Fast ion beams from intense, femtosecond laser irradiated nanostructured surfaces

We present results on hot electron and energetic ion (keV--MeV) generation from polished and nanostructured metallic surfaces excited by p-polarized, femtosecond laser pulses in the intensity range of 1X1015--1.5X1017 Wcm-2. A clear enhancement in the hard X-ray spectrum from nanoparticle-coated surfaces is observed, indicating 'hotter' electron production in nanoparticle-produced plasma until the intensity of 2X1016 Wcm-2 is reached. Contrary to the existing perception, we find that the hotter electrons do not lead to hotter ion emission. The total ion flux and the ion energy integrated over the 4--1400 keV energy range are found to be enhanced by 50% and 16%, respectively, for nanostructured targets in comparison to those from polished targets. 55% enhancement in yield is observed for ions at the lower end of the energy range, while hotter ions are actually found to be suppressed by ~40%. The surface modulations present on the nanoparticle-coated targets are observed to reduce the maximum energy of the ions and showed an intensity-dependent increase in the divergence of the ion beam.