Observation of attosecond light localization in higher order harmonic generation

The superposition of selected higher order harmonics and continuous backgro und, produced in PLL. 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, f ollowed by a subfemtosecond beating, indicative for the production of train s of sub-fs XUV pulses, is clearly observable in the resulting temporal tra ce. The trace Fourier transforms to a frequency spectrum composed of harmon ics from the selected ensemble and a continuous background that correlates with the observed ultrashort feature. Numerical simulation spectra encompas s similar temporal localization.