Attosecond Pulse Trains Generated Using Two Color Laser Fields

We investigate the spectral and temporal structure of high harmonic emission from argon exposed to an infrared laser field and its second harmonic. For a wide range of generating conditions, trains of attosecond pulses with only one pulse per infrared cycle are generated. The synchronization necessary for producing such trains ensures that they have a stable pulse-to-pulse carrier envelope phase, unlike trains generated from one color fields, which have two pulses per cycle and a $\pi$ phase shift between consecutive pulses. Our experiment extends the generation of phase stabilized few cycle pulses to the extreme ultraviolet regime.

Figure 1

(a) A one color driving field results in a frequency offset of $\delta ={\omega }_1$. (b) Phase locking produces an APT (solid line) with periodicity equal to half that of the driving field (dashed line) and a $\pi$ phase shift between consecutive pulses. (c) A two color field gives $\delta =0$. (d) Phase locking yields an APT with periodicity equal to that of the driving field, and a stable carrier envelope phase.

Figure 2

Modulation in the total harmonic yield as a function of $\phi$. The insets show the harmonic spectra for maximum (left) and minimum (right) yields.

Figure 3

(a) Photoelectron spectra versus delay between APT generated with a two color field and IR probe. The periodicity is one IR cycle. (b)–(e) Integrated electron yield above 18.3 eV for four different APTs. (b) $\phi =0$; (c) $\phi =\pi /2$; (d) $\phi =\pi$. In (e) an APT generated with only the IR pump pulse is shown.

Figure 4

Calculated harmonic generation when the ionization is restricted to occur only when the electric field is positive (thick green line) or negative (thin gray line). The intensities are $2\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ (IR) and $1\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ (blue), and $\phi$ is $0.7\pi$ in (a) and (b). (a) The two color electric field together with the temporal structure of the generated APT. (b) The corresponding spectra for ionization when the field is positive or negative. (c) Cutoff position and yield as a function of $\phi$ when the ionization is restricted to positive fields. The results for the negative half-cycles are identical except for a translation by $\pi$.