Title: Ion acceleration from micrometric targets immersed in an intense laser field

Abstract: We report on an experimental study of proton acceleration by intense laser irradiation of micrometric bar targets, whose dimensions are transversely immersed in the laser focal volume and longitudinally smaller than half its wavelength. Using only 120 mJ of laser energy, we recorded proton energies in excess of 6 MeV, exceeding those reported with any other experimental method by a factor of 3. 3D particle-in-cell simulations revealed that the efficient energy transfer from the diffracted laser fields to electrons on both sides of the target, combined with its reduced surface area, results in a thicker electron sheath and higher accelerating electric field gradients. We demonstrated numerically how this ion acceleration technique opens up the possibility of laser ion acceleration in multiple stages, allowing manipulation of the accelerated ion spectrum by optical means.