Observation of sub - fs localization by higher order harmonic generation

Using two time delayed 60 fs pulses at 800 nm interacting with noble gases, sub Femtosecond beating and an isolated less than 100 attosecond sharp Feature is observed in the measured trace of a selected superposition of higher harmonics as a function of the delay time. The trace Fourier-transforms to a frequency spectrum composed of harmonics from the selected ensemble and a continuous background that correlates with the observed ultra short feature. Numerical simulations of the field amplitude spectrum of the superposition show similar temporal localization. Fourier transformed spectra of traces obtained for different atomic systems and different laser intensities result cut-off energies in agreement with the formula Ecut-off = I.P + 3.17U(p) . demonstrating the applicability of the experimental approach for the characterization of higher order harmonics.