Gain dynamics of a free-space nitrogen laser pumped by circularly polarized femtosecond laser pulses

Abstract

We experimentally demonstrate ultrafast dynamic of generation of the 337-nm nitrogen laser by injecting an external seed pulse into a femtosecond laser filament pumped by a circularly polarized laser pulse. In the pump-probe scheme, it is revealed that the population inversion between the C(3)Π(u) and B(3)Π(g) states of N(2) for the free-space 337-nm laser is firstly built up on the timescale of several picoseconds, followed by a relatively slow decay on the timescale of tens of picoseconds, depending on the nitrogen gas pressure. By measuring the intensities of 337-nm signal from nitrogen gas mixed with different concentrations of oxygen gas, it is also found that oxygen molecules have a significant quenching effect on the nitrogen laser signal. Our experimental observations agree with the picture of electron-impact excitation.

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

“…In the first type of air lasers, population inversion is achieved by dissociation of molecular oxygen or nitrogen followed by excitation of the atomic fragments using high-peak-intensity picosecond ultraviolet (UV) lasers, which gives rise to bidirectional amplified spontaneous emissions (ASEs) at either 845 nm from oxygen atoms or 870 nm from nitrogen atoms [8][9][10]. The second type of air lasers are realized by focusing intense ultrafast laser pulses in air which created population inversion conditions either in neutral N 2 molecules or in N 2 ions [4][5][6][7][11][12][13][14][15]. Although the pump mechanism behind the air laser from neutral N 2 molecules has been clarified which can be attributed to electron collisional excitation, the mechanism of the air laser from N 2 ions is still far from being fully understood and under intensive investigation.…”

“…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.…”

Characterization of Forward ASE Pulses Generated in Nitrogen with a Circularly Polarized Femtosecond Laser

“…Therefore, we conclude that the ASE pulses are randomly polarized, which is typical for ASE sources. It is already known that the buildup time for population inversion generated in the nitrogen molecules with a circularly polarized femtosecond laser is on the order of ~10 ps [3]. This buildup time is significantly longer than the pulse durations of supercontinua generated in nitrogen gas by femtosecond laser filamentation, thereby excluding any possibilities of seed amplification initiated by the generated white light.…”

“…Recently, it was found that free-space nitrogen lasers at 337 nm wavelength can be efficiently generated either by a linearly polarized 1053-nm picosecond laser [1] or by circularly polarized 800-nm femtosecond lasers [2,3]. These findings suggest that inelastic collision between the electrons produced by the pump pulses and the neutral nitrogen molecules plays a key role in producing the population inversion.…”

Nonperturbative generation of above-threshold harmonics from pre-excited argon atoms in intense mid-infrared laser fields

“…The pump-probe experimental setup is schematically illustrated in Figure 1, which is similar to that used in our previous works [23][24][25] . Briefly, the mid-infrared femtosecond laser pulses were provided by optical parametric amplifier (OPA, HE-TOPAS, Light conversion, Ltd.) pumped by a commercial Ti:sapphire laser system (Legend Elite-Duo, Coherent Inc.).…”