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The Most Imperfect Mirror

Publié le 1 juillet, 2008 | Pas de commentaires
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Scientists in New York have recently constructed a blanket of carbon nanotubes shown to be the blackest material ever made, or, put another way, a nearly perfect anti-mirror. Nanotubes seem to have found their way into the spotlight yet again, but this time, these tubes are taking it all for themselves.

 Mirrors in the Raindrops on the Windshield
Kevin Krejci, Mirrors in the Raindrops
on the Windshield
, 2006
Certains droits réservés.

As Plato proposed some two thousand years ago, the idea is always more perfect than its corresponding physical manifestation. We think of a circle and, in so doing, idealize in the mind’s eye a collection of geometrical points equidistant from a common center; but, as any casual, or even careful sketch of a circle demonstrates, the transition from thought to paper quickly reveals imperfections.

Though all scientific disciplines make use of this distinction, perhaps none rely on it as much as physics. First year students in Physics 101 courses often encounter problems involving frictionless surfaces, perfect vacuums, and many other idealized cases of real world examples. This trend towards abstracting away the actual continues even into advanced treatments where most problems remain unsolvable unless certain imperfections are ignored or considered negligible. In fact, it seems fair to suggest that many of the major advances in theoretical physics would have never happened without the ability to disregard many little details of the physical world. And yet, physics purports to be a description of that very same physical world.

Considering the prominent place of the ideal in the historical landscape of science, efforts in experimental researches strive to form the perfect from the imperfect, sometimes towards the goal of better technologies, other times simply for the sake of the thing in itself. And thus, a claim that a new material or method approximates the theoretical ideal deserves attention. Roughly ten years ago, researchers at the MIT department of material science and engineering created what was to be dubbed by the popular press as a perfect mirror. [1]

Very recently another achievement has made similar advances in the exact opposite direction. Materials scientists in Troy, New York have documented the design and construction of what they claim to be the most perfect absorber of electromagnetic radiation (i.e. light): the most perfect anti-mirror yet observed. [2] Whereas a mirror will reflect incoming light back towards the observer, this anti-mirror absorbs almost all photons that strike its surface. Classical electromagnetic theory allows for a simple characterization of this effect. By comparing the ratio of light arriving at a surface to the amount reflected, one can obtain a ratio for the reflectance of the material. The perfect mirror should have a reflectance of 100 percent while the perfect anti-mirror should have a reflectance of 0 percent.

To get a feel for the numbers involved to quantify this achievement, consider: a standard household silver mirror will reflect about 95 percent of the light that falls on it, compared to a piece of amorphous carbon (essentially soot) which reflects only 8.5 percent of the incoming photons. [3] Even a NIST (National Institute of Standards and Technology) certified ‘black’ sample reflects about 1.4 percent of incoming light. This new material has a total reflectance of just 0.045 percent.

The material that has this exciting property is none other than the often heralded, yet rarely applied, carbon nanotube. Research labs around the world have been creating carbon nanotubes for about two decades. At the atomic level, a nanotube is nothing more than a sheet of carbon atoms rolled into a tube. A few billionths of a meter in diameter (about 10nm in this case), these tiny tubes have been lauded as potential saviors in nearly every facet of technology: from faster transistors to stronger tennis rackets, and have even found a role in the infamous space elevator. These tubes are just one of the many novel materials emerging from recent advances in nanotechnology.

The nanotubes used in the anti-mirror are grown in the form of small forests, somewhat resembling a microscopic carpet or a piece of sod. These forests are obtained by decomposing a carbon based gas such as ethylene at high temperatures in the presence of nanoscale catalyst particles. The carbon from the gas organizes itself into a tube structure in the presence of the catalyst. By controlling the density of the nanotubes, the researchers were able to alter the optical properties of the films. At relatively low densities (about 50 nm between each tube), the nanotube carpets were found to have the most pronounced absorption, that is, the lowest optical reflection value.

Since their discovery nearly twenty years ago, fruitful applications for carbon nanotubes have been much easier to invent on paper than to actualize in industry. Proposed technologies for these special films of near zero reflectance include improved solar panels for energy conversion. Time will tell if such ideas can make the transition to practical applications. In the meanwhile, we can be content knowing we might now have a reply to Nigel Tufnel’s timeless question concerning the cover of Spinal Tap’s last album: “that’s so black, it’s like, ‘how much more black could this be?’” Well, apparently this much.

References

[1] Fink, et al. “A Dielectric Omnidirectional Reflector.” Science 282 (1998): 1679-1682

[2] Yang, et al . “Experimental Observation of an Extremely Dark Material By a Low-Density Nanotube Array.” NanoLetters 8.2 (2008): 446-451

[3] Lee and Hernández-Andrés “Virtual tunnels and green glass: The colors of common mirrors.” American Journal of Physics 72.1 (2004): 53-59

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