  Ion acceleration resulting from the interaction of 11 fs laser pulses of ~35
mJ energy with ultrahigh contrast (<10^-10), and 10^19 W/cm^2 peak intensity
with foil targets made of various materials and thicknesses at normal
(0-degree) and 45-degree laser incidence is investigated. The maximum energy of
the protons accelerated from both the rear and front sides of the target was
above 1 MeV. A conversion efficiency from laser pulse energy to proton beam is
estimated to be as high as ~1.4 % at 45-degree laser incidence using a 51
nm-thick Al target. The excellent laser contrast indicates the predominance of
vacuum heating via the Brunels effect as an absorption mechanism involving a
tiny pre-plasma of natural origin due to the Gaussian temporal laser pulse
shape. Experimental results are in reasonable agreement with theoretical
estimates where proton acceleration from the target rear into the forward
direction is well explained by a TNSA-like mechanism, while proton acceleration
from the target front into the backward direction can be explained by the
formation of a charged cavity in a tiny pre-plasma. The exploding Coulomb field
from the charged cavity also serves as a source for forward-accelerated ions at
thick targets.
