Measurement of the Subcycle Timing of Attosecond XUV Bursts in High-Harmonic Generation

The absolute timing of the high-harmonic attosecond pulse train with respect to the generating IR pump cycle has been measured for the first time. The attosecond pulses occur $190\pm 20\mathrm{a}\mathrm{s}$ after each pump field maxima (twice per optical cycle), in agreement with the “short” quantum path of the quasiclassical model of harmonic generation.

Figure 1

Photoelectron spectrum of argon exposed to harmonics 11–19 and the IR probe. The single-photon peaks are labeled by the order of the harmonic. The sidebands appear in between. The inset shows the relevant quantum paths.

Figure 2

Energy-integrated ionization signal (top panel) and sideband signals (from $E_0+18\hslash \omega$ to $E_0+12\hslash \omega )$ vs the pump-probe delay. Sideband signals were normalized to the corresponding total signal.

Figure 3

Intensity envelope of the attosecond beats (thick line) and the IR pump field (thin line). The beat spikes, 266 as FWHM, occur 190 as after the field maxima. A wave packet ejected just after a peak of the field is recolliding with the nucleus (producing the XUV bursts) after about 1520 as.