Laser-driven proton acceleration via excitation of surface plasmon polaritons into TiO(2)nanotube array targets

In this paper we report the measurement of laser-driven proton acceleration obtained byirradiating nanotube array targets with ultrashort laser pulses at an intensity in excess of 1020W cm−2. The energetic spectra of forward accelerated protons show a larger flux and a higherproton cutoff energy if compared to flat foils of comparable thickness. Particle-In-Cell 2Dsimulations reveal that packed nanotube targets favour a better laser-plasma coupling andproduce an efficient generation of fast electrons moving through the target. Due to theirsub-wavelength size, the propagation of e.m. field into the tubes is made possible by theexcitation of Surface Plasmon Polaritons, travelling down to the end of the target and assuring acontinuous electron acceleration. The higher amount and energy of these electrons result in turnin a stronger electric sheath field on the rear surface of the target and in a more efficientacceleration of the protons via the target normal sheath acceleration mechanism.