We have developped a novel procedure for the characterization of attosecond pulses, termed iPROOF (improved Phase Retrieval by Omega Oscillation Filtering) for its similarity with the already existing PROOF technique. The method relies on the conversion of the attosecond pulse into electron wave-packets through photoionization of atoms in the presence of a weak IR field. The spectral phases of the frequency components making up the pulse are determined from the temporal modulation of electron signal detected in one direction along the polarization of the light, as the delay between the attosecond pulse and the IR field is varied. The main advantage of our method over other techniques is that it allows for the unique determination of these spectral phases by accurately taking into account the atomic physics of the photoionization process. The phases are evaluated by optimizing the fit of a state-of-theart two-photon perturbation theory calculation to the experimental result. Because only two parameters, the phase of a particular frequency component and a normalization factor proportional to the IR electric field strength, are used in the fit, our semi-analytical retrieval procedure is much faster than other techniques based on evolutionary algorithms which use all the phases as parameters for the optimization. The retrieval algorithm is also robust and does not need any initial assumption about the phases. The measurement is easy to setup at low cost as it only needs the measurement of the photoelectron emission in one direction along the polarization vector, which can be easily done with a conventional time-of-flight spectrometer.
G. Laurent et al. Optics Express 21, 16914 (2013)