Observation of Carrier-Envelope Phase Phenomena in the Multi-Optical-Cycle Regime

So far the role of the carrier-envelope phase of a light pulse has been clearly experimentally evidenced only in the sub-6-fs temporal regime. Here we show, both experimentally and theoretically, the influence of the carrier-envelope phase of a multi-optical-cycle light pulse on high-order harmonic generation. For the first time, we demonstrate that the short and long electron quantum paths contributing to harmonic generation are influenced in a different way by the pulse carrier-envelope phase.

Figure 1

Measured single-shot harmonic spectra in argon generated by 20-fs driving pulses: (a) gas jet located $\sim 2\mathrm{mm}$ after the laser focus; (b) gas jet located around the laser focus; the inset shows an enlarged view of the spectral region between two adjacent odd harmonic peaks. The black and blue curves correspond to two different driving pulses.

Figure 2

Harmonic spectra in argon calculated using a nonadiabatic three-dimensional numerical code for two different CEPs (pulse duration 20 fs; pulse peak intensity $2\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$; gas jet thickness 0.2 mm; argon pressure 50 torr).

Figure 3

Portion of the plateau region of harmonic spectra calculated using the saddle-point method; the red curves correspond to $\phi =0$; the black curves correspond to $\phi =5\pi /8$. (a) Coherent superposition of the contributions from the short quantum paths. (b) Coherent superposition of the contributions from the long quantum paths; the calculated intensity has been multiplied by a factor of 2 for visual convenience. (c) Complete single-atom emission spectra produced by the coherent superposition of the contributions from the short and the long quantum paths.