Subcycle Phase Matching Effects in Short Attosecond Pulse Trains

Attosecond pulses produced by high-order harmonic generation in gases driven by intense laser fields have become a cornerstone technique for probing ultrafast electronic motion in matter. These applications require a good knowledge of the temporal and spectral properties of the emitted radiation. In this work, we generate a train of 2 to 3 attosecond pulses that we characterize using 2-color laser-assisted photoionization. An unexpected spectral behavior, with more pulses at high energies than at low energies, is observed when the carrier-to-envelope phase of the laser field is changed by 90 degrees. High-order harmonic generation simulations indicate that the time-dependent phase matching of the harmonics contributes in a nontrivial way to the structure of the pulse train. Two-color laser-assisted photoionization enables us to unravel the dynamical influence of subcycle phase matching on the spectral properties of the attosecond pulse train, going beyond the predictions of the response of a single atom to a strong laser field.