecting from the ends of cables.
Grigorenko is now looking at optimizing the design of the material to create a perfect lens, but he admits that this will be quite a challenge. "There remain significant hurdles to overcome, such as a reduction in the losses in the system," he commented. "We are now trying to increase the coupling to the light by increasing the density of the pillars, making them more conical and using a dielectric between them."
Grigorenko and his team exploit the same theoretical concept as the Shalaev paper and Pendry is upbeat about their recent results. "They have succeeded not only in creating a negative electric and magnetic response at the same frequency, which is already a considerable achievement, but in addition they have sufficient control to adjust the precise values to give an impedance match to free space," he commented. "This will be important if they want to make practical use of their structure."
According to Grigorenko, there are numerous potential applications of these metamaterials in the fields of optoelectronics and biochemical sensing. "Our samples can be used as selective optical filters, antireflection coatings and very high-frequency modulators," he explained. "As the plasmon modes are very susceptible to subtle changes in the environment, our fabricated nanomaterial can detect very small changes in the ambient index of refraction. This can be used for developing biosensors."
As the electromagnetic field between the nanopillars is strongly enhanced near the plasmon resonances, it also means that the material could be used to create substrates for surface-enhanced Raman spectroscopy. Other opportunities include nanolasers made from the nanopillars.