Observation of Attosecond Light Localization in Higher Order Harmonic Generation

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Abstract
The superposition of selected higher order harmonics and continuous background, produced in Ar gas through two time delayed 60 fs pulses at 800 nm, is measured as a function of their delay time. An isolated less than 100 as sharp feature, indicative for pulse production in the attosecond regime, followed by a subfemtosecond beating, indicative for the production of trains of sub-fs XUV pulses, is clearly observable in the resulting temporal trace. The trace Fourier transforms to a frequency spectrum composed of harmonics from the selected ensemble and a continuous background that correlates with the observed ultrashort feature. Numerical simulation spectra encompass similar temporal localization.