Laser-driven proton acceleration via excitation of surface plasmon polaritons into TiO2 nanotube array targets

G. Cristoforetti, F. Baffigi, F. Brandi, G. D’Arrigo, A. Fazzi, L. Fulgentini, D. Giove, P. Koester, L. Labate, G. Maero, D. Palla, M Romé, R. Russo, D. Terzani, P. Tomassini and L.A. Gizzi

Plasma Phys. Control. Fusion 62 (2020) 114001

Abstract

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