Observation and analysis of an interferometric autocorrelation trace of an attosecond pulse train

We report the direct observation of phase locking between adjacent pulses in an attosecond pulse train (APT) via interferometric autocorrelation (IAC). In this measurement, the Coulomb explosion of N₂ caused by two-photon absorption is utilized as correlated signals between two replicas of the APT that are the outcome of the spatial division of the APT in the interferometer. The analysis of IAC by the spatial division of the APT is consistent with the experimental trace of the IAC, and yields the duration of the pulse in the APT of 3200.3em0exattoseconds, which corresponds to a 1.30.3em0excycle period of the carrier frequency of the harmonic field.