High-order harmonic generation from atoms and molecules

We experimentally investigate the high order harmonic generation (HHG) from aligned CO₂ molecules and demonstrate that the modulation inversion of harmonic yield with respect to molecular alignment can be altered dramatically by the intensity of the driving laser pulse. The laser intensity dependence of the angular distributions for the 19 to 27 harmonics is also directly measured. HHG has been proven as an effective approach for soft X-ray attosecond emission. However, the attainable attosecond pulse duration is limited by the intrinsic chirp of HHG, which is originated from the laser-intensity-dependent atomic dipole phase. We propose and demonstrate experimentally a scheme to compensate dynamically the intrinsic chirp of the attosecond harmonic pulses.