Nat. Commun. 7, 12891 (2016)

When a plasma interacts with a high-intensity laser pulse, the resulting collective motion of the electrons in the plasma can generate an accelerating electric field, providing conditions for the electrons to rapidly 'surf' away. Based on this insight, a number of different laser–plasma particle acceleration schemes have been proposed.

Bruno Gonzalez-Izquierdo and colleagues have looked at the plasma that is formed when a high-intensity femtosecond laser pulse is incident on a 10-nm-thick aluminium target coated with hydrocarbon layers. The onset of the pulse locally generates a plasma in the aluminium foil, which then serves as an aperture for the remainder of the pulse, causing it to diffract. The diffracted beam leads to an accelerating electric field, in this case driving away protons, which are freed from the hydrocarbon coating during irradiation.

To assess the potential of this scheme as a compact proton accelerator, the authors varied the polarization of the laser light — linear, elliptic or circular — and found that it significantly affected the diffractive behaviour of the aperture, and ultimately the proton beam's intensity profile. Tuning the polarization's ellipticity resulted in different distributions, featuring ring, bubble or striped features.