Enhanced Coherent Emission from Ionized Nitrogen Molecules by Femtosecond Laser Pulses

The forward emission spectra were experimentally measured for ionized nitrogen molecules by an 800 nm pump laser and a delayed seed laser. It was found that emission lines around both 428 and 391 nm are greatly enhanced upon use of a 391 or 428 nm seed laser. The emission lines around 391 and 428 nm can be assigned to the rotational transitions of N₂⁺ [B²Σ_u⁺(v' = 0) → X²Σ_g⁺(v = 0)] and N₂⁺ [B²Σ_u⁺(v' = 0) → X²Σ_g⁺(v = 1)], respectively. They originate from seed-induced superfluorescence and resonant stimulated Raman scattering. The genetic algorithm was utilized to simulate the experimental observations and determine the relative population of B²Σ_u⁺(v' = 0), X²Σ_g⁺(v = 1), and X²Σ_g⁺(v = 0). The result verifies that vibrational population inversion is achieved between B²Σ_u⁺(v' = 0) and X²Σ_g⁺(v = 0) by the 800 nm pump laser. Our finding provides new insights into controlling the coherent emission of ionized nitrogen molecules, which has promising application in filamentation-based remote atmospheric sensing.