Frequency shift induced by multielectron polarization in high-order harmonic generation from polar molecules

Multielectron polarization (MEP) has been identified as an essential factor that must be considered for understanding nonlinear effects resulting from interactions of intense lasers with highly polarizable targets. While recent studies have mostly focused on intensity modulation induced by MEP in high-order harmonic generation (HHG), this work explores a significant MEP effect on another critical HHG feature: harmonic frequency. In particular, we discover MEP-induced frequency shifts up to 0.8 order (1.24 eV) for harmonics in the cutoff region when a polar molecule CO interacts with a few-cycle laser pulse. These frequency shifts are clarified through their relationship with the MEP-induced phase changes of attosecond bursts and then validated numerically by solving the time-dependent Schr"odinger equation. Moreover, we also verify them with available experimental data. Furthermore, we demonstrate the universality of these frequency shifts regardless of varying laser parameters. Importantly, we find a close connection between the MEP-induced harmonic frequency shift discovered in this study and MEP-induced intensity modulation of odd-even HHG reported previously. We claim that they are the two manifestations of the MEP effect on HHG, both stemming from the distortion of the phase difference between the two adjacent attosecond bursts. This comprehensive understanding of the MEP effect on HHG from polar molecules is significant for characterizing intrinsic molecular properties and probing molecular dynamics from HHG.