Laser-driven mini x-ray sources could reach brightness of synchrotrons
The proposed device relies on shooting electrons and ultrafast light pulses at a metasurface.
Apr 30th, 2019
Example medical x-ray application (Image: A*STAR) A compact ultrafast-laser-based metasurface x-ray device has the potential to produce x-rays at a brightness comparable to existing x-ray medical-imaging devices, and ultimately at the extremely high brightness level of a synchrotron, according to a theoretical study by researchers at the A*STAR Singapore Institute of Manufacturing Technology (SIMTech; Singapore), and collaborators from the Massachusetts Institute of technology (MIT; Cambridge, MA), Technion (Haifa, Israel), and the University of Mons (Mons, Belgium).1 Synchrotron x-ray sources are bright enough to allow detailed study of nanoscopic structures such as proteins or complex crystals. However, they are large installations -- typically tens of meters in scale, requiring entire buildings to house them.Liang Jie Wong from A*STAR and his team envisage a tabletop apparatus for their X-ray generators, which rely on the interaction between a few-cycle-pulse laser emitting wavelengths between infrared and ultraviolet, and electron energies around five MeV, a regime achievable by current state-of-the-art electron guns. The arena for the interaction between the laser and the electrons is a silver metasurface structure on a glass slide. The laser is directed at the surface at an angle, creating plasmon polaritons. The electrons are then shot parallel to the surface into the plasmon polaritons, which interact with the free electrons, causing their trajectories to undulate, generating x-rays. The upconversion to X-ray energies is a result of the properties of plasmon polaritons. These hybrid particles are strongly confined on the surface, which concentrates the intensity. As the spatial dimension is greatly reduced, the polariton's momentum is greatly increased at a given energy, resulting in the conversion from few-eV plasmon polaritons into keV X-rays, using MeV electron energies. The team explored a range of configurations for the metamaterial, with groups of structures ranging in size and spacing from 5 to 26 nm and spaced regularly around 90 nm apart. The results showed it was possible to control the spatial and temporal characteristics of the x-rays by changing parameters such as the geometry of the metasurface or the shape of the electron wave-packets. The ability to control the beam features is a huge benefit because x-rays are challenging to focus and steer. Wong points out that with the right configuration, highly directional X-rays that are coherent can be generated. Generating coherent x-rays gives the process a big advantage over conventional medical imaging because it allows phase-contrast imaging, a technique that can give higher contrast than the absorption processes that form conventional x-ray scans. The team developed software to make ab initio calculations using classical electromagnetic theory and then cross-checked them with a second approach based on quantum electrodynamics. They found excellent agreement between the two approaches, which has given them confidence to take the next step. Wong and his co-workers now plan to conduct proof-of-principle experiments with the new X-ray source. Source: https://research.a-star.edu.sg/research/8039/future-bright-for-mini-synchrotrons REFERENCE: 1. Gilles Rosolen et al., Light: Science & Applications (2018); doi: 10.1038/s41377-018-0065-2.