Theoretical treatment on externally-seeded superradiance from N2+ in femtosecond laser filamentation in low-pressure nitrogen

Abstract

We perform a theoretical investigation of a collective emission in tunnel-ionized nitrogen molecules triggered by a coherent seed pulse. A semiclassical theory of superradiance that includes the superradiant temporal profile, characteristic duration, time delay, intensity is achieved. The theoretical predictions of 391-nm forward emission corresponding to the transition between N2+(B2Σu+,ν′=0) and N2+(X2Σg+,ν=0) as a function of nitrogen gas pressure are compared with the recent experimental data [Ding P. Lasing effect in femtosecond filaments in air, Ph.D. thesis, Université Paris-Saclay (2016)]. The good agreement demonstrates that the time-delayed optical amplification inside the molecular nitrogen ions is superradiance.