Optimization of ion acceleration in the interaction of intense femtosecond laser pulses with ultrathin foils

Ion emission is investigated using particle-in-cell simulations where a Gaussian laser pulse with duration 50 fs and intensity 1.37x10(19) W/cm(2) is incident obliquely onto ultrathin solid foils. When the foil is thicker than 0.1 mum, it is opaque to the laser light and the highest ion energy drops exponentially with target thickness. Optimization of ion acceleration occurs for a target with a thickness of 0.04 mum when it becomes transparent to the laser light. The behaviors of the high-energy electrons oscillating in the charge separation potential at the front and the rear of the target, as well as the enhanced electron acceleration in the laser pulse, play dominant roles for the observed features of ion emission. The relation of the optimal target thickness with parameters of the incident laser pulse and foil targets is also discussed.