Enhanced proton acceleration using hollow silica nano-sphere coated targets

Acceleration of protons by transient strong electric fields formed with intense ultrashort laser pulses is important for advancements in radiography and biomedical applications. Controlling the absorption mechanisms by material modification or adding structural features to the solid substrate is important to enhance ion energies for a given laser intensity. We present here an experimental demonstration of enhanced proton acceleration when a BK-7 glass target is coated with 150nm diameter silica hollow spheres. The hollow particle coated target yielded a maximum proton energy of >= 800keV at a peak intensity of 10(18) W cm(-2) while the maximum energy is only up to 200keV with a plain glass target under otherwise identical conditions. Two-dimensional particle in cell simulations demonstrate the role of local fields in the hollow spherical cavities that lead to the enhanced proton energies comparable to the experiments.