Abstract
The continuous progress in N+2 lasing recently stimulates a great deal of interest in nonlinear and quantum optics of molecular ions,while a complete description of the ionic polarization is still lacking to date.In this work,we are dedicated to construct-ing the fundamental ionic polarization theory where several ubiquitous strong-field processes including ionization,electronic couplings and molecular alignment jointly determine the spatial arrangement of ions.With the model,the elusive polarization of N+2 lasing can be well interpreted.Our results show that the different electronic transition rules for strong-field ionization and resonant couplings result in peculiar population distributions of various electronic states of N+2 in space.Meanwhile,the spatial nonuniformity of population distribution can be aggravated or mitigated during field-free evolutions of coherent molec-ular rotational wave packets.Furthermore,when a follow-up resonant seed pulse interacts with the prepared ionic system,the anisotropic quantum coherence determining the polarization of subsequent N+2 lasing can be established.The qualitative agree-ment between experiments and simulations confirms the validity of the proposed model.The findings provide critical insights into the polarization and radiation mechanisms of molecular ions constructed via ultrafast laser pulses.
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