We investigate the spatiospectral coupling appearing in the spatially resolved high-order-harmonic spectra generated in gases. When ionization is weak, harmonic generation in the far field often exhibits rings surrounding a central spot centered on each odd harmonics in the spatiospectral domain. The nature of these structures is debated. They could stem from interferences between the emission of short and long trajectories, or could be the signature of the temporal and spatial dependence of the longitudinal phase matching of long trajectories (Maker fringes). We conducted spectrally and spatially resolved measurements of the harmonic spectra as a function of pressure, intensity, and ellipticity. In addition, we performed calculations where only a single emission plane is included (i.e., omitting deliberately the longitudinal phase matching), reproducing the features experimentally observed. This study has been completed by the spatiospectral coupling when strong ionization occurs leading to complex patterns which have been compared to calculations using the same model and also show good agreement. We conclude that many spatiospectral structures of the harmonic spectrum can be interpreted in terms of spatial and temporal transverse coherence of the emitting medium without resorting to longitudinal phase matching or quantum phase interference between short and long trajectories.

• Received 28 July 2016

DOI:https://doi.org/10.1103/PhysRevA.94.063401