Spectral-width variation of N2+ lasing

Abstract

The complicated collisional processes inside a femtosecond laser-induced plasma column can show a great impact on the light emissions from it. A known example is that the lifetime of noncoherent fluorescence emitting from gas plasma can be greatly decreased due to the fierce ultrafast plasma collisions. However, for coherent stimulated emissions in plasma surrounding, to what extent it can be influenced by the collisional process has not been fully understood. Herein, we report an experimental spectral width analysis on coherent ${\mathrm{N}}_{2}^{+}$ lasing that is generated in weakly ionized gas plasma with a frequently adopted pump-probe scheme. It is found unexpectedly that the spectral width of ${\mathrm{N}}_{2}^{+}$ lasing evidently shortens with the increasing delay between the pump and the probe pulses at a pressure of 40 mbars, whereas an invariable spectral width appears when the gas pressure is decreased below 10 mbars. Moreover, it was also observed that the spectral width narrows with the increment of polarization angle between the pump and the probe pulses. These observations update the understandings of molecular spectral manipulation on the basis of the previous that the spectral modulations mainly occur in the vicinity of molecular alignment. Our results indicate that by combining the spectral width and intensity analysis, a more comprehensive understanding regarding the impact of ultrafast electronic plasma-ion collisions on the ${\mathrm{N}}_{2}^{+}$ lasing generation can be obtained. These findings also shed light on the spectral manipulation for coherent irradiations produced during strong laser-matter interaction.