A particle accelerator just 0.2 millimetres long is the smallest device of its kind ever built. It is the first tiny accelerator that can produce fast and well-focused bunches of electrons, and could eventually be incorporated into a pen tip for use in medicine.
Particle accelerators, like the Large Hadron Collider or those in medical facilities for treating cancer, speed up particles using electric fields and magnets. Those electric fields are typically generated using radio-frequency waves that have wavelengths measured in metres or centimetres. Peter Hommelhoff at the University of Erlangen-Nuremberg in Germany and his collaborators chose to accelerate particles using a different kind of electromagnetic wave – light – which has a wavelength measured in hundreds of nanometres. This allowed them to shrink the size of their accelerator from kilometres to under a millimetre.
They made it from silicon shaped into thousands of 2-micrometre-tall pillars that were arranged into two parallel lines, each 0.2 millimetres long. To run the accelerator, the researchers shone laser light on this pillar-lined “runway” from above while injecting electrons into it from the side. The light waves from the laser interacted with the pillars in just the right way to create an electromagnetic field that made the electrons cluster together in narrow bunches. These clusters then accelerated through the structure at speeds of over a hundred thousand kilometres per second.
The team experimented with adding more pillars to the runway. When they built a 0.5-mm-long version, they found that they could accelerate the electrons at even greater rates, increasing the energy they carried by 43 per cent. This indicates that the accelerator is scalable and can be made more powerful while remaining small enough to be integrated on chips or even directly on the end of an optical fibre, says Hommelhoff.
Pietro Musumeci at the University of California, Los Angeles says that some tiny accelerators have been built before, but this is the first device of this size that not only accelerates electrons but also keeps them constrained into a relatively narrow beam that can be used in scientific experiments. “An accelerator is not just a scheme that gives energy to a particle; you also need to be able to transversely confine the particles,” he says.
At the moment, the new accelerator only gives electrons about a millionth of the energy that they gain in larger accelerators. But Hommelhoff says there may be ways to boost each electron’s energy. He thinks making the pillars from a glass material called fused silica, which can withstand more intense laser light, might help.
Hommelhoff says scientists first proposed using light to shrink accelerators in the 1960s, but engineering challenges made it difficult to produce the technology at the time.
“We think we can eventually shrink accelerators so that they will fit into the tip of a pen. Then, you can really think of new treatment tools for doctors or small-scale sterilisation tools for biology labs,” says Hommelhoff. “We think that we haven’t even yet conceived of all the possible [applications].”