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Graphene provides new route to X-ray generation

24 November 2015

MIT researchers report a phenomenon that they believe might lead to more compact, low-dose, tunable X-ray devices made of graphene.

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)

The most widely used technology for producing X-rays – used in everything from medical and dental imaging, to testing for cracks in industrial materials – has remained essentially the same for more than a century. But based on a new analysis by researchers at MIT, that might potentially change in the next few years.

The finding, based on a new theory supported by accurate 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 new work is reported this week in the journal Nature Photonics, in a paper by MIT professors Marin Soljacic and John Joannopoulos and post-doctoral students, Ido Kaminer, Liang Jie Wong (now at the Singapore Institute of Manufacturing Technology), and Ognjen Ilic.

Soljacic says that 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 says 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 and expensive particle accelerators.

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 is struck 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.

“The reason this is unique is that we’re substantially bypassing the problem of accelerating the electrons,” says Kaminer. “Every other approach involves accelerating the electrons. This is unique in producing X-rays from low-energy electrons.”

In addition, the system would be unique in terms of its tunability, able to deliver beams of single-wavelength light all the way from infrared, through visible light and ultraviolet, on into X-rays. And there are three different inputs that can be used to control the tuning of the output: the frequency of the laser beam to initiate the plasmons, the energy of the triggering electron beam, and the 'doping' of the graphene sheet.

So far, the work is theoretical, based on precise simulations, but the group is now in the process of building a device to test the system in the lab, starting initially with producing ultraviolet sources and working up to the higher-energy X-rays. “We hope to have solid confirmation of the principles within a year, and X-rays, if that goes well, optimistically within three years,” Soljacic says.

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