A Smooth Plasmon Accelerator
Shining an ultrahigh-intensity laser onto a metal foil creates an electron plasma that oscillates in sync with the laser’s electric field. If the laser’s wave vector parallel to the surface has a certain value—determined by the metal’s properties—this interaction becomes resonant. Then, the collective oscillations form a surface plasma wave (SPW), which propagates along the foil’s surface. SPWs can be used to accelerate charged particles and generate light pulses at multiples of the initial laser frequency. But, so far, exciting them has required the patterning of a nanoscopic diffraction grating on the foil, which can be delicate, expensive to fabricate, and lead to energy losses. Now Adrian McCay at Queens University Belfast in the UK and colleagues have demonstrated SPW generation without the need for gratings or other components [1].
The team’s demonstration followed a theoretical proposal from 2021 [2] in which the laser beam is directed at the foil edge on rather than obliquely, as is usually the case. In this geometry, the laser’s wave vector is parallel to the foil’s surface, making phase matching between the laser’s electric field and the electronic oscillations easier. Accordingly, electrons that were excited by the laser and injected into the SPWs at the foil’s front edge were accelerated to very high energies, exiting the rear of the foil as a collimated beam with an energy of more than 100 MeV. The team also found that this acceleration process was enhanced by naturally occurring surface contaminants, which extended the reach of the SPWs.
Next, McCay and colleagues intend to develop the technique to accelerate protons, an application for which the edge-on geometry was initially proposed.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK.
References
- A. McCay et al., “Surface wave electron acceleration from flat foils at parallel laser incidence,” Phys. Rev. Lett. 135, 145001 (2025).
- X. F. Shen et al., “Monoenergetic high-energy ion source via femtosecond laser interacting with a microtape,” Phys. Rev. X 11, 041002 (2021).