\textit{Ab initio} theoretical investigation of the frequency comb structure and coherence in the vuv-xuv regimes via high-order harmonic generation
Juan J. Carrera, Sang-Kil Son, and Shih-I Chu

We present a fully \emph{ab initio} quantum investigation of the frequency comb structure and coherence within each order of the high harmonic generation (HHG) spectrum in the high-frequency VUV/XUV regime. The HHG spectrum driven by a train of equal-spacing short laser pulses is calculated by propagating the time-dependent Schr\"{o}dinger equation accurately and efficiently by means of the time-dependent generalized pseudospectral method. We explore the comb structure and coherence by varying the laser pulse separation $\tau$, the number of pulses $N$, and the laser intensity. We found that a nested comb structure appears within each of the harmonics, ranging from the first harmonic all the way to the cut-off harmonic, and this global pattern persists regardless of the values of $\tau$ and $N$ used. The comb structure of the harmonics originates from quantum interferences among induced dipole pulses. Finally, the frequency comb structure prevails even in the presence of appreciable ionization.

© 2008 The American Physical Society.