In principle, the temporal beating of superposed high harmonics obtained by focusing a femtosecond laser pulse in a gas jet can produce a train of very short intensity spikes, depending on the relative phases of the harmonics. We present a method to measure such phases through two-photon, two-color photoionization. We found that the harmonics are locked in phase and form a train of 250-attosecond pulses in the time domain. Harmonic generation may be a promising source for attosecond time-resolved measurements.

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Journal Science
Citation
Paul, P. M, Toma, E. S, Breger, P, Mullot, G, Augé, F, Balcou, P, … Agostini, Pierre. (2001). Observation of a train of attosecond pulses from high harmonic generation. Science, 292, 1689–1692.