Resonance-enhanced high-order harmonic generation by combination of an infrared laser field and a time-delayed attosecond XUV pulse

We investigate theoretically the high-order harmonic generation of the molecular ion ${\text{H}}_{3}{}^{2+}$ by numerically solving the two-dimensional time-dependent Schr"odinger equation. The results show that a resonant harmonic with large ellipticity is generated and the harmonic efficiency is significantly enhanced when the infrared laser field is combined with a time-delayed attosecond extreme-ultraviolet (XUV) laser pulse. The generation of resonant harmonics can be understood as the electron transition between the ground and excited states through the absorption of XUV photons. By varying the time delay and amplitudes of the two laser pulses, we can obtain the harmonics close to circular polarization in the resonant region. The input XUV pulse is linearly polarized, while the generated resonant harmonics are elliptically polarized, which means that the input XUV pulse can be reshaped. Moreover, we investigate the dependence of the resonant harmonic on the laser polarization, which shows the sensitivity of the harmonic polarization to the laser polarization and the resonance state can also be changed by changing the direction of polarization of the laser field.