Energetic protons generated by ultrahigh contrast laser pulses interacting with ultrathin targets

A regime of laser acceleration of protons, which relies on the interaction of ultrahigh contrast laser pulses with ultrathin targets, has been validated using experiments and simulations. Proton beams were accelerated to a maximum energy of ∼7.3 MeV from targets as thin as 30 nm irradiated at 10 ¹⁸ W cm^-2 μm² (1 J, 320 fs) with an estimated peak laser pulse to pedestal intensity contrast ratio of 10 ¹¹. This represents nearly a tenfold increase in proton energy compared to the highest energies obtainable using non contrast enhanced pulses and thicker targets (>5 μm) at the same intensity. To obtain similar proton energy with thicker targets and the same laser pulse duration, a much higher laser intensity (i.e., above 10¹⁹ W cm^-2 μm ²) is required. The simulations are in close agreement with the experimental results, showing efficient electron heating compared to the case of thicker targets. Rapid target expansion, allowing laser absorption in density gradients, is key to enhanced electron heating and ion acceleration in ultrathin targets.