Towards attosecond pulses with high harmonics

The high harmonics produced by focusing an intense femtosecond laser in a gas are theoretically shown to be locked in phase. The physics of this locking is discussed and a new method based on quantum interference in two-photon, two-color ionization allowing to retrieve the relative phase of harmonic pairs is described. The main result is that the 5~harmonics of orders 11--19 produced in argon generate a train of subfemtosecond pulses with a period of 1.35~fs and a duration of 250~attoseconds.