Double Optical Gating of High-Order Harmonic Generation with Carrier-Envelope Phase Stabilized Lasers

We demonstrated a novel optical switch to control the high-order harmonic generation process so that single attosecond pulses can be generated with multiple-cycle pulses. The technique combines two powerful optical gating methods: polarization gating and two-color gating. An extreme ultraviolet supercontinuum supporting 130 as was generated with neon gas using 9 fs laser pulses. We discovered a unique dependence of the harmonic spectra on the carrier-envelope phase of the laser fields, which repeats every $2\pi$ radians.

Figure 1

The driving filed components for PG correspond to (a) without and (b) with the second harmonic field, respectively. The driving field is shown as the red line. The two vertical lines represent the gate width. Here, the filled curves are the attosecond pulses emitted when the driving fields alone are applied. The background color shows the strength of the PG. In figure (c) the ionization probability of an argon atom in the field of PG pulses is compared with that in the field of DOG. The longest pulses that can be used are those at which the probability reaches one.

Figure 2

Comparison of harmonic spectra and pulses. (a) The harmonic spectrum images obtained by various gating methods. The lineout plots in (b) compare the spectral profile and intensity obtained from DOG (blue line) with those from PG for delays of 12 fs (red line) and 15 fs (green line). The results from numerical simulations are shown in (c). (d) Shows the Fourier transform for the DOG and (e) shows the Fourier transforms for the PG and two-color spectra. These were done assuming flat phase.

Figure 3

The CE phase dependence on the DOG spectra. (a) shows the intensity plot of the harmonic spectrum as the CE phase was scanned through $2\pi$. The normalized integrated signal is shown as the red line in (b). The blue line corresponds to the theoretical predictions. (c) shows the semiclassical predictions of the waveform within the gate width when the CE phase is scanned. The color indicates the strength of the ellipticity.

Figure 4

Results of harmonics generated with a neon target. (a) and (b) show the harmonic spectra and the line profile, respectively. (c) The temporal component derived from the Fourier transform assuming flat phase.