Harmonic-seeded remote laser emissions in N2-Ar, N2-Xe and N2-Ne mixtures: a comparative study

We report on the investigation on harmonic-seeded remote laser emissions at 391 nm wavelength from strong-field ionized nitrogen molecules in three different gas mixtures, i.e., N2-Ar, N2-Xe and N2-Ne. We observed a decrease in the remote laser intensity in the N2-Xe mixture because of the decreased clamped intensity in the filament; whereas in the N2-Ne mixture, the remote laser intensity slightly increases because of the increased clamped intensity within the filament. Remarkably, although the clamped intensity in the filament remains nearly unchanged in the N2-Ar mixture because of the similar ionization potentials of N2 and Ar, a significant enhancement of the lasing emission is realized in the N2-Ar mixture. The enhancement is attributed to the stronger third harmonic seed, and longer gain medium due to the extended filament.

Different from the ASE schemes mentioned above, we recently demonstrated a novel approach to realize switchable multi-wavelength laser in air [14] and simultaneous multi-wavelength-generated ultraviolet laser in carbon dioxide [15]. In our scheme, molecular nitrogen and carbon dioxide ions induced by ultrafast laser filamentation of femtosecond infrared laser pulses serve as the gain medium with the population inversion established in an ultrafast time scale comparable to the pump laser pulse.
Besides, the lasing emissions originate from stimulated amplification seeded by self-generated harmonics of the pump laser during filamentation, which enables production of stronger laser emissions than those reported in ref. [6][7][8][9][10][11][12] and simpler operation than that in ref. [13]. In a broad sense, the discoveries of the strong harmonic-seeded remote laser emissions from N 2 and CO 2 gas media are clear evidences of the advantages of investigating femtosecond laser filamentation with intense, wavelength-tunable mid-infrared ultrafast sources. The research field is still in its infancy, but some exciting results have already been achieved [16][17][18][19].
In this paper, we systematically study the behaviors of the harmonic-seeded nitrogen laser inside a femtosecond laser-induced filament in nitrogen mixed with xenon, argon or neon. In different gases the clamped laser intensity inside the filament core would vary, depending on the nonlinear Kerr refractive index and the ionization potential. The highest clamped intensity is produced in the N 2 -Ne mixture, which is followed by the N 2 -Ar mixture and then the N 2 -Xe mixture. It is reasonable to expect a higher yield of 391 nm lasing emission in the N 2 -Ne mixture because of the higher clamped intensity.
Interestingly, we observed that the enhancement in the N 2 -Ar mixture is more remarkable. The mechanisms responsible for the observations will be discussed in details. Moreover, we also show in this study simultaneous laser emissions at 357 and 391 nm, or at 420 nm, 424 nm, and 428 nm by pump lasers with different wavelengths.

Experiment
The experimental setup is shown in Fig. 1. Wavelength-tunable mid-IR femtosecond laser pulses are generated by an optical parametric amplifier (OPA) (HE-TOPAS, Light Conversion, Inc.) pumped by a commercial Ti: sapphire laser system (Legend Elite-Duo, Coherent, Inc.) with a repetition rate of 1 kHz and a central wavelength of ~800 nm. The mid-IR pulse is focused into a vacuum chamber filled with different gas mixtures (N 2 -Ne, N 2 -Ar, or N 2 -Xe) by a fused silica lens with a focal length of 24.3 cm. The focal length is measured at 1184 nm wavelength taking into account the 3-mm-thick window of the vacuum chamber. The diameter of the laser beam impinged on the lens is ~8 mm at 1/e 2 of the energy fluence. Output signals are collimated by a lens with a focal length of 25 cm, and then sent into an imaging grating spectrometer (Shamrock 303i, Andor) with a 1200 grooves/mm grating. The entrance slit of the spectrometer is set to be 50 μm. Side fluorescence is imaged by a lens with a focal length of 4cm and collected by the same spectrometer (Shamrock 303i, Andor). In this experiment, nitrogen is mixed respectively with three different gases: A. xenon; B. neon; C. argon. The ionization potentials ( p I ) and the second-order Kerr refractive index coefficients ( 2 n ) of all the gases used are listed in Table 1 [20][21][22][23]. In these three different gas mixtures, the pump laser wavelength is fixed at 1184 nm with an average power of 960 mW. Fluorescence signals of ionized nitrogen molecules at 391 nm are recorded from the side of filament with 1 s exposure time, corresponding to 1000 laser shots. Forward 391nm-laser is attenuated to 1% and is recorded with 0.1s exposure time, corresponding to 100 laser shots.  [22,23] 10 [22,23] 0.74 [20,21]

