  The influence of laser frequency on laser-driven ion acceleration is
investigated by means of two-dimensional 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 to 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 of maximum proton energy and of the number of high-energy protons by a
factor of 2 after the interaction of an ultrashort laser pulse of maximum
intensity $7 \times 10^{21}~\rm{W/cm^2}$ with a fully ionized plastic foil of
realistic density and of optimal thickness between $100~\rm{nm}$ and
$200~\rm{nm}$ when switching from the fundamental frequency to the third
harmonics.
