  A new regime in the interaction of a two-colour ($\omega$,$2\omega$) laser
with a nanometre-scale foil is identified, resulting in the emission of
extremely intense, isolated attosecond pulses - even in the case of multi-cycle
lasers. For foils irradiated by lasers exceeding the blow-out field strength
(i.e. capable of fully separating electrons from the ion background), the
addition of a second harmonic field results in the stabilization of the foil up
to the blow-out intensity. This is then followed by a sharp transition to
transparency that essentially occurs in a single optical cycle. During the
transition cycle, a dense, nanometre-scale electron bunch is accelerated to
relativistic velocities and emits a single, strong attosecond pulse with a peak
intensity approaching that of the laser field.
