Measuring the Coulomb time shift in high-order harmonic generation

High-order harmonic generation (HHG), a key nonlinear phenomenon in strong-field physics, enables ultrafast detection on the attosecond timescale. Quantifying ionization/recombination times is essential for trajectory-resolved high-harmonic spectroscopy and for benchmarking its temporal resolution. In this review, we summarize our recent studies Phys. Rev. A105 L041103 (2022), Phys. Rev. A106 023117 (2022), Phys. Rev. A107 063102 (2023), Phys. Rev. A111 039902 (2025) on the role of electron–core interactions in HHG. Employing the classical trajectory model, analytical R-matrix theory, and numerical solutions of time-dependent Schrödinger equations for helium, we reveal how Coulomb attraction induces subtle shifts in ionization and recombination times. Such effects emerge as observable signatures under orthogonally polarized bichromatic fields at high probe frequencies. Because of the direct experimental relevance of these findings, this review seeks to stimulate further experimental efforts to control and resolve electron dynamics in HHG. In the future, it will be of great interest to (i) refine retrieval methods by incorporating Coulomb corrections beyond the static-field approximation, and (ii) advance two-color detection techniques with the capability to reconstruct complete quantum trajectories in HHG.