Generation of elliptically polarized nitrogen ion laser fields using two-color femtosecond laser pulses

National Basic Research Program of China [2014CB921300]; National Natural Science Foundation of China [11127901, 11134010, 11204332, 11304330, 11404357, 61405220]; Shanghai 'Yang Fan' program [14YF1406100]Recently, lasing action produced in population-inverted assembles, such as nitrogen/oxygen atoms(1,2), nitrogen molecular ions(3,4) and neutral nitrogen molecules(5,6), has attracted a great deal of research interests because of its promising potentials in remote sensing applications(7,8). Among them, the mechanism behind the establishment of the population inversion in neutral nitrogen molecules has been well identified, which is due to the impact excitation of neutral nitrogen molecules from the ground state to the C3 Pi(+)(u) state by energetic free electrons produced in the intense laser fields(9-11). However, a widely accepted model accounting for the lasing action in nitrogen molecular ions (N-2(+)) is still lacking. Yao et al. proposed the seed-amplification model and verified the population inversion between the B-2 Sigma(+)(u) state and the X-2 Sigma(+)(g) state of N-2(+) via energy amplification of a time-delayed seed pulse(3), which was further described as a result of the couplings of the ground and two excited states of N-2(+) in the strong laser fields(12). Kartashov et al. proposed that the different rotational periods of aligned molecular ions on the ground and excited electronic states can lead to transient laser gain and thus the creation of the coherent emissions(13). Liu et al. considered the coherent emission as a super-radiant emission whose population transferred to the excited state is caused by the field-induced multiple recollisions(14). These efforts have significantly enhanced the understanding of the physics of tunnel-ionized molecules in intense laser fields. In this work, we report on another unusual behavior of the N-2(+) laser at the wavelength of 391 nm. Previous results show that the N-2(+) lasers induced by tunnel ionization possess the same polarization direction as that of the seed pulses when the pump and seed pulses are either parallelly or perpendicularly polarized to each other(4). This can be well understood from a seed-amplification point of view. In such a case, the laser signal generated by the seed-amplification mechanism typically inherits all the characteristics of the seed pulses. Interestingly, when the angle between the polarization directions of the two linearly polarized pump and seed pulses is variable in the range of 0 degrees-90 degrees, we find that the N-2(+) laser field becomes elliptically polarized with a variable ellipticity depending on the angle between the polarization directions of the two pulses. Moreover, the P-branch and R-branch lines in the N-2(+) laser show dramatically different behaviors with the varying polarization direction of the seed pulses (i.e., the polarization direction of the pump is always fixed). We attempt to provide a plausible explanation to qualitatively understand this unexpected observation