Subfemtosecond pulses

High-harmonic emission is very sensitive to the polarization of a fundamental laser beam. By combining two short perpendicularly polarized pulses with frequencies centered at ω1 and ω2, where (ω1 − ω2)/ω1 ≪ 1, we can create a pulse with polarization that sweeps from circular through linear back to circular. It is possible to make the duration of the segment of the pulse capable of emitting short-wavelength radiation so small that emission is possible only for a time less than the period of the fundamental.