Proton acceleration from nano-engineered targets using high-intensity short pulse lasers*

A theoretical and experimental study of proton acceleration from planar gold targets covered with gold nanowires is presented. The nano-engineered foils composed of a flat substrate with thickness ranging from 7 to 20 μm and protruding gold nanowires having length equal to the foil thickness were irradiated by a short pulse high intensity laser (Hercules: 3 J, 1.5x10²¹ W/cm², 30 fs, 1.5 μm focal spot). The nanowires enhance significantly the laser energy coupled to the plasma and maximum proton energies. Different nanowire and substrate parameters were tested showing that the most important parameter is the number of nanowires per focal spot, N. Theoretical analysis and experimental data indicate that optimum is reached for nanowire density of N=1. Proton spectra are compared to simulations using a 2D-3V Particle-In-Cell (PIC) code which reproduces the experimental data with a good agreement.