  Sub-10-attosecond pulses with half-cycle electric fields provide exceptional
options to detect and manipulate electrons in the atomic timescale. However,
the availability of such pulses is still challenging. Here, we propose a method
to generate isolated sub-10-attosecond half-cycle pulses based on a cascade
process naturally happening in plasma. A 100s-attosecond pulse is first
generated by shooting a moderate overdense plasma with a one-cycle femtosecond
pulse. After that, the generated attosecond pulse cascadedly produce a
sub-10-attosecond half-cycle pulse in the transmission direction by unipolarly
perturbing a nanometer-thin relativistic electron sheet naturally form in the
plasma. Two-dimensional particle-in-cell simulations indicate that an isolated
half-cycle pulse with the duration of 8.3 attoseconds can be produced. Apart
from one-cycle driving pulse, such a scheme also can be realized with a
commercial 100-TW 25-fs driving laser by shaping the pulse with a relativistic
plasma lens in advance.
