The influence of laser frequency on laser-driven ion acceleration is investigated by means of twodimensional particle-in-cell simulations. When ultrashort intense laser pulse at higher harmonic frequency irradiates a thin solid foil, the target may become relativistically transparent for significantly lower laser pulse intensity compared with irradiation at fundamental laser frequency. The relativistically induced transparency results in an enhanced heating of hot electrons as well as increased maximum energies of accelerated ions and their numbers. Our simulation results have shown the increase in maximum proton energy and increase in the number of high-energy protons by a factor of 2 after the interaction of an ultrashort laser pulse of maximum intensity 7 × 1021 W/cm2 with a fully ionized plastic foil of realistic density and of optimal thickness between 100 nm and 200 nm when switching from the fundamental frequency to the third harmonics.
One output from particle simulations of ultra-short laser pulses interacting with ionized solid targets. (a) shows that laser impulse at fundamental frequency has predominantly reflected on ionized solid target. (b) shows that large portion of laser impulse at third harmonics frequency has got through the target as a consequence of relativistc transparency. Laser puls is aimed at the target (marked with dashed line) from below.
1FNSPE, Czech Technical University in Prague, Czech Republic 2ELI-Beamlines project, Institute of Physics, ASCR, Czech Republic
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