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Abstract

In this work, the coherent control of attosecond pulse and pulse train production by high-order harmonic generation (HHG) is investigated both experimentally and numerically. For this purpose, intense, few-cycle femtosecond laser pulses with a stable carrier-envelope phase (CEP) are focused into argon. After the examination of pure CEP effects, the controllability of HHG is qualitatively enhanced by the addition of a time delay either between two variable spectral sections or with respect to a second driver field at the doubled frequency. The concept of independent control of several properties of the produced attosecond pulse at once by an equally large number of control variables is introduced as an efficient multidimensional (multi-parameter) scheme to control HHG both comprehensively and based on fundamental physics principles. The experimental realization of the split-spectrum time-delay method leads to control over manifold spectral properties of the produced high harmonics. The effects of the individual control variables are separated and the control mechanisms are uncovered on the single-atom level and confirmed by numerical simulations. In particular, it is found that a time-delay dependent change of the instantaneous frequency at the intensity maximum of the driver pulse causes a quantitatively determined modulation of the harmonic energies. Moreover, the interference of the two subfields enables the creation of two driver intensity maxima. The spectral interference of the two time-delayed attosecond pulse trains, produced at these intensity maxima, results in the controlled generation of fractional high-harmonic combs. These combs can, in future, be applied to high-precision spectral interferometry in the extreme ultraviolet.