We experimentally demonstrate the generation of narrow-bandwidth emissions with excellent coherent properties at ∼391 and ∼428 nm from N +) inside a femtosecond filament in air by an orthogonally polarized two-color driver field (i.e. 800 nm laser pulse and its second harmonic). The durations of the coherent emissions at 391 and 428 nm are measured to be ∼2.4 and ∼7.8 ps, respectively, both of which are much longer than the duration of the pump and its second harmonic pulses. Furthermore,

We investigate lasing action in aligned nitrogen molecular ions (N 2 þ) produced in an intense laser field. We find that, besides the population inversion between the B 2 AE u þ À X 2 AE g þ states, which is responsible for the observed simulated amplification of a seed pulse, a rotational wave packet in the ground vibrational state (v ¼ 0) of the excited electronic B 2 AE u þ state has been created in N 2 þ. The rotational coherence can faithfully encode its characteristics into the amplified seed pulses, enabling reconstruction of rotational wave packets of molecules in a single-shot detection manner from the frequency-resolved laser spectrum. Our results suggest that the air laser can potentially provide a promising tool for remote characterization of coherent molecular rotational wave packets.

Recently, amplification of harmonic-seeded radiation generated through femtosecond laser filamentation in air has been observed, giving rise to coherent emissions at wavelengths corresponding to transitions between different vibrational levels of the electronic B(2)Σ(u)(+) and X(2)Σ(g)(+) states of molecular nitrogen ions [Phys. Rev. A. 84, 051802(R) (2011)]. Here, we carry out systematic investigations on its physical mechanism. Our experimental results do not support the speculation that such excellent coherent emissions could originate from nonlinear optical processes such as four-wave mixing or stimulated Raman scattering, leaving stimulated amplification of harmonic seed due to the population inversion generated in molecular nitrogen ions the most likely mechanism.

Recently, Yao et al. demonstrated the creation of coherent emissions in nitrogen gas with two-color (800 nm + 400 nm) ultrafast laser pulses [New J. Phys. 15, 023046 (2013)]. Based on this two-color scheme, here we report on systematic investigation of temporal characteristics of the coherent emission at)) by experimentally examining its evolution with the increase of the plasma channel induced by the intense 800 nm femtosecond laser pulses at a nitrogen gas pressure of ∼25 mbar. We reveal unexpected temporal profiles of the coherent emissions, which show significant superradiance signatures owing to the quantum coherence via cooperation of an ensemble of excited N 2 + molecules. Our findings shed more light on the mechanisms behind the laser-like emissions induced by strong-field ionization of molecules.

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.

Abstract:We report, for what we believe to be the first time, on the generation of remote self-seeding laser amplification by using only one 800 nm Ti:Sapphire femtosecond laser pulse. The laser pulse (∼40 fs) is first used to generate a filament either in pure nitrogen or in ambient air in which population inversion between the

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