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Graphene hints at novel method to make X-rays

30 Nov 2015  | David L. Chandler

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The applications of X-rays extend from medical and dental imaging, to testing for cracks in industrial materials. Now, researchers from MIT and in Singapore have found that the most widely used technology for producing X-rays, which has essentially remained the same for quite a long time, may enter a transformative phase in the coming years.

The finding, based on a new theory backed by exact simulations, shows that a sheet of graphene could be used to generate surface waves called plasmons when the sheet is struck by photons from a laser beam. These plasmons in turn could be triggered to generate a sharp pulse of radiation, tuned to wavelengths anywhere from infrared light to X-rays.

What's more, the radiation produced by the system would be of a uniform wavelength and tightly aligned, similar to that from a laser beam. The team says this could potentially enable lower-dose X-ray systems in the future, making them safer. The work is reported in the journal Nature Photonics, in a paper by MIT professors Marin Soljacic and John Joannopoulos and postdocs Ido Kaminer and Ognjen Ilic, and Liang Jie Wong at the Singapore Institute of Manufacturing Technology.

Novel method for generating X-rays

By using plasmons to 'wiggle' a free electron in a sheet of graphene, researchers have developed a method for generating X-rays. In this image of one of their simulations, the colour and height represent the intensity of radiation (with blue the lowest intensity and red the highest), at a moment in time just after an electron (grey sphere) moving close to the surface generates a pulse. (Courtesy of the researchers)

Soljacic said there is growing interest in finding new ways of generating sources of light, especially at scales that could be incorporated into microchips or that could reduce the size and cost of the high-intensity beams used for basic scientific and biomedical research. Of all the wavelengths of electromagnetic radiation commonly used for applications, he said, "coherent X-rays are particularly hard to create." They also have the highest energy. The new system could, in principle, create ultraviolet light sources on a chip and table-top X-ray devices that could produce the sorts of beams that now require huge, multimillion-dollar particle accelerators.

To make focused, high-power X-ray beams, "the usual approach is to create high-energy charged particles [using an accelerator] and 'wiggle' them," stated Kaminer. "The oscillations will produce X-rays. But that approach is very expensive," and the few facilities available nationwide that can produce such beams are highly oversubscribed. "The dream of the community is to make them small and inexpensive," he indicated.

Most sources of X-rays rely on extremely high-energy electrons, which are hard to produce. But the new method gets around that, using the tightly-confined power of the wave-like plasmons that are produced when a specially patterned sheet of graphene gets hit by photons from a laser beam. These plasmons can then release their energy in a tight beam of X-rays when triggered by a pulse from a conventional electron gun similar to those found in electron microscopes.

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