Isolated attosecond pulses provide an extremely high temporal resolution for studying ultrafast electron dynamics. The high-order harmonic generation (HHG) process by atomic or molecular gas targets remains the most efficient method for obtaining ultrashort isolated attosecond pulses in experiments. The harmonic spectra of molecular gas targets can be significantly extended and exhibit abundant and complex phenomena that merit attention and research. Because of the intrinsic dipole moment of asymmetric structured molecules such as CO, variations in molecular orientation in the laser field cause the ionization process to be very different, which can be used to modulate HHG and produce a supercontinuum spectrum. Furthermore, a two-color few-cycle laser drive can effectively enhance and control the ionization of molecules in the experiment, allowing the platform area to be extended and the intensity of harmonic radiation to be increased. The relative phase structure of the two-color field can be changed to further control and optimize its interaction with molecules. Based on the foregoing, this study investigates the effects of CO orientation differences and full relative phase modulation of two-color fields on achieving ultrashort isolated attosecond pulses.

This study calculates the attosecond pulse obtained by driving CO molecules to generate high-order harmonics in a two-color laser field using the extended Lewenstein model to the oriented molecule CO. The two-color line polarization fields are used in the calculations. One has a pulse duration of 9 fs, peak intensity of 2.0×10¹⁴ W/cm², and wavelength of 1600 nm; the corresponding values of the other are 5 fs, 1.0×10¹³ W/cm², and 800 nm, respectively. First, the microscopic ionization properties of CO molecules in the laser field with different orientations (molecular axis and laser field orientation angle from 0° to 180°) are considered in the case of a two-color laser field with a relative phase of 0. Then, after evaluating the intensity, conversion efficiency, and super-continuum of the plateau region under different conditions, the orientation angles 0° and 180° are chosen as representatives for the next step of the analysis. The effect of the change in relative phase on the molecular ionization is then induced at different molecular orientations by varying the carrier phase of the driving and control fields separately to analyze the mechanism of harmonic generation and the results for isolated attosecond pulses.

Results and Discussions Any relative phase can cause supercontinuum broadening of the harmonic plateau region at different molecular orientations. When the driving field s carrier-envelope phase changes and the molecular orientation is 0°, the ionization rates at different relative phases exhibit a double-peaked structure, with the intensity of each peak varying. At 0 and π, the ionization rate is the lowest (Fig. 3). When the molecular orientation is 180°, the ionization rate has a single-peak structure between the relative phases [0, π/2], whereas between [π/2, π], it has a double-peak structure with similar peak intensities, and the lowest ionization rate occurs at π/2. When the ionization rate is high, the cut-off energy for generating the HHG spectrum is low, and interference effects exist between the half-cycles, which makes acquiring ultrashort attosecond pulses difficult. This issue can be overcome by changing the carrier-envelope phase of the control field to yield shorter isolated attosecond pulses at a lower ionization rate (Fig. 6). As a result, ultrashort, isolated attosecond pulses can be easily obtained at lower ionization levels without constructing a specific relative phase to control the extent of field asymmetry (Fig. 4 and Fig. 7).

By driving CO molecules, two-color few-cycle laser pulses can achieve supercontinuum broadening of the harmonic plateau at any relative phase. Lower ionization rates are observed when the driving field s carrier-envelope phase shifts at molecular orientations of 0° and 180° with relative phases 0 and π/2, respectively. As a result, the plateau region s many harmonics were superimposed to produce isolated attosecond pulses with a duration of 94 as. Modulating the relative phase of the combined field causes a significant change in the ionization of electrons when the molecule orientation and laser field polarization direction adopt distinct orientations, making it simpler to construct a super continuous broadening plateau region with lower ionization rates. That is, the pulse emission energy can change more quickly over time, making it easier to obtain ultrashort attosecond pulses.