Introducing an effective method for extending the high harmonic spectrum plateau from gas targets

An effective semi-classical method is introduced for controlling the high-order harmonic generation process and extending the cutoff frequency. This method is capable of defining the proper specification of the driving laser for maximizing the cutoff frequency. This method is evaluated by examining the high harmonic spectrum from the hydrogen atom and the fluorine (F₂) molecule irradiated by single-, two-, and three-color laser fields. This study is done using the time-dependent density functional theory in a three-dimensional space. The results show that the single-, two-, and three-color laser pulses tuned by proper specifications could extend the cutoff frequency by up to 85%, 176%, and 241% compared to their non-tuned forms, respectively. Also, single attosecond pulses with a duration of 161 as and 129 as are obtained by applying the tuned three-color laser for the hydrogen atom and the fluorine molecule, respectively. (paper)