SUBFEMTOSECOND PULSES

High-harmonic emission is very sensitive to the polarization of a fund amental laser beam. By combining two short perpendicularly polarized p ulses with frequencies centered at omega(1) and omega(2), where (omega (1)-omega(2))/omega(1) much less than 1, we can create a pulse with po larization that sweeps from circular through linear back to circular. It is possible to make the duration of the segment of the pulse capabl e of emitting short-wavelength radiation so small that emission is pos sible only for a time less than the period of the fundamental.