Abstract
Delayed cavity-free forward lasing at the wavelengths of 391 and 428 nm was observed in recent experiments in air or pure nitrogen pumped with an intense femtosecond laser pulse at wavelength of 800 nm. The mechanism responsible for the lasing is highly controversial. In this article we propose a model explaining the delayed lasing, which contains two parts: (i) ionization of neutral nitrogen molecules and subsequent excitation of nitrogen ions in a strong pump laser pulse, and (ii) coherent emission of excited ions due to the presence of long-lived polarizations maintained by a weak laser post-pulse and coupling simultaneously ground state to states A2Π u and of singly ionized nitrogen molecules . Two regimes of signal amplification are identified: a signal of a few picosecond duration at low gas pressures and a short (sub-picosecond) signal at high gas pressures. The theoretical model compares favorably with results obtained by different experimental groups.
Highlights
Several explanations for this lasing have been proposed so far
Such a V-scheme, studied in references [19,20,21] and more recently in reference [22], can be applied to the interpretation of lasing of nitrogen molecular ions N+2 driven by an ultrashort laser pulse with peak intensity in the range of a few 1014 W cm−2, but one needs to explain how the third level is driven, what are the populations at the excited levels and what are the conditions for obtaining optical amplification
The signal amplification in the B–X transition of nitrogen molecular ions is described by a two-step process: (i) the interaction of a short and intense pump laser pulse at the wavelength 800 nm with a nitrogen gas leads to partial ionization of the nitrogen molecules and partial excitation of the molecular ions to the upper states A and B
Summary
Republic of China 4 CAS Center for Excellence in Ultra-intense Laser Science, Shanghai, 201800, People’s Republic of China 5 Laboratoire d’Optique Appliquee, ENSTA Paris, CNRS, Ecole Polytechnique, Institut Polytechnique de Paris, 91762 Palaiseau, France ∗ Author to whom any correspondence should be addressed
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