Interference effects in harmonic ellipticity from H₂⁺ molecules in an orthogonally polarized two-color laser field

We theoretically investigate the elliptically polarized harmonics from aligned molecular ion H₂⁺ in orthogonally polarized two-color laser pulses. Our results show that a striking peak appears in the ellipticity curve of odd-order harmonics, and this peak shows obvious alignment dependence and shifts toward higher energy as the amplitude ratio of the two colors increases. The location of the peak is close to that of the interference minimum predicted by using a modified dipole model, indicating strong two-center interference effects in ellipticity. Through adjusting the frequency ratio of the two colors, large ellipticity is obtained for both odd and even harmonics in a wide energy region. As a result, we synthesize a bright, elliptically polarized, isolated attosecond pulse with an ellipticity up to 0.63.