A spectral-dip fingerprint of quantum path interference in high harmonic generation

Abstract High harmonic spectroscopy, as a self-referenced attosecond technique, is widely used to probe electronic structure and dynamics in both atoms and polyatomic molecules. In this study, we theoretically and experimentally investigate the high-harmonic spectra of argon at various laser intensities, revealing a distinctive spectral minimum in the near cutoff region. This feature, attributed to the destructive interference between short and long electron trajectories, serves as a spectroscopic fingerprint of quantum path interference. By analyzing the relative contributions and phases of these quantum paths, we demonstrate that the spectral minimum represents a new modulation pattern of quantum path interference with a period of 0.2 × 10 14 W cm − 2 . This provides new insights into the dynamics of electron trajectories and offers a potential diagnostic tool for observing and controlling ultrafast electronic processes in laser-matter interactions.