Experimental simulations show that high-order harmonic generation from a long intense femtosecond laser can produce multiple shorter attosecond sub-bursts within each half optical cycle, which is different from the well-known attosecond pulse trains. The duration of each sub-burst scales approximately with the driving laser wavelength, and their origin is attributed to the interference of quantum orbits from the first two returns of the recombining electron. These sub-bursts can be phase matched and observed experimentally under specific laser focusing conditions and gas cell positioning.
High-order harmonics generated by a long intense femtosecond laser are known experimentally to create attosecond pulse trains (APTs). In the time domain, an APT consists of a sequence of sharp attosecond bursts that are equally separated by each half optical cycle. Here we show that such well-known features can be modified when a longer wavelength driving laser is used. From our simulations, we show that multiple shorter attosecond sub-bursts exist in the femtosecond pulse train within each half optical cycle and the duration of each sub-burst scales approximately as lambda-2 0 with the driving laser wavelength lambda 0. We show that such sub-bursts can be found using quantitative rescattering model for harmonics generated from a single atom, and their origin is due to the interference of the quantum orbits from first two returns of the recombining electron. We further show that such sub-bursts can be phase matched under proper laser focusing condition and the position of the gas cell, thus, such new features should be observable experimentally.
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