Results
In Fig  We notice that the enhancement introduced by argon can also exist when we tune the pump wavelength to 1258 nm at which the lasing signal at 391 nm is very weak (barely observable) in pure nitrogen. This is illustrated in Figs. 3(a-b). To verify that the 391 nm emission is generated by amplification of the harmonic seed rather than by spontaneous emission or amplified spontaneous emission (ASE), we measured its polarization property as presented in the inset of Fig. 3(b). A nearly perfect linear polarization is obtained as shown in the inset of Fig. 3(b), indicating that the 391 nm signal results from amplified seed action because both spontaneous emission and ASE show isotropic polarization. To demonstrate the enhancement of 391 nm laser emission in the N 2 -Ar mixture, the 391 nm intensity is depicted in Fig. 3(c) as a function of the argon pressure, which gives an almost linear dependence without saturation.

Discussion
In order to interpret the different behaviors of the 391 nm laser emission, we describe the output of the 391 nm lasing signal from the gas mixture by the well-known small signal gain equation for simplicity, harmonics [14,15]. It is therefore strongly dependent on the ionization rate of N 2 and accordingly the clamped intensity in the filament.
In different kinds of gases, the clamped intensity I varies following the relationship n and e N are proportional to pressure, the clamped intensity is independent on pressure. According to this formula, the clamped intensity in nitrogen is higher than that in xenon, lower than that in neon, and comparable with that in argon. In the case of a gas mixture, the clamped intensity depends on the concentration ratio of different gas species in the mixture.
In the N 2 -Xe mixture, the clamped intensity will decrease with the increase of xenon pressure and gradually approach the value obtained in pure xenon at high xenon pressure. To confirm this, we measure fluorescence intensity of 391 nm from the side of the filament in Fig. 4(a). The 391 nm fluorescence stems from the radiative decays of the excited state + Σ u B 2 of N 2 + [27] and thus depends on the ionization rate of nitrogen and correspondingly the clamped intensity inside the filament.
Therefore, the clamped intensity can be indicated from the fluorescence intensity at 391 nm. In Fig.   4(a), the 391 nm fluorescence decays rapidly with the increase of xenon pressure and stabilizes at high xenon pressure, suggesting the clamped intensity inside the filament sharply decreased as the xenon pressure increased and finally reach the value in pure xenon. Note that the filament width also impacted on the fluorescence intensity. In Fig. 4 (d), respectively. From Fig. 4(d) we can see that the filament in the N2-Xe mixture gets much broader than that in the pure nitrogen, which may reflect the ionization potential of different gases, that is, the lower ionization potential is, the broader the filament diameter becomes. We speculate that the central bright line in N 2 -Xe mixture is from the filament core and the weak luminescence is from the energy reservoir. This broader filament diameter will result in a larger number of fluorescent nitrogen ions and in turn stronger fluorescence intensity. Judging from the decay curve in Fig. 4(a), the influence of the broadened filament width on the fluorescence signal should be relatively weak as compared with the influence of the decreasing clamped intensity in the filament.
The sharply reduced clamped intensity will lead to a great reduction in the generation of nitrogen ions, and strongly suppress the population inversion n Δ . Although the high nonlinearity of xenon enables efficient generation of the 3 rd harmonic which is clearly shown in Fig. 4(e), and a larger volume of filament that can be clearly observed in Figs. 4(b-d), the strongly suppressed population inversion leads to the quenching of the 391 nm lasing emission, as demonstrated in Figs. 2(a) and 2(d).
In the N 2 -Ne mixture, the clamped intensity is expected to increase with the increase of neon pressure and gradually approach the value obtained in pure neon. However, within the pressure range of our experiment, the increase of clamped intensity is small, which is evidenced by the slight increase of the 391 nm fluorescence in Fig. 4(a). This is because the high ionization potential of neon makes it hard to be ionized, so that its contribution to the electron density is negligible. To exclude the influence of filament width on the fluorescence intensity, we note that the filament is mainly supported by nitrogen molecules in this case, so that the change in the filament size is not significant since the pressure of nitrogen is fixed. This is also confirmed by the lateral and longitudinal plasma luminescence profiles in Figs. 4(c-d).
The slightly increased clamped intensity (in N 2 -Ne mixture) consequently leads to the slight increase of the 391 nm laser emission. The reasons are as follows. First of all, the increased clamped intensity will promote the ionization of nitrogen and the population inversion n Δ . Secondly, note that the 3 rd harmonic in a filament scales as ) [25,28], therefore the increase of both the neon pressure and the clamped intensity can contribute to the enhancement of the 3 rd harmonic intensity which serves as the seed.
In the N 2 -Ar mixture, the clamped intensity should remain nearly the same with changing gas To further clarify the role of the 3 rd harmonic intensity in the enhancement of the 391 nm laser emission, we compare the 391 nm laser peak intensity and its seed intensity in Fig. 4 ) in the two gas mixtures is the peak intensity in Fig. 2(e) and Fig. 2(f), respectively. The seed intensity is the interpolation value of the 3 rd harmonic intensity at the peak wavelength of the 391 nm lasing emission.
As we can see from Fig. 4(f), the 391 nm laser peak intensity closely follows the seed intensity. In the N 2 -Ar mixture, it stops growing at pressures above 200 mbar due to saturation of the seed inside the core of the filament. We ascribe the saturation of the 3 rd harmonic to the ring structure of the 3 rd harmonic beam profile at high pressure [18,19,29]. In this case, the increase of the 3 rd harmonic with argon pressure will be contained in the ring due to plasma defocusing, leading to the saturation of the 3 rd harmonic inside the core. Since the 391 nm laser emission emerges from the core of the filament, the saturation of the 3 rd harmonic together with the nearly constant intensity of the pump inside the filament core inevitably results in the saturation of the 391 nm laser emission.
Comparing the seed and 391 nm laser signals obtained in N 2 -Ar with those obtained in N 2 -Ne as illustrated in Fig. 4(f), the laser emission at 391 nm is much stronger in the N 2 -Ar mixture than that in the N 2 -Ne mixture. This is ascribed to the much stronger seed and the longer filament length as shown in Figs. 4(b) and 4(c).

Simultaneous multi-wavelengths harmonic seeded N 2 + remote laser
In the early work in Ref [14], the 5 th harmonic seeded N 2 + remote laser is obtained separately at 330, This is not reported in Ref. [14]. The simultaneous multi-wavelength laser emissions mentioned above stem from the enhanced spectral broadening of the harmonic seed in a long filament [25], because in such case, the spectrum of the harmonic seed enables covering the wavelengths of multiple lasing lines [15].

Conclusion
In conclusion, we have systematically investigated the behavior of the 3 rd harmonic seeded remote lasers in different types of gas mixtures. We show that although the signal intensity can be slightly increased in the N 2 -Ne mixture due to the increased clamping intensity, the enhancement in the N 2 -Ar mixture is much more significant. The enhancement mainly stems from the increase of both the 3 rd harmonic intensity and the filament length with increased argon pressure, as the ionization rate of nitrogen almost remain unchanged owing to the intensity clamping. Simultaneous laser emissions have also been achieved in pure nitrogen.
The experiment carried out in this work not only demonstrates enhancement of the remote lasing signal using gas mixtures, but also provides a new approach for gaining deep insight into the underlying physics of the establishment of population inversion. In generally, inside a filament induced in a certain type of gas medium, it is not easy to tune the peak intensity of the pump laser in a wide range because of the well-known intensity clamping [25,26]. This creates difficulty in studying the intensity dependence of the remote lasing signal. However, with different combination of gas mixtures, we are able to significantly vary the peak intensity in a filament, taking advantage of the different ionization potentials of different inert atoms or molecules. Therefore, the use of gas mixture provides us an efficient way for tuning the pumping conditions in a filament that is not reachable in a pure nitrogen gas, which benefits the investigation on the mechanism behind population inversion. The results obtained above clearly shows that the key factors for efficiently achieving the harmonic-seeded remote lasing are high pump intensity and strong self-generated harmonic seed. Only in the nitrogen-argon mixture, both conditions are satisfied, thus a dramatic enhancement of the lasing signal at 391 nm wavelength has been observed.