Enhanced ion acceleration by using femtosecond laser pulses at the third harmonic frequency

Relativistically induced transparency regime has been already achieved experimentally by the expansion of ionized solid target during laser-target interaction for relatively longer laser pulses (about 500 fs), but not for ultrashort pulses (about 30 fs), where the expansion is not significant, and, thus, too high laser intensity (close to 1023 W/cm(2)) is required. However, 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 the irradiation at fundamental laser frequency. The relativistically induced transparency results in an enhanced heating of hot electrons as well as increased maximum energy of accelerated ions and their numbers. When fundamental laser frequency is converted to the third harmonics, we observed the increase of maximum energy of accelerated protons and numbers of high energy protons by factor of 2 in our two-dimensional particle-in-cell simulations. Lengthening of the laser pulse due to frequency conversion should not decrease the energy and numbers of accelerated protons. The proposed scheme enables to accelerate protons to energies over 250 MeV by ultrashort laser pulse of maximum intensity below 10(22) W/cm(2) irradiating 200 nm thick solid foil.