Smallest Optical Particle Accelerator

Layout of the experiment demonstrating the inelastic ponderomotive scattering of electrons at a high-intensity optical travelling wave in vacuum. (Source: Hommelhoff et al., FAU)

The largest particle acce­lerator in the world – the Large Hadron Collider at CERN in Switzer­land – has a circum­ference of around 26 kilo­metres. Researchers at Friedrich-Ale­xander Univer­sität Erlangen-Nürnberg FAU, Germany, are attemp­ting to go to the other extreme by building the world’s smallest machine of this kind – a particle acce­lerator that fits on a microchip. The research team has now taken another step towards achieving this ambition. The funda­mental idea behind the minia­ture par­ticle acce­lerator’s develop­ment is to enable scientists to use laser beams to acce­lerate electrons. What sounds decep­tively simple in theory raises a whole series of challenges in practice, extending across various fields of physics. For example, the scientists need to be able to control the oscil­lation of light and the movement of electrons with great precision in order to ensure that they meet each other at just the right moment.

One way of envi­saging this is to imagine a ship on a stormy sea; to safely ascend the wave and come down on its other side, the helmsman has to watch the oncoming wave and judge when it will meet the vessel. It is equally crucial for the scientists to ascer­tain when and where the maximum crest of a light wave will hit a packet of electrons so that they can influence the outcome to a highly specific degree. This means they need to enable light and electrons to coincide within atto­seconds.

In an exciting first, this is exactly what the research group around Peter Hommel­hoff have succeeded in achieving. The team has developed a new technique involving the inter­section of two laser beams oscil­lating at different fre­quencies in order to generate an optical field whose proper­ties the researchers can influence to an extremely precise degree. The key property of this optical field is that it retains contact with the electrons, effec­tively moving with them – hence its being termed a travelling wave – so the electrons can conti­nuously sense, or surf, the optical field. In this way, the optical field transmits its pro­perties exactly to the particles.

Not only does this process cause the particles to precisely reflect the field structure, it also acce­lerates them to a strikingly high degree. This effect is crucial to the minia­ture particle acce­lerator’s practical appli­cation, as it relates to how much energy can be trans­ferred to the electrons across what distance. The acce­leration gradient, which indi­cates the maximum measured electron energy gain versus distance covered, reaches the extremely high value of 2.2 giga-electron-volts per metre, much higher than that attained by conventional acce­lerators. However, the acce­leration distance of only 0.01 millimetres cur­rently available to the research team in Erlangen is not sufficient for them to generate the energy needed for achieving results of relevance to practical appli­cations. “Despite this, for particle acce­lerators in medicine, we would only need a tiny acce­leration length of less than a milli­metre,” explains Martin Kozák, who carried out the labora­tory experi­ment.

Peter Hommel­hoff considers acce­lerator minia­turisation to be a technical revo­lution analogous to the develop­ment of computers, which went from occupying entire rooms to fitting on people’s wrists. “This approach will hope­fully enable us to make this inno­vative particle acce­leration technique usable in a range of research areas and fields of appli­cation such as materials science, biology and medicine; one example might be particle therapies for cancer patients.” (Source: FAU)

Reference: M. Kozák et al.: Inelastic ponderomotive scattering of electrons at a high-intensity optical travelling wave in vacuum, Nat. Phys., online 9 October 2017; DOI: 10.1038/nphys4282

Link: Laserphysics, Dept. of Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg FAU, Erlangen, Germany

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