Free-space ^Ν₂⁺ lasers generated in strong laser fields: the role of molecular vibration

We investigate free-space lasing actions from molecular nitrogen ions (N₂⁺ ) at the wavelengths of ~391 nm and ~428 nm. Our results show that pronounced gain can be measured at either 391 nm or 428 nm laser wavelength with a pump laser centered at 800 nm wavelength, whereas the gain at 391 nm laser wavelength completely disappears when the wavelength of the pump laser is tuned to 1500 nm. Our theoretical analysis reveals that the different gain behaviors can be attributed to the vibrational distribution of populations in X²Σ⁺_g (v = 0) and X²Σ⁺_g (v = 1) states as the N₂⁺ ions are generated by photoionization in the laser fields, giving rise to more robust (i.e., less sensitive to the pump laser wavelength) population inversion for generating the 428 nm laser.