Inconsistencies between two attosecond pulse metrology methods: A comparative study

The two basic approaches underlying most of the metrology of attosecond pulse trains are compared in the spectral region $~14-24$ eV, that is, the second-order intensity volume autocorrelation and the resolution of attosecond beating by interference of two photon transitions (RABITT). They give rather dissimilar pulse durations. It is concluded that for the present experimental conditions RABITT may underestimate the duration under measurement, due to variations of the driving intensity, but in conjunction with theory allows an estimation of the relative contributions of two different electron trajectories to the extreme-ultraviolet (XUV) radiation.

Figure 1

Experimental setup showing the 2-IVAC and the RABITT arrangements.

Figure 2

(a) Harmonic signal as a function of the focus position relative to the xenon gas jet. The arrows show the positions $z_{\mathrm{BJ}},z_{\mathrm{OJ}}$, and $z_{\mathrm{AJ}}$, where the 2-IVAC and RABITT measurements were performed. (b)–(d) 2-IVAC traces. The gray dots are the measured data. The yellow circles correspond to a 10-point running average. The dark purple line in (b) is a 12-peak sum of Gaussians fit to the raw data. The green-filled area in (b) is one of the twelve Gaussian pulses of the fitted function.

Figure 3

(Left panel) RABITT traces measured at the three focus positions normalized to the corresponding total signal. The gray dots are the measured data. The yellow circles correspond to a running average over 15 points for the sidebands and 40 points for the total signal. The dark purple lines are sinusoidal fits to the raw data over 13 oscillations on the sideband traces and over 6 laser oscillations on the total signal. (Right panel) Reconstructed pulse trains.

Figure 4

(a) Harmonic phases ${\phi }_q$ and (b) electron return times $t_{\mathrm{e}}$ as a function of the harmonic order $q$ obtained from the RABITT traces of Fig. 3. The dark purple lines in (a) are quadratic and in (b) are linear fits to the data. The $t_{\mathrm{e}}$ is set to zero for the cutoff harmonics. The large gray circle corresponds to the position of the 19th harmonic, which is assumed to belong to the cutoff region.

Figure 5

(a)–(c) Calculated RABITT traces for different relative contributions of the short and long trajectories. The solid yellow, the dashed blue, and the solid black lines match the measured RABITT traces at $z_{\mathrm{BJ}},z_{\mathrm{OJ}}$, and $z_{\mathrm{AJ}}$, respectively. (d) The line is the superposition of several asec pulse trains calculated, using equal contributions of the short and long trajectories. The green-filled area shows the asec pulse train that RABITT would result in for the above superposition.