Medieval philosophers debated how many angels could dance on the head of a pin; modern scientists are getting close to sticking a particle accelerator on one instead.

Particle physicists have announced the demonstration of a nano-accelerator that uses lasers to push electrons to high speeds. Naturally tiny machines like this are not going to replace vast machines the size of cities any time soon, but they should address questions that don’t require pushing matter to such extremes, and could make medical treatments far more targeted.

Along with the giant particle accelerators such as at CERN, a variety of smaller ones exist down to those that can sit on a (large) benchtop and move charged particles to more modest speeds. They need to be this big because they use radio frequency waves, whose wavelengths are too far apart for anything shorter, to do the accelerating. In an age of miniaturization, physicists have dreamed of going smaller still by using optical light as the force to save money and energy. The gains in energy achieved on microscopic devices were too small to be useful, however.

Now, teams at Freidrich-Alexander-Universität (FAU) and Stanford each claim to have provided useful amounts of energy using particle accelerators barely large enough to see. To do it, they had to combine recent advances with a once-popular idea that has more recently been neglected.

Nanophotonic accelerators, also known as dielectric laser accelerators, are less than half a millimeter (0.02 inches) long and push electrons down a 225 nanometer (0.000009 inch) wide channel. Very short bursts of laser pulses accelerate the particles. Unlike metallic surfaces, which cannot cope with wavelengths shorter than the radio spectrum, dielectric materials can operate with optical light.

“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 Dr Tomáš Chlouba of FAU in a statement. Chlouba and co-authors acknowledge they are not there yet, but claim to have made major progress. “For the first time, we really can speak about a particle accelerator on a chip,” said Dr Roy Shiloh.

If you only want to accelerate a single charged particle at a time it can be relatively easy, but that is seldom a useful thing to do. Particle accelerators, small and large, face the challenge of keeping particles of the same charge direction together while their mutual repulsion makes them spread out. 

The team addressed this problem using alternating phase focusing (APF), an approach physicists had toyed with when building the first accelerators. Physical laws dictate that it is impossible to focus charged particles in all three directions at once. APF gets around this by using lasers to focus a group of electrons in one dimension while allowing them to defocus in another, before reversing the dimensions. The net effect is to focus in both directions and can be repeated until the particles are tightly bunched.

Two years ago, the FAU team showed that by passing electrons between a series of pillars, electrons can be repeatedly focused or defocused in successive cells on an almost unimaginably short timescale. “By way of comparison,” Dr Johannes Illmer said at the time, “the large Hadron collider at CERN uses 23 of these cells in a 2,450 metre [8,040 foot] long curve. Our nanostructure uses five similar-acting cells in just 80 micrometres.”

Now, the FAU team have built a fully functional particle accelerator on a chip using this technique, adding 12 kiloelectron volts, a 43 percent increase in energy for the electrons involved. The next goal is to increase this energy gain to the point where it can be used in medicine, such as for irradiating tumors, which will require a 100-fold increase in energy. “We will have to expand the structures or place several channels next to each other,” Chlouba said. 

The FAU team’s work is published in Nature. The Stanford team’s work is still under peer review, but a preprint is available at arXiv.