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Two researchers say they have built a cylinder that acts as an ersatz electromagnetic black hole, soaking up radiation in the microwave regime like the astrophysical version sucks up matter and light.
Qiang Cheng and Tie Jun Cui of the State Key Laboratory of Millimeter Waves at Southeast University in Nanjing, China, detailed their creation in a paper posted to the online physics preprint Web site arXiv.org last week. Cheng and Cui report engineering a thin cylinder 21.6 centimeters in diameter comprising 60 concentric rings of so-called metamaterials—composite structures specifically crafted to possess unique light-bending capabilities.
Unlike ordinary magnifying glasses, lenses made from metamaterials can have a negative index of refraction, meaning that refracted light bends to the same side of the "normal," the imaginary line perpendicular to the surface of the lens, as does the incident light. In the past few years, research groups around the world have harnessed metamaterials to create "superlenses" as well as for so-called invisibility cloaking, in which light is bent around an object as if it were not there.
The laboratory black hole is based on a similar approach—establishing a graded index of refraction to bend electromagnetic radiation inward to the cylinder's core. The core, in turn, is an efficient absorber of electromagnetic radiation. In one possible application, the core would be replaced with a "payload" such as a solar cell, with the outer layers funneling light inward. But Cui cautions that such an implementation is a long way off, requiring both that the device be modified to work at visible wavelengths and that the two-dimensional ring be extended to three dimensions.
Cheng and Cui's work represents the preliminary realization of a theoretical proposal put forth just this year by Evgenii Narimanov and Alex Kildishev of Purdue University for a metamaterial structure that could absorb incident light from all directions.
Narimanov, a professor of electrical and computer engineering, says that in the wake of his work with Kildishev, as well as the many studies into extreme light manipulation with metamaterials, he is not surprised to see the theoretical made real. "It's impressive, though, how quickly they have done it," he says.
John Pendry, a physicist at Imperial College London who was among the first to harness the unusual properties of metamaterials, says the new research "constitutes an entirely novel way of constructing an absorber, but at the same time keeping control of the absorbed radiation."
Nevertheless, Pendry notes, the analogy to black holes is imperfect. "Black holes absorb incident radiation and other objects, but the key point about real black holes is the prediction of Hawking radiation emitted by the black hole," he says, referring to physicist Stephen Hawking's hypothesis that is rooted both in general relativity and quantum mechanics. Were it observed, Hawking radiation would provide critical insight into the complicated boundary of the two theories. "A real black hole powers the radiation through its gravitational energy," Pendry says, "but the device reported in this paper has no internal source of energy and therefore cannot emit Hawking radiation."
Besides, the metamaterial black hole is not as ruthlessly voracious as the gravitational kind. Cui estimates that the demonstration black hole only absorbs 80 percent of the microwaves that hit it but that increasing the frequency of the incident light—to visible wavelengths, for instance—will increase absorption. Such an artificial black hole for optical light is in the works and might even be developed by the end of the year, Cui says—a prediction that may raise a few eyebrows in the field. "I think that the authors are rather optimistic in projecting into the visible region," Pendry says. "But I would be very happy to be proved wrong."
