Second-order autocorrelation measurements of attosecond XUV pulse trains

The unequivocal demonstration of attosecond XUV pulse train generation based on the synthesis of a comb of harmonics of an infrared femtosecond laser pulse is inevitably linked to a reliable method of pulse characterization in this temporal regime. The most commonly used method to date relies on a cross-correlation technique between the fundamental laser frequency and the XUV light from which a measurement of the relative phases of the consecutive harmonics is obtained and the attosecond temporal structure in a train of XUV pulses is inferred. Here we report on an alternative method that renders possible the direct observation of a train of attosecond pulses in form of a second-order autocorrelation trace. The method is an extension in the XUV spectral range of the well-established technique commonly used for the characterization of femtosecond laser pulses. Besides being the first direct visualization of a periodic attosecond structure, the approach supplies quantitative information on the overall pulse ...