Spectral Signature of Short Attosecond Pulse Trains

We report experimental measurements of high-order harmonic spectra generated in Ar using a carrier-envelope-offset (CEO) stabilized 12 fs, 800 nm laser field and a fraction (less than 10%) of its second harmonic. Additional spectral peaks are observed between the harmonic peaks, which are due to interferences between multiple pulses in the train. The position of these peaks varies with the CEO and their number is directly related to the number of pulses in the train. An analytical model, as well as numerical simulations, support our interpretation.

Figure 1

Experimental harmonic spectrum. The dashed light blue lines indicate the position of the harmonics, starting from the 14th. The top pictures illustrate the composition of the pulse train corresponding to a selection of an approximately 3 eV spectral bandwith. The relative amplitude of the pulses is not known.

Figure 2

Calculated harmonic spectrum and structure of the pulse train at different energies. The position of the harmonics 14 to 25 is indicated by the dashed light blue lines.

Figure 3

(a) Experimental harmonic spectrum as a function of the phase difference between the infrared and blue fields, $\phi$. (b) Spectra taken at the three phases marked in (a); the green and gray curves are shifted 0.6 and 1.2 intensity units, respectively. The dashed light blue lines indicate approximately the positions of harmonics 15 to 31.

Figure 4

(a) Experimental harmonic spectrum as a function of ${\varphi }_0$. (b) Calculated harmonic spectra as a function of ${\varphi }_0$ including amplitude effects. (c) Phase variations for the short trajectories added to the calculation in (b). (d) Phase effects corresponding to the long trajectories added to the calculation in (b).