Ion acceleration from micrometric targets immersed in an intense laser field

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 are longitudinally smaller than half its wavelength. With only 120 mJ of laser energy, we recorded proton energies in excess of 6~MeV, three times higher than those achieved with flat-foil irradiation using similar pulse energies. 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 acceleration gradients. We demonstrated numerically how this technique opens up the possibility of laser-ion acceleration in a cascaded manner, allowing manipulation of the ion spectrum by optical means.