  Half-cycle picosecond pulses have been produced from thin photo-conductors,
when applying an electric field across the surface and switching on conduction
by a short laser pulse. Then the transverse current in the wafer plane emits
half-cycle pulses in normal direction, and pulses of 500 fs duration and 1e6
V/m peak electric field have been observed. Here we show that single half-cycle
pulses of 50 as duration and up to 1e13 V/m can be produced when irradiating a
double foil target by intense few-cycle laser pulses. Focused onto an
ultra-thin foil, all electrons are blown out, forming a uniform sheet of
relativistic electrons. A second layer, placed at some distance behind,
reflects the drive beam, but lets electrons pass straight. Under oblique
incidence, beam reflection provides the transverse current, which emits intense
half-cycle pulses. Such a pulse may completely ionize even heavier atoms. New
types of attosecond pump-probe experiments will become possible.
