We demonstrate a harmonic-seeded switchable multiwavelength laser in air driven by intense midinfrared femtosecond laser pulses, in which population inversion occurs at an ultrafast time scale (i.e., less than $\sim${}200 fs) owing to direct formation of excited molecular nitrogen ions by strong-field ionization of inner-valence electrons. The bright multiwavelength laser in air opens the perspective for remote detection of multiple pollutants based on nonlinear optical spectroscopy.

Powerful femtosecond laser pulses propagating in transparent materials result in the formation of self-guided structures called filaments. Such filamentation in air can be controlled to occur at a distance as far as a few kilometers, making it ideally suited for remote sensing of pollutants in the atmosphere. On the one hand, the high intensity inside the filaments can induce the fragmentation of all matters in the path of filaments, resulting in the emission of characteristic fluorescence spectra (fingerprints) from the excited fragments, which can be used for the identification of various substances including chemical and biological species. On the other hand, along with the femtosecond laser filamentation, white-light supercontinuum emission in the infrared to UV range is generated, which can be used as an ideal light source for absorption Lidar. In this paper, we present an overview of recent progress concerning remote sensing of the atmosphere using femtosecond laser filamentation.

We investigate lasing action in aligned nitrogen molecular ions ($\mathrm{N}_2{}^{+}$) produced in an intense laser field. We find that, besides the population inversion between the ${\mathrm{B}}^2\Sigma _u{}^{+}-{\mathrm{X}}^2\Sigma _g{}^{+}$ states, which is responsible for the observed simulated amplification of a seed pulse, a rotational wave packet in the ground vibrational state ($v=0$) of the excited electronic ${\mathrm{B}}^2\Sigma _u{}^{+}$ state has been created in $\mathrm{N}_2{}^{+}$. 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, Yao et al. demonstrated the creation of coherent emissions in nitrogen gas with two-color (800 nm + 400 nm) ultrafast laser pulses [J. Yao, G. Li, C. Jing, B. Zeng, W. Chu, J. Ni, H. Zhang, H. Xie, C. Zhang, H. Li, H. Xu, S. L. Chin, Y. Cheng, and Z. Xu, New J. Phys. 15, 023046 (2013)]. Based on this two-color scheme, here we report on systematic investigation of temporal characteristics of the radiation emitted at 391 nm [N${}_2{}^{+}$: $B{}^2{\Sigma }_u^{+}\left(\nu =0\right)-X{}^2{\Sigma }_g^{+}\left(\nu =0\right)$] by experimentally examining its temporal profiles 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 cooperation of an ensemble of excited N${}_2$ ${}^{+}$ molecules that are coherently radiating in phase. Our findings shed more light on the mechanisms behind the coherent laserlike emissions induced by strong-field ionization of molecules.

We show that the intensity of self-seeded ${\mathrm{N}}_2^{+}$ lasing at 391 nm, assigned to the $B^2{\mathrm{\Sigma }}_u^{+}(v^{\prime }=0)\to X^2{\mathrm{\Sigma }}_g^{+}(v^{\prime \prime }=0)$ emission, is enhanced by 2 orders of magnitude by modulating in time the polarization of an intense ultrashort near-IR (40 fs, 800 nm) laser pulse with which ${\mathrm{N}}_2$ is irradiated. We find that this dramatic enhancement of the 391 nm lasing is sensitive to the temporal variation of the polarization state within the laser pulse while the intensity of the spontaneous fluorescence emission at 391 nm is kept constant when the polarization state varies. We conclude that a postionization multiple-state coupling, through which the population can be transferred from the $X^2{\mathrm{\Sigma }}_g^{+}$ state of ${\mathrm{N}}_2^{+}$ to the first electronically excited $A^2{\mathrm{\Pi }}_u$ state, leads to the depletion of the population in the $X^2{\mathrm{\Sigma }}_g^{+}$ state, and consequently, to the population inversion between the $X^2{\mathrm{\Sigma }}_g^{+}$ state and the $B^2{\mathrm{\Sigma }}_u^{+}$ state.

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 B²Σ_u⁺ and X²Σ_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.