Probing the spectral and temporal structures of high-order harmonic generation in intense laser pulses

Abstract

We present an ab initio three-dimensional quantum study of high-order harmonic generation ~HHG! of atomic H in intense pulsed laser fields. Accurate time-dependent wave functions are obtained by means of the time-dependent generalized pseudospectral method recently developed and wavelet transform is used to perform time-frequency analysis of the resulting HHG power spectra. The results reveal striking details of the spectral and temporal fine structures of HHG, providing insights regarding HHG mechanisms in different energy regimes and benchmark data for testing the validity of existing HHG models. PACS number~s!: 42.65.Ky, 32.80.Wr, 42.50.Hz Recently a great deal of attention has been devoted to the study of multiple high-order harmonic generation ~HHG! processes in intense short laser pulses @1‐3#. Besides its fundamental interest for strong-field atomic and molecular physics, the HHG provides a potential tunable coherent light source in the extreme ultraviolet ~XUV! region, a so-called ‘‘tabletop synchrotron’’ @2#. Further, the HHG may lead to a promising way for generating subfemtosecond ~attosecond! pulses of radiation of high frequency @4,5#. For optimal control of the HHG processes, it is essential to have a thorough understanding of the spectral and temporal structures of the high-order harmonics and the detailed underlying mechanisms for HHG. The first experimental observation of the temporal coherence of HHG has recently been reported @6#. In this Rapid Communication, we present an ab initio threedimensional ~3D! precision quantum calculation of the time