Researchers from A*STAR Singapore Institute of Manufacturing Technology (SIMTech) and the Massachusetts Institute of Technology (MIT) develop small and powerful X-ray sources from graphene, plasmons and electrons

Since their discovery in 1895, X-rays have led to significant advances in science, medicine and industry. From probing distant galaxies to screening at airport security and facilitating medical diagnosis, X-rays have allowed us to look beyond the surface and see what lies beneath.

Now, a collaboration between the A*STAR Singapore Institute of Manufacturing Technology (SIMTech) and the Massachusetts Institute of Technology (MIT) in the United States has proposed a versatile, directional X-ray source that potentially could fit on a laboratory bench and is based on the intriguing two-dimensional material graphene.

Compact yet tunable

X-rays are high-frequency electromagnetic waves that can be generated using X-ray tube technology dating back to the 19^th century or from huge sources like synchrotrons and kilometer-long free electron lasers.

X-ray tube sources, popularly used in medical diagnostics, emit radiation in all directions. But since only X-rays generated in the forward direction can be used for imaging, a significant amount of the generated X-rays are wasted. Moreover, they are not ‘tunable’, meaning that a different X-ray source has to be installed in a diagnostic device for each desired wavelength.

Kilometer-long free electron lasers, on the other hand, can produce intense, tunable X-rays by accelerating free electrons to extremely high energies and then causing them to ‘wiggle’ with an undulator stage. But these enormous X-ray sources only exist in a few places in the world and are housed in very large, expensive and over-subscribed facilities.

An X-ray source that is both small and powerful has been much sought after for some time. This challenge intrigued Liang Jie Wong from SIMTech and his collaborators at MIT: Ido Kaminer, Ognjen Ilic, John Joannopoulos and Marin Soljačić. “We wanted to combine the best of both worlds by creating something that is compact and also capable of producing very intense X-rays,” says Wong. “Essentially, we wanted to implement the concept behind the enormous free-electron-laser sources on a scale small enough to fit on a laboratory table or even a microchip.” This goal led the researchers to the wonder material graphene, the subject of the 2010 Nobel Prize for Physics.

X-ray wiggles

Graphene is a one-atom-thick sheet of carbon atoms arranged in a honeycomb-like fashion. Its suite of enviable properties include high mechanical strength and near optical transparency, as well as excellent thermal and electrical conductivity. But more importantly for Wong and his MIT collaborators, the two-dimensional material can also support plasmons — collections of electronic oscillations that can be used to confine and manipulate light on scales of around ten nanometers.

The idea for a graphene plasmon-based free electron X-ray source was born toward the end of Wong’s postdoctoral fellowship at MIT, where he met several scientists studying plasmons in graphene. Together, they realized that graphene might be the key to realizing the long-standing goal of a compact and powerful X-ray source. “Graphene plasmons were a natural option because they are capable of confining electromagnetic radiation to very small scales,” Wong explains.

In November 2014, Wong joined SIMTech and began working on the project together with his MIT collaborators. His first steps were to develop a robust ab initio simulation tool that models the exact physics of electrons interacting with a plasmon field, which is sustained on a graphene sheet deposited on a piece of dielectric. By performing numerical simulations, the SIMTech–MIT team showed that this set-up induces a ‘wiggling’ motion in electrons fired through the graphene plasmons, causing the electrons to produce high-frequency X-ray radiation (see image). The simulations agreed with the analytical theory the team had developed to explain how electrons and plasmons interact to produce X-rays.

One standout characteristic of such a source will be its directionality, or ‘pointability’, which will increase efficiency and hence lower costs by ensuring that all the generated radiation goes where it is supposed to. This will make the source promising for medical treatments as it could be used to target tumors more precisely and hence minimize damage to surrounding organs and cells, notes Wong.

Perhaps most attractive will be the source’s versatility. The output radiation frequency can be tuned in real time from longer infrared rays to shorter X-rays by modifying various elements of the source, such as the speed of the electrons, the frequency of the graphene plasmons and the conductivity of the graphene.

This flexible, compact source is promising as a cost-effective alternative to the high-intensity beams used for fundamental scientific and biomedical research. “Although there is a long way to go to actual realization, this is a very exciting research direction.” says Wong. “Developing an intense X-ray source that can fit on a table or be held in one’s hand would potentially revolutionize many areas of science and technology.” The team next plans to experimentally verify their concept with proof-of-principle trials.

About the Singapore Institute of Manufacturing Technology

The A*STAR Singapore Institute of Manufacturing Technology (SIMTech) develops high-value manufacturing technology and human capital to contribute to the competitiveness of Singapore’s industry. It collaborates with multinational and local companies in the precision engineering, electronics, semiconductor, medical technology, aerospace, automotive, marine, oil and gas, logistics and other sectors.