Theory of a filament initiated nitrogen laser

Abstract

We present the theoretical model for a single-pass, discharge-type standoff nitrogen laser initiated by a femtosecond filament in nitrogen gas. The model is based on the numerical solution of the kinetic equation for the electron energy distribution function self-consistently with balance equations for nitrogen species and laser equations. We identify the kinetic mechanisms responsible for a buildup of population inversion in the filament afterglow plasma and determine the dependence of population inversion density and the parameters of nitrogen lasing at a 337 nm wavelength corresponding to the transition between the C3Πu (v = 0) excited and the X1Σg (v = 0) ground electronic states in a nitrogen molecule on the polarization and wavelength of the driver laser pulse used to produce the filament. We show that population inversion is achieved on an ultrafast time scale of ≈10 ps and decays within the time: <100 ps. We derive the low-signal gain 2.2 cm−1 for lasing from a circularly polarized 0.8 μm near-IR filament and 0.16 cm−1 for a linearly polarized 4 μm mid-IR filament. The results of the numerical simulations demonstrate good quantitative agreement with the experimental measurements.

Formation Dynamics of Excited Neutral Nitrogen Molecules inside Femtosecond Laser Filaments

“…In the STP condition, the mean collision time between electrons and nitrogen molecules can be estimated to be around 0.35 ps for electrons with energy of 11.1 eV, the threshold energy for electron collision excitation for the C 3 Π þ u state [27]. Therefore, in a temporal window of 4 ps, tens of inelastic collision events can occur, which can gradually populate level N 2 ðC 3 Π þ u Þ and qualitatively explain our observation [28,29]. For a more quantitative analysis, we simulate the formation dynamics of excited neutral molecules using a rate equation for the different neutral nitrogen excitation processes.…”

Backward nitrogen lasing actions induced by femtosecond laser filamentation: influence of duration of gain

“…It should be noted that the optimal gas mixture (300 mbar N 2 and 900 mbar Ar) for generating the strongest lasing signal presented in figure 3(a) is different from that (1 bar N 2 and 5 bar Ar) using 3.9 μm mid-infrared pumping [6]. One possibility leading to the difference is that according to the calculation in [16], the electron energy will be higher and distributed in a broader range with the 3.9 μm pump laser, which can lead to more efficient collisional excitation. Also, we speculate that at the longer wavelength, higher gas pressure can be used due to the reduced ionization effect (i.e., the same electron kinetic energy can be achieved at much lower laser intensity for the long wavelength driver laser, which reduces photoionization rate), which further leads to the enhancement of lasing.…”

“…Also, we speculate that at the longer wavelength, higher gas pressure can be used due to the reduced ionization effect (i.e., the same electron kinetic energy can be achieved at much lower laser intensity for the long wavelength driver laser, which reduces photoionization rate), which further leads to the enhancement of lasing. The results in figure 3(b) indicate that pumping N 2 molecules with excited metastable Ar atoms can lead to a much longer duration of gain than that achievable with the electron collisional excitation in pure N 2 gas, even intense, ultrashort laser pulses are used in both cases [14,16,19]. The difference in turn leads to essentially different laser behaviors, i.e., the former scheme has successfully generated strong ASE N 2 laser in both forward and backward directions, whereas with the latter scheme only forward laser signals can be generated with a decent pulse energy.…”

“…A careful examination shows that although high gain coefficients can be observed in the forward (FW) direction for both the atomic and molecular air lasers, they have distinctly different gain dynamics. Noticeably, the duration of gain in the atomic and molecular air lasers are on the nanosecond and picosecond levels, respectively [10,14,16]. The duration of gain is defined as the duration of time during which amplification of a seed pulse at the lasing wavelength can be observed in the gain medium.…”

Testing the Role of Recollision in N2+ Air Lasing

“…The filament is accompanied by a variety of emissions, with on-axis components at frequencies covering a wide range from terahertz to harmonics of the fundamental beam [3]. The directional emissions observed from the neutral nitrogen molecule [5][6][7][8][9][10] and the nitrogen molecular cation [10][11][12][13][14][15][16][17], known as "air lasing," are of particular interest for remote sensing.…”