Ion acceleration with an intense short-pulse laser and large-area suspended graphene in an extremely thin target regime

Graphene is an atomic thin 2D material, known as the strongest material with such a thin regime. Free-standing, large-area suspended graphene (LSG) has been developed for a target of laser-driven ion acceleration. The LSG has shown remarkable durability against laser prepulse, producing MeV protons and carbons by direct irradiation with an intense laser without a plasma mirror, yet no optimization has been concerned. Here we investigate the optimization of the laser-driven ion acceleration with LSG, paying special attention to the target conditions. We irradiate nanometer-thick LSG with an intense laser, where the incident angle and the target thickness are controlled. The maximum proton energy increases with increasing the number of LSG layers, where $25\pm 0.3\mathrm{MeV}$ protons at maximum are consistently observed with Thomson parabola spectrometer and diamond-based detectors. For comparison purposes, we perform ideal numerical simulations using particle-in-cell (PIC) code without consideration of the prepulse. In the PIC simulation, the protons are successively accelerated by the electric field associated with laser radiation pressure and the surface sheath field, yet the maximum proton energies are overestimated. The maximum proton energies from the experiment asymptotically approach the ideal PIC expectations, indicating that the thinner LSG is more affected by the prepulse. We expect higher proton energy with the optimized LSG conditions and a plasma mirror.