Subfemtosecond-resolved modulation of superfluorescence from ionized nitrogen molecules by 800-nm femtosecond laser pulses

Superfluorescence emission around 391 nm is generated when nitrogen molecules are irradiated by a strong 800-nm pump laser and a delayed seed laser. The emission corresponds to the transition between N2+(B²Σu+,ν^″=0) and N2+(X²Σg+,ν=0). When another weak 800-nm probe laser is injected and scanned after the pump laser, the superfluorescence intensity is observed to exhibit periodical modulation. The period is determined to be ~2.63 fs, corresponding to the transition frequency between N2+(A²Π_u,ν^'=2) and N2+(X²Σg+,ν=0). Based on theoretical derivation, these observations can be attributed to the laser-induced population transfer and polarization variation between the relevant electronic states of ionized nitrogen molecules.