![]() Such beams could have applications in crystallography, the team says, which is used in many scientific fields to determine the precise atomic structure of molecules. And there are three different inputs that can be used to control the tuning of the output, Kaminer explains – the frequency of the laser beam to initiate the plasmons, the energy of the triggering electron beam, and the “doping” of the graphene sheet. In addition, the system would be unique in its tunability, able to deliver beams of single-wavelength light all the way from infrared, through visible light and ultraviolet, on into X-rays. This is unique in producing X-rays from low-energy electrons.” “Every other approach involves accelerating the electrons. “The reason this is unique is that we’re substantially bypassing the problem of accelerating the electrons,” he says. 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. 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. ![]() Most sources of X-rays rely on extremely high-energy electrons, which are hard to produce. “The dream of the community is to make them small and inexpensive,” he says. But that approach is very expensive,” and the few facilities available nationwide that can produce such beams are highly oversubscribed. To make focused, high-power X-ray beams, “the usual approach is to create high-energy charged particles and ‘wiggle’ them,” says Kaminer. 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. 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. Soljačić 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. The new work is reported this week in the journal Nature Photonics, in a paper by MIT professors Marin Soljačić and John Joannopoulos and postdocs Ido Kaminer and Ognjen Ilic, and Liang Jie Wong at the Singapore Institute of Manufacturing Technology. ![]() The team says this could potentially enable lower-dose X-ray systems in the future, making them safer. 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. These plasmons in turn could be triggered to generate a sharp pulse of radiation, tuned to wavelengths anywhere from infrared light to X-rays. The finding, based on a new theory backed by exact simulations, shows that a sheet of graphene – a two-dimensional form of pure carbon – could be used to generate surface waves called plasmons when the sheet is struck by photons from a laser beam. But based on a new analysis by researchers at MIT and in Singapore, that might potentially change in the next few years. 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.
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