TEMPORAL PHASE-CONTROL OF SOFT-X-RAY HARMONIC EMISSION

We report a strong dependence of the soft-x-ray spectra generated by h igh harmonic emission on the chirp of the excitation pulse, when an ul trashort laser drives the process. For identical pulse durations, dist inct harmonic peaks can be observed for positively chirped excitation pulses, while for negatively chirped pulses, the harmonic peaks become irregular. This behavior is explained by simulations that combine the chirp of the laser with the intrinsic phase shift of the harmonics. T his work resolves an outstanding discrepancy between theory and experi ment by demonstrating that high-order harmonic generation driven by sh ort-duration pulses can result in distinct harmonic peaks. This work c onclusively demonstrates the role of the intrinsic phase in determinin g harmonic emission spectra, and control this phase during the emissio n process.