Among currently available optical spectroscopic methods, Raman spectroscopy has versatile application to investigation of dynamical processes of molecules leading to chemical changes in the gas and liquid phases. However, it is still a challenge to realize an ideal standoff coherent Raman spectrometer with which both high temporal resolution and high-frequency resolution can be achieved, so that one can remotely probe chemical species in real time with high temporal resolution while monitoring the populations in their respective rovibronic levels in the frequency domain with sufficiently high spectral resolution. In the present study, we construct an air-laser-based Raman spectrometer, in which near-infrared femtosecond (fs) laser pulses at 800 nm and cavity-free picosecond N2+ air-laser pulses at 391 nm generated by the filamentation induced by the fs laser pulses are simultaneously used, enabling us to generate a hybrid ps/fs laser source at a desired standoff position for standoff surveillance of chemical and biochemical species. With this prototype Raman spectrometer, we demonstrate that the temporal evolution of the electronic, vibrational, and rotational states of N2+ and the coupling processes of the rovibrational wave packet of N2 molecules can be probed.

https://downloads.spj.sciencemag.org/ultrafastscience/2022/9867028.pdf

Air-Laser-Based Standoff Coherent Raman Spectrometer

We experimentally demonstrate an externally seeded ${\mathrm{N}}_{2}{}^{+}$ lasing action on the transition between the $B{\phantom{\rule{4pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{u}{}^{+}(v=0)$ and $X{\phantom{\rule{4pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}{}^{+}({v}^{\ensuremath{'}}=0)$ states at 391.4 nm by irradiating a ${\mathrm{N}}_{2}$ gas with an intense 400-nm pump laser pulse and reveal that the populations in the rotational levels of ${J}^{\ensuremath{'}}=7--17$ in the $B{\phantom{\rule{4pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{u}{}^{+}(v=0)$ state are responsible for the lasing based on the rotational revival structure in the lasing intensity recorded by the pump-probe measurements. By introducing additionally an 800-nm control pulse, we find that the lasing intensity is suppressed when the timing of the control pulse is set between the 400-nm pump and 400-nm seed laser pulses while it can be enhanced when the control pulse overlaps temporally the seed pulse. By solving the time-dependent Schr\"odinger equation including the continuous ionization of ${\mathrm{N}}_{2}$ and multistate coupling among the ${B}^{\phantom{\rule{4pt}{0ex}}2}{\mathrm{\ensuremath{\Sigma}}}_{u}{}^{+}$, ${A}^{\phantom{\rule{4pt}{0ex}}2}{\mathrm{\ensuremath{\Pi}}}_{u}$, and $X{\phantom{\rule{4pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}{}^{+}$ states in ${\mathrm{N}}_{2}{}^{+}$ induced by the control pulse, we show that the population inversion in ${\mathrm{N}}_{2}{}^{+}$ can be achieved by the 400-nm pump laser pulse and can be further modulated by the control pulse. We show also that the ${\mathrm{N}}_{2}{}^{+}$ lasing intensity can be suppressed or enhanced depending on the timing and intensity of the control pulse originating from the competition between the two dynamical processes, that is, the ionization of ${\mathrm{N}}_{2}$ and the population transfer among the three electronic states of ${\mathrm{N}}_{2}{}^{+}$. 

Modulation of population inversion in 
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 by a pump-control-seed scheme

The processes leading to the N2 + lasing are rather complex and even the population distribution after the pump laser excitation is unknown. In this paper, we study the population distribution at electronic and vibrational levels in N2 + driven by ultra-short laser pulse at the wavelengths of 800 nm and 400 nm by using the quantum-mechanical time-domain incoherent superposition model based on the time-dependent Schrödinger equation and the quasi-classical model assuming instantaneous ionization injection described by density matrix. It is shown that while both models provide qualitatively similar results, the quasi-classical instantaneous ionization injection model underestimates the population inversions corresponding to the optical transitions at 391 nm, 423 nm and 428 nm due to the assumption of quantum mixed states at the ionization time. A fast and accurate correction to this error is proposed. This work solidifies the theoretical models for population at vibrational states in N2 + and paves the way to uncover the mechanism of the N2 + lasing.

Quantum and quasi-classical effects in the strong field ionization and subsequent excitation of nitrogen molecules.

Lasing of [Formula: see text] at 428 nm is studied as a function of the delay dt between two 800 nm ultrashort pumping pulses from a Ti:sapphire laser. This lasing corresponds to a population-inverted transition between levels [Formula: see text](0) and [Formula: see text](1) of a singly ionized nitrogen molecule. An important erasing of laser emission is observed under pumping with two consecutive pulses separated by large delays −20 ps < dt < +20 ps. From an analysis of the quenching results, we conclude that the mechanism of lasing corresponds to a coherent two-photon amplification scheme. Our results show that a two-photon process can dominate over a population-inverted one-photon amplification.

Laser emission at 428 nm in 
N2+: Competition between two- and three-level amplification

Air lasing induced by laser filamentation opens a new route for research on atmospheric molecular physics and remote sensing. The generation of air lasing is composed of two processes, i.e., building up optical gain of air molecules in femtosecond time scale and emitting coherent radiation in picosecond time scale. Here, we focus on the emission mechanisms of N2 + air lasing and reveal, by examining the intensities and temporal profiles of N2 + lasing at 391 nm generated respectively in a time-varying polarization-modulated and a linearly polarized pump laser field under different nitrogen gas pressures, that the N2 + lasing can emit through either triggered super-radiance or seed amplification. We find that the two pressure-sensitive factors, i.e., the dipole dephasing time T2 and the population inversion density n, determine which of these two mechanisms dominates the N2 + lasing emission process, enabