We theoretically investigate the impact of spectral leakage on harmonic intensity and phase by numerically solving the one-dimensional time-dependent Schrödinger equation (TDSE) for a model ${\mathrm{He}}^{+}$ ion. We show that spectral leakage is an intrinsic and ubiquitous issue in TDSE-based simulations of high-order harmonic generation. Its impact becomes increasingly pronounced with increasing laser peak intensity and wavelength. By analyzing the coupling terms in the dipole acceleration, we identify the physical origin of this effect as post-pulse oscillations of the dipole response. These oscillations give rise to artificial spectral leakage in the calculated harmonic spectra. By applying window functions to dipole acceleration in the time domain, the impact of spectral leakage can be efficiently reduced. However, most window functions cause a reduction in harmonic intensity and introduce errors in harmonic phase. Here, we demonstrate that several window functions are well suited for TDSE simulations, as they effectively mitigate spectral leakage while accurately regaining both the harmonic intensity and the phase.

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• High-order harmonic generation
• Ultrafast phenomena
• Atoms
• Dipole approximation
• Schroedinger equation