Particle accelerators range in size from a few square meters to large research centers. Using lasers to accelerate electrons within a photonic nanostructure constitutes a microscopic alternative with the potential of generating significantly lower costs and making devices considerably less bulky. Until now, no substantial energy gains have been demonstrated. In other words, it has not been shown that electrons have increased in speed significantly. A team of laser physicists at the University of Erlangen-Nuremberg (FAU) has now succeeded in demonstrating the first nanophotonic electron accelerator — at the same time as colleagues from Stanford University. While the term “particle accelerator” will often bring to mind the Large Hadron Collider in Geneva, an approximately 27-km-long ring-shaped tunnel, enormous accelerators such as those are the exception. More common are those used in medical imaging procedures or during radiation to treat tumors. Those devices, still, are several meters in size, with room for improvement both in terms of space consumption and performance. Researchers have succeeded in measurably accelerating electrons in structures that are only a few nanometers in size. Courtesy of Julian Litzel/FAU. To reduce the size and improve the performance of existing devices, researchers have pursued dielectric laser acceleration, or nanophotonic accelerators. The structures they use are just 0.5 mm in length, and the channel the electrons are accelerated through measures roughly 225 nm in width. Particles are accelerated by ultrashort laser pulses illuminating the nanostructures. “The dream application would be to place a particle accelerator on an endoscope in order to be able to administer radiotherapy directly at the affected area within the body,” said Tomáš Chlouba, one of the four lead authors of the recently published paper. While that dream may still extend beyond the grasp of the team led by physics professor Peter Hommelhoff, demonstrating a nanophotonic electron accelerator brings that dream closer to reality. “For the first time, we really can speak about a particle accelerator on a chip,” said Roy Shiloh of the Institute of Condensed Matter Physics. About two years ago, the team succeeded in using the alternating phase focusing (APF) method from the early days of acceleration theory to control the flow of electrons in a vacuum channel over long distances, the first major step toward building a particle accelerator. The next step was acceleration, to gain major amounts of energy. “Using this technique, we have now succeeded not only in guiding electrons but also in accelerating them in these nanofabricated structures over a length of half a millimeter,” said researcher Stefanie Kraus. Though this may not sound like much, it is a major step in the field of accelerator physics. “We gained energy of 12 kiloelectron volts. That is a 43% gain in energy,” said researcher Leon Brückner. In order to accelerate the particles over such large distances (when seen from the nanoscale), the FAU physicists combined the APF method with specially developed pillar-shaped geometrical structures. The aim now is to increase the gain in energy and electron current to such an extent that the particle accelerator on a chip is sufficient for applications in medicine. For this to happen, the gain in energy would have to be increased by a factor of approximately 100. “In order to achieve higher electron currents at higher energies at the output of the structure, we will have to expand the structures or place several channels next to each other,” Chlouba said. The advancement by the FAU team was demonstrated almost simultaneously by researchers at Stanford University, whose results are currently under review. The two teams are working together on the realization of the accelerator on a chip in a project funded by the Gordon and Betty Moore Foundation. The research conducted by FAU was published in Nature (www.doi.org/10.1038/s41586-023-06602-7).