Generation of isolated sub-40-as pulses from gas-phase CO molecules using an intense few-cycle chirped laser and a unipolar pulse

Abstract

We theoretically investigate the high-order-harmonic generation (HHG) from the oriented gas-phase CO molecule exposed to the combination of an intense few-cycle chirped laser and a unipolar pulse. We show that the HHG is dramatically extended and an ultrabroad extreme ultraviolet supercontinuum spectra in the plateau is generated by adding a unipolar pulse to the few-cycle chirped laser at a proper time. Both the classical trajectory simulation and quantum time-frequency analysis show that a dominant short quantum path is selected to contribute the HHG spectra in the plateau. By superposing the supercontinuum harmonics centered at several different central frequencies, we can directly produce several isolated pulses in which the shortest one achieves 38 as.