Strong and coherent forward emissions from molecules driven by femtosecond infrared laser pulses

Strong narrow-band coherent emission lines of the B²Σ⁺ _u –X²Σ⁺ _g transition of N⁺ ₂ in air and the A²Π_u–X²Π_g transition of CO⁺ ₂ in CO₂ gas are observed from filaments generated by intense infrared femtosecond laser pulses (1150– 2000 nm, <200 fs) in air. The coherent emission stimulated by self-generated third and/or fifth harmonics of the infrared femtosecond laser pulses is found to propagate in the forward direction along the infrared laser beam and has the same polarization direction as the third and fifth harmonics serving as the seed light. The intensity of the strong forward emissions of N⁺ ₂ and CO⁺ ₂, exhibiting excellent temporal and spatial coherence properties, is two orders of magnitude stronger than that of the fluorescence of molecules, and is strongly dependent on the pump laser intensity, the intensity of the seed harmonic radiation, and the filament length. The possible mechanisms responsible for the generation of the strong coherence emissions are discussed.