Accelerating electrons by emitting laser light into a nanophotonic cavity

The laser driven particle accelerator on silicon chips was created by two independent research groups. With further improvements, this dielectric laser accelerator can be used in medicine and industry, and even in high-energy particle physics experiments.

Accelerating electrons to high energy is usually accomplished over long distances in large and expensive facilities. For example, the electron accelerator at the European X-ray Free Electron Laser Center in Germany is 3.4 kilometers long, and the Stanford Linear Accelerator in California is 3.2 kilometers long.

Therefore, the practical application of electron accelerators in medicine and industry is strictly limited. Size and cost are also factors in particle physics based on accelerators. As collision energy increases, facilities become larger and more expensive.

Surfers on the waves
In traditional accelerators, the microwave oscillation of the electric field in the metal cavity accelerates electrons like a surfer on a traveling wave. The maximum acceleration gradient is usually several tens of megavolts per meter, defined by the maximum electric field that may exist between metal components in the cavity.

No one knows for sure what happens on the surface of the [cavity], which is still an active research field... but when the field becomes too large, things like tiny little pyramids grow on the surface, and electrons eject, the field will decompose, "said Peter Hommelhoff of Friedrich Alexander University in Erlangen Nuremberg, Germany.

The cost and technical challenges of traditional accelerators mean that researchers are enthusiastic about developing alternative acceleration methods. In this latest study, oscillating electric fields are generated by emitting laser pulses into tiny optical cavities made of silicon nanostructures.

Hommelhoff said it took physicists nearly thirty years to realize that electron acceleration can also be achieved using nanophotonic cavities driven by optical frequency light. The use of optical light helps to shrink the device, as the wavelength of radiation is much shorter than that of microwaves.

No metal required
Hommelhoff pointed out another important benefit of this method: "When you drive these frequencies with a laser, you don't need a metal structure. He added, "If you only use ordinary glass, it's enough... you can generate the same modes as microwave cavities and microwave fields.

Due to the fact that the cavity is an insulator, high concentrations of charges will not appear at points on the surface. Therefore, the only limitation of the acceleration gradient is the electrical breakdown field of the material.

In principle, this allows the integration of nanophotons in particle accelerators to generate electron beams in tiny, precisely focused beam lines. However, there are also practical challenges. The electrons in each beam repel each other, and maintaining a beam of electrons together requires external force focusing. Furthermore, compressing a pile in one direction can cause it to propagate in the same direction.

Exclusion issues
In previous work, researchers including Hommelhoff and Olav Solgaard from Stanford University in California have demonstrated that this exclusion problem can be alleviated through alternating phase focusing. In this technology, electrons are alternately confined in one direction and then confined in another direction, resulting in an oscillating field distribution.

Now, two independent research groups have completed new work on these accelerators. One of them is led by Homelhoff of Friedrich Alexander University. Another group is a collaboration between scientists from Stanford University led by Solgaard and researchers from the Technical University of Darmstadt in Germany led by Uwe Niedermeyer. Both teams have created nanophoton dielectric laser accelerators that can increase the energy of electron beams without splitting. Solgaard and Niedermeyer's team manufactured two accelerators - one designed at Stanford University and one designed at Darmstadt University of Technology. An accelerator at only 708 μ The energy of 96 keV electrons was increased by 25% within a distance of m. This is about ten times the thickness of human hair.

I think I exert more force on electrons than anyone else, "Solgaard said.
The Hommelhoff group's device operates at a lower energy level of 500 μ Accelerate the electrons from 28.4 keV to 40.7 keV. As Hommelhoff explained, this presents its own challenges. "When you want to accelerate non relativistic electrons - in our case, they only travel at one-third of the speed of light - this is not easy, and the efficiency of generating optical modes that travel with electrons is low.

Higher subdivision fields
Researchers are now seeking to achieve higher field gradients by manufacturing devices in materials with higher breakdown fields than silicon. They believe that in the short term, their acceleration plan can find applications in medical imaging and dark matter search.

Solgaard said that he "may be one of the few people who believes that this will play a role in high-energy physics," but the technology should be applicable to materials such as quartz, with a breakdown field almost 1000 times that of traditional accelerators. Our millimeters have become one meter, "he said; When we reach one meter, we should match SLAC in terms of energy... Consider installing an accelerator in my office that matches SLAC.

I think these two teams have demonstrated an important new step towards a true accelerator on a chip, "said accelerator scientist Carsten Welsch from the University of Liverpool in the UK. However, he warned that there is still much work to be done in beam control and micro diagnosis. In terms of application, he said, "I share their optimistic attitude towards medical applications similar to catheters, bringing electrons where they are needed, especially for micro light sources that I personally believe have the greatest potential. The combination of high-quality electron beams and light can truly open up new research opportunities and applications.
However, Welsch still does not believe in applications such as particle colliders, pointing out that such machines require high brightness and high beam quality. The next Large Hadron Collider will not be a dielectric laser accelerator, "he concluded.
Hommelhoff and colleagues described their work in the journal Nature. Solgaard, Niedermeyer, and colleagues described their work on arXiv.

Source: Laser Network