Attosecond synchronization of high-order harmonics from midinfrared drivers

The group delay dispersion, also known as the attochirp, of high-order harmonics generated in gases has been identified as the main intrinsic limitation to the duration of Fourier-synthesized attosecond pulses. Theory implies that the attochirp, which is inversely proportional to the laser wavelength, can be decreased at longer wavelength. Here we report the first measurement of the wavelength dependence of the attochirp using an all-optical, in situ method. We show that a 2μm driving wavelength reduces the attochirp with respect to 0.8μm at comparable intensities.