Dynamical mechanism of resonance-like enhancement in atomic harmonic spectra

Resonance-like enhancement (RLE) structures in high-order harmonic generation (HHG) are typical quantum phenomena arising from the interaction of intense laser pulses with atoms or molecules. Although its existence is well-established, the dynamical mechanisms underlying these enhancements are still under debate. In this study, we apply the empirical mode decomposition method to achieve time-frequency analysis of the nonlinear and non-stationary quantum time-dependent dipole moments of hydrogen atoms in different laser fields, which are obtained by numerically solving the three-dimensional time-dependent Schrödinger equation. Combining semiclassical three-step model simulations and Morlet wavelet time-frequency analysis, it has been proven that these approaches allow for the quantitative extraction of individual quantum trajectories contributing to HHG, including the first and the second return trajectories, and further analysis indicates that the RLE structure in HHG spectra stems from the constructive interference between the first and the second return trajectories.