Comparative investigations of the spontaneous and stimulated emissions from nitrogen molecules in air with femtosecond laser excitation pulses

Abstract

We report on experimental investigations on the spontaneous and stimulated emissions from excited nitrogen molecules generated in both linearly and circularly polarized intense laser fields. The spontaneous emission is measured from the side direction of the laser-induced filament whereas the stimulated emission generated by seed amplification is measured in the forward direction of the laser propagation. The comparison between the signal intensities of the spontaneous fluorescence emission and the seed-amplified stimulated emission provides an insight into the population inversion generated in nitrogen molecules with circularly polarized femtosecond laser pulses.

Recent Advances in Air Lasing: A Perspective from Quantum Coherence

“…It has been recently demonstrated that backward emission from the to transition can also be achieved by pumping the nitrogen with circularly polarized femtosecond 800 nm pulses, which provides a potential method for generating the backward air lasing. Pulses with intensity around 10 mJ have been focused into gas chamber filled with pure nitrogen or a mixture of nitrogen and oxygen gases.…”

Air lasing: Phenomena and mechanisms*

“…[43] The generation of the intense backward N 2 lasing emission in the circularly-polarized femtosecond laser fields was also demonstrated. [44,45] Figure 3(a) shows backward emission spectra of the pure N 2 gas induced by circular-and linearpolarized laser filaments (800 nm/50 fs/9.3 mJ) at the atmospheric pressure. [44] It can be seen that the 337-nm spectral intensity in the circularly-polarized case is higher by one order of magnitude than that in the linearly-polarized case.…”

“…The dramatic enhancement of backward N 2 fluorescence in the circularly-polarized laser field was explained based on the ASE phenomenon with the population inversion of N 2 induced by the inelastic electron impact excitation. [44,45] To verify this, the electron kinetic energy distributions in the linearly-and circularly-polarized laser fields were calculated, as shown in Figs. 3(c) and 3(d), respectively.…”