About a week ago, I reported on another 'teaser' in the media about 'optical cloaks', hypothetical devices which would in principle make objects contained in their core completely invisible. Such devices have gotten a lot of attention, both scientifically and in the press, since the publication of two fascinating theoretical papers in 2006. I recently wrote a post, which can be found here, summarizing those original two papers.
The press reports a week ago suggested another major breakthrough in cloaking research, with headlines such as "Science close to unveiling invisible man" and "Invisibility cloak closer than you think." They were a little confused about what exactly had been accomplished, however: had the researchers made a three-dimensional invisibility cloak, a cloak that works at visible frequencies, or both?
Well, it turns out that they've done neither! This is another example of the press hunting for the best 'hook' for the story, no matter how tangentially related to the actual research. What has been accomplished, however, is the development of low-loss, three-dimensional negative refractive index materials which work for visible wavelengths, which is an important and interesting accomplishment in and of itself. I give a brief answer to the question, "What is a metamaterial?" below the fold, followed by an explanation of the actual results of the recent Berkeley papers and an analysis of how the press got themselves confused again.
The term 'metamaterial' is a bit ambiguous, but may be loosely described as a material whose optical properties arise from the structure of the material, rather than its chemical composition. This definition sounds good but is not quite complete, because many natural materials do have optical properties which arise from structure: for instance, carbon comes in two common solid forms (allotropes): diamond (highly transparent to visible light), and graphite (completely opaque). Probably because of this, the term is usually restricted to materials which are engineered to have optical properties not found in nature.
The idea of metamaterials really took off in 2000, when J.B. Pendry published a paper with the provocative title, "Negative refraction makes a perfect lens."1 The paper speculated that a material with a negative refractive index (something not found in nature) could result in an imaging system which could resolve objects spaced much closer than the wavelength of light. Ordinary imaging systems fail to resolve objects which are closer than a wavelength apart.
Negative refraction is a fascinating idea, first proposed theoretically in the 1960s by Russian scientist V. Vesalago.2 In short, if one can produce a material with a negative refractive index, light will be refracted in the opposite direction than an ordinary positive index material:
This ability to make light bend in unusual directions opens up many new possible applications, among them the idea of 'superlenses' and, of course, 'optical cloaking'. I'll discuss negative refraction and its applications in more detail in a future post.
To make a material with a negative refractive index, however, one must be able to control the structure of the material on a scale significantly smaller than a wavelength. If one is working with microwaves, with a wavelength on the order of centimeters, this is straightforward, albeit not necessarily easy. If one is working with visible light, however, with a wavelength smaller than a micrometer (a millionth of a meter), it is extremely difficult.
Furthermore, it is difficult at any wavelength to fabricate a fully three-dimensional metamaterial structure. Most early work on metamaterials has used a thin 'slice' of metamaterial, though most conceived applications require a 'chunk' of metamaterial.
Both of these difficulties (small wavelengths, three-dimensional) are illustrated by the work of Schurig et al.3, which resulted in the first demonstration of a 'cloaking' effect.
The image, taken from their paper, shows their cloak, designed to operate at microwave frequencies and designed to cloak in a two-dimensional (flat) geometry, with a total diameter of 12 cm. The inset shapes are illustrations of the fundamental building block of the cloak: so-called 'split ring resonators', structures which produce both an electric and magnetic response to an illuminating light wave. To make such a device work at visible light frequencies, one would need to produce similar structures with a size less than a millionth of a meter. To make such a device work for a three-dimensional geometry, one would need to make these structures fill a volume of space.
The researchers at Berkeley, though they did not make a cloak, did demonstrate an ability to fabricate a negative refractive index material at visible light frequencies. Furthermore, they were able to make these materials in 'bulk', i.e. they made a fully three-dimensional 'chunk' of metamaterial. Two papers appeared simultaneously in Science and Nature, each of which provided a different technique for making such a bulk, visible light metamaterial.
In the Science paper, negative refraction is achieved by the use of an array of silver nanowires. The wires are individually 60 nm wide and have a 110 nm center-to-center distance. Two samples were made, with thicknesses of 5 μm and 11 μm. A schematic of such an array is shown below (adapted from the paper):
Negative refraction was demonstrated at wavelengths of 660 nm and 780 nm for this structure.
In the Nature paper, negative refraction was demonstrated by the use of a layered 'fishnet' structure, also constructed from silver, illustrated schematically below:
The separation between 'unit cells' of the structure was 860 nm. The device demonstrated negative refraction at a wavelength of 1775 nm. The device consisted of 21 layers of fishnets, with about 80 nm for each layer.
Both devices demonstrate a big step forward for metamaterials. The development of three-dimensional negative refraction in the visible range of wavelengths paves the way for more research and applications including, in the far future, optical cloaking. Furthermore, the nanowire device works over a broad range of visible light frequencies, which most earlier materials fail to do.
A "Harry Potter" cloak looks to still be a long, long way off, though. Although these papers demonstrate 'bulk' metamaterials, the devices are still extremely thin: 11 millionths of a meter for the nanowire device! Furthermore, although these new materials have relatively low absorption compared to previously considered metamaterials, it is still much too high to make an optical cloak: an exponential decay rate of 0.43/μm is listed for the nanowire array! Light would be completely absorbed after traveling only a millimeter in such a material.
In defense of the authors, they hardly mention cloaking in their work: the Science paper briefly cites the work of Pendry and Schurig as a possible application of metamaterials, while the Nature paper does not mention cloaking at all! This research is properly called a significant advance in the fabrication of metamaterials, and is only remotely related to the development of optical cloaks.
Why, then, did we get headlines such as, "Science close to unveiling invisible man"? It seems that reporters, always looking for a 'hook' to popularize their articles, have found a dream-come-true in cloaking: one needs metamaterials to make an optical cloak, so any progress in the field of metamaterials = cloaking device, in their eyes! I fully expect to see every new result in metamaterials development transformed into an invisible man story.
1 J.B. Pendry, "Negative refraction makes a perfect lens," Phys. Rev. Lett. 85 (2000), 3966.
2 V.G. Veselago VG (1968), "The electrodynamics of substances with simultaneously negative values of ε and μ," Sov. Phys. Usp. 10 (4) (1968), 509–14.
3 D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, D. R. Smith, “Metamaterial electromagnetic cloak at microwave frequencies,” Science 314 (2006), 977-980.
Yao, J., Liu, Z., Liu, Y., Wang, Y., Sun, C., Bartal, G., Stacy, A.M., Zhang, X. (2008). Optical Negative Refraction in Bulk Metamaterials of Nanowires. Science, 321(5891), 930-930. DOI: 10.1126/science.1157566
Valentine, J., Zhang, S., Zentgraf, T., Ulin-Avila, E., Genov, D.A., Bartal, G., Zhang, X. (2008). Three-dimensional optical metamaterial with a negative refractive index. Nature DOI: 10.1038/nature07247