Second Order Autocorrelation of an XUV Attosecond Pulse Train

Temporal widths of an attosecond (asec) XUV radiation pulse train, formed by the superposition of higher order harmonics, have been recently determined utilizing a 2nd order autocorrelation measurement. An assessment of the validity of the approach, for the broadband XUV radiation of asec pulses, is implemented through ab initio calculations modeling the spectral and temporal response of the two-XUV-photon He ionization detector employed. The measured width of the asec bursts is discussed in terms of the spectral phases of the individual harmonics, as well as in terms of the spatially modulated temporal width of the radiation, and is found in reasonable agreement with the expected duration.

Figure 1

All possible two-XUV-photon ionization channels by the superposition of the 7th to the 15th harmonic and the spectral ionization yield distribution calculated by solving the He TDSE (black bars) or calculated for a detector with perfectly flat spectral response (gray bars).

Figure 2

(a) Calculated interferometric 2nd order AC traces for three different laser wavelengths, 790 nm (solid line), 767.62 nm (dashed line), and 759.45 nm (dash-dotted line), and compared to the interferometric trace of a detector with instantaneous response (filled gray area). (b) Position of the 13th harmonic of the three laser wavelengths with respect to the $1s2p$ intermediate state of He.

Figure 3

(a) Classically calculated emission times $t_e$ for four different driving laser intensities across the radial laser beam profile. (b) Attosecond pulse trains estimated using the experimental harmonic amplitudes, and the phases resulting from the calculated emission times $t_e$.

Figure 4

Calculated peak intensity (solid line) and radially averaged (dashed line) 2nd order interferometric AC traces of a superposition of harmonics with amplitudes equal to the amplitudes used in the experiment [4, 5], and phase distributions classically calculated [6, 7].