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Updated: Dec 31, 2025

Femtosecond Laser Filaments for Use in Sub-Diffraction-Limited Imaging and Remote Sensing
Published on: April 25, 2019
Formation Dynamics of Excited Neutral Nitrogen Molecules inside Femtosecond Laser Filaments
Rostyslav Danylo1, Xiang Zhang1, Zhengquan Fan1
1Engineering Research Center of Optical Instrument and System, The Ministry of Education; Shanghai Key Lab of Modern Optical System, University of Shanghai for Science and Technology, 516, Jungong Road, 200093 Shanghai, China.
Abstract:
Nitrogen molecules are promoted to excited neutral states during femtosecond laser pulse filamentary propagation in atmosphere, leading to a characteristic UV fluorescence. Using a laser-induced fluorescence depletion technique, we measure the formation dynamics of these excited neutral nitrogen molecules with femtosecond time resolution. We find that the excited neutral molecules are formed in an unexpected ultrafast timescale of ∼4 ps at 1 bar and ∼120 ps at 30 mbar pressure. From this observation we deduce that the excitation of neutral N_{2} occurs via multiple collisions with hot free electrons. Numerical simulations based on rate equations reproduce well this ultrafast formation time and its dependence on gas pressure, and thus support this interpretation.
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