Ellipticity of harmonics from symmetric molecule H+2 in strong and short-wavelength laser fields

Ellipticity properties of high-order harmonic generation (HHG) from symmetric molecules H+2 in strong and short wavelength (less than 800nm) laser fields are numerically investigated. In this study, the ellipticity of harmonic is compared with the corresponding harmonic spectrum and dipole, and the calculation results are analyzed and compared between different laser intensities, different laser wavelengths, different internuclear distances and different orientation angles. Our numerical simulations show that the influence of laser intensity, laser wavelength, internuclear distances and orientation angle on the ellipticity of harmonic is different. Especially in the two-center interference region, the excited state plays an important role in the HHG, but the effect of the excited state on the ellipticity of harmonic is different at different orientation angles. Further analysis shows that these different effects are due to the influence of the excited state on the harmonic yields. Using the numerical scheme, it is determined that in the two-center interference region, the excited state plays an important role in the parallel harmonic spectrum, while the effect of the excited state on the perpendicular harmonics at different angles is very small, which result in different phase differences between the accurate harmonic spectrum and the harmonic spectrum only returning to the ground state. Overall, the relative yields of the accurate perpendicular harmonics are lower (higher) than the accurate parallel ones, but the intensity of the perpendicular harmonics, which only returns to the ground state, is comparable to (or farther away from) the parallel which is only to return to ground state in the two-center interference regions. Therefore, the small (large) intensity ratio between the accurate perpendicular and accurate parallel harmonics can be attributed to the contributions of the excited state to harmonics. Then we can conclude that the harmonic spectra that only transitions back to the ground state show high (small) ellipticity, whereas the accurate harmonic spectra show small (high) ellipticity, resulting in a strong angle dependence of the influence of the excited state on the ellipticity of harmonic. In addition, in the high-order harmonic plateau region, the relative yields of harmonics can be well predicted by the corresponding dipoles, indicating the applicability of tunneling pictures and plane wave approximation in the strong and short-wave laser fields. When the ellipticity of harmonic occurs in the interference region due to the two-center characteristics of the symmetric potential, the results show that the polarization measurement can also be used to detect the structure of symmetric molecules and track the dynamics of excited states.