Generation of multiple attosecond sub-bursts in femtosecond pulse trains by long-wavelength driving lasers

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.

Automated summary

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.