Resonance-enhanced high-harmonic generation and attosecond transient absorption spectroscopy of hydrogen atoms

Using ab initio simulations, we investigate high-harmonic generation in hydrogen atoms driven by a near-infrared (NIR) laser pulse combined with an attosecond extreme-ultraviolet (XUV) pulse. The efficiency of high-harmonic generation is significantly enhanced when the XUV pulse excites atoms from the ground state to an excited p state. Furthermore, by varying the time delay between the two laser pulses, we calculate the attosecond transient absorption spectra of hydrogen atoms based on numerical solutions of the time-dependent Schrödinger equation (TDSE). Combining attosecond transient absorption spectra with high-harmonic spectra provides a powerful approach for revealing laser-induced energy-level shifts and the splitting structure of resonant harmonic emission.