Spectral and temporal structures of high-order harmonic generation of Na in intense mid-ir laser fields

Abstract

We present a three-dimensional quantum study of high-order harmonic generation (HHG) of the Na atom in intense mid-ir laser fields. An accurate one-electron model potential is constructed for the description of the Na atom. The time-dependent Schr\"odinger equation is solved by means of the time-dependent generalized pseudospectral method, allowing nonuniform and optimal spatial grid discretization and accurate and efficient propagation of the wave function in space and time. Excellent agreement of the HHG spectrum in the length and acceleration forms is obtained from the lowest harmonics up to the cutoff. The HHG power spectrum shows fine structures and significant enhancement of the intensities of the lower harmonics due to the strong coupling of the $3s\ensuremath{-}\mathrm{np}$ states and the $3s\ensuremath{-}3p$ multiphoton resonance. We use a wavelet transform to present a detailed time-frequency analysis of the whole range of the HHG power spectrum. The results reveal striking details of the spectral and temporal fine structures of HHG, providing insights into different HHG mechanisms in different energy regimes of Na atoms at long wavelengths.