  We present the experimental results on ion acceleration by petawatt
femtosecond laser solid interaction and explore strategies to enhance ion
energy. The irradiation of micrometer thick (0.2 - 6.0 micron) Al foils with a
virtually unexplored intensity regime (8x10^19 W/cm^2 - 1x10^21 W/cm^2)
resulting in ion acceleration along the rear and the front surface target
normal direction is investigated. The maximum energy of protons and carbon
ions, obtained at optimised laser intensity condition (by varying laser energy
or focal spot size), exhibit a rapid intensity scaling as I^0.8 along the rear
surface target normal direction and I^0.6 along the front surface target normal
direction. It was found that proton energy scales much faster with laser energy
rather than the laser focal spot size. Additionally, the ratio of maximum ion
energy along the both directions is found to be constant for the broad range of
target thickness and laser intensities. A proton flux is strongly dominated in
the forward direction at relatively low laser intensities. Increasing the laser
intensity results in the gradual increase in the backward proton flux and leads
to almost equalisation of ion flux in both directions in the entire energy
range. These experimental findings may open new perspectives for applications.
