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ncreasing the carrier frequency of attosecond pulses would greatly extend the range of attosecond effects that can be studied in time-resolved experiments. Although generation of harmonic photons with energies exceeding 1 keV has been demonstrated with intense few-cycle laser pulses, two problems have to be solved to make these harmonics suitable for the attosecond spectroscopy: the conversion efficiency must be improved and a practical scheme for generation of isolated attosecond pulses in this spectral region must be found. This talk addresses both of these issues.
In the first part of the talk the potential of using mid-IR few-cycle pulses for generation of keV harmonics and attosecond pulses will be discussed. Our simulations are based on the semi-classical Lewenstein's model for the single-atom dipole response combined with numerical solution of propagation equations. We demonstrate that increasing the wavelength of the driving field may increase the conversion efficiency in spite of the rapid decrease in the strength of the single-atom dipole response due to the improved phase-matching. We discuss conditions, when this is possible and analyse the potential of using a longer wavelength of the driving field to generate isolated attosecond pulses. In the second part of the talk we will discuss how generation of attosecond pulses from high-order harmonics can be controlled by using a sequence of gas jets. We demonstrate that the quasi-phase-matching provided by a multi-jet system can be limited to a sub-femtosecond time window, while the central frequency of the generated isolated attosecond pulse can be adjusted by tuning parameters of the jets. Additionally, an efficient method to simulate the high-frequency part of the single-atom dipole response will be presented, which significantly facilitates modelling generation of keV harmonics. |
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