Fig. 1. (a) Schematic diagram of the experimental setup. BS, beam splitter; GT, Glan–Taylor prism; DM, dichroic mirror; HWP, half-wave plate; L₁ and L₂, lenses; filter, neutral density filter. (b) Typical N₂⁺ lasing intensity as a function of the pump-seed delay.

Fig. 2. (a) Measured polarization of N₂⁺ lasing at a non-alignment delay t₃ under the circumstances of three different seed intensities. The intensities of Seed 1, Seed 2, and Seed 3 are 9 × 10⁹, 2.1 × 10⁸, and 3.7 × 10⁷ W/cm², respectively. The pump and seed polarization directions are denoted by the red and blue arrows, respectively. (b) The intensity ratio defined as I(ϕ = 90°)/I(ϕ = 0°) versus the seed intensity. Inset: comparison of the three seed spectra.

Fig. 3. Measured polarization of N₂⁺ lasing at the revival moments of t₁ and t₂ by triggering with a relatively (a) strong seed intensity (6.3 × 10⁸ W/cm²) and (b) weak seed intensity (4.2 × 10⁷ W/cm²). The gray curves depict the polarization induced by the pump alone. (c) Typical spectra of the lasing components parallel to the pump (ϕ = 90°) polarization and the seed (ϕ = 0°) polarization at the delays of t₁ and t₂ for the case of a strong or weak seed pulse.

Fig. 4. Similar polarization measurements of N₂⁺ lasing at various seed intensities while the pump-seed delay was fixed at the alignment delay of t₁.