Observation of the optical X2Σg+–A2Πu coupling in N2+ lasing induced by intense laser field

We propose a simple pump-coupling-seed scheme to examine the optical X^2Σg^+–A^2Πu coupling in N2^+ lasing. We produce the N2^+ lasing at 391 nm, corresponding to the B^2Σu^+(v = 0)–X^2Σg+(v = 0) transition, by externally seeding the N^2+ gain medium prepared by irradiation of N2 with an intense pump pulse. We then adopt a weak coupling pulse in between the pump and seed pulses, and show that the intensity of the 391-nm lasing can be efficiently modulated by varying the polarization direction of the coupling pulse with respect to that of the pump pulse. It is found that when the polarization directions of the pump and coupling pulses are perpendicular, the 391-nm lasing intensity is more sensitive to the coupling laser energy, which reflects the inherent nature of the perpendicular X^2+Σg^–A^2Πu transition.

Optical free induction decay emission carrying orbit angular momentum from ionic nitrogen enabled by multiple-photon resonance

Coherent optical emission of free induction decay (FID) excited by the multiple-photon resonance has been recently observed in nitrogen ions produced by intense femtosecond laser pulses.Here we report that orbit angular momentum (OAM) of the pump laser pulses can be transferred to the much longer pulsed free induction decay emission with the OAM conserved.It was found that when the pump laser frequency approaches the fraction of the 391.4 nm emission (ν_(0)/5),an FID emission is emitted whereas its topological charge was identified to be 5 times of the pump laser.Due to the reduced laser intensity with a vortex pump compared to the conventional Gaussian beam,the resonant pump laser wavelength presents a redshift,and the FID signal is significantly weaker.

Anisotropic coherence induced nonuniform amplification in N_(2)^(+)

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 constructing 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 molecular 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 agreement 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.