Spectral characteristics across the spatial profile of a high-harmonic beam

The spectral characteristics and degree of spatial coherence of a few-order high-harmonic beam generated by femtosecond laser pulses in an argon gas cell are studied in situ by applying the maximum-entropy method to the interference pattern from a Young double-slit pair. Power spectra of the harmonics taken across the beam profile are presented for different effective interaction lengths and different diameters of an aperture applied to the fundamental laser beam. The role of the effective focused intensity and the associated fraction of generated plasma on the high-harmonic generation process is discussed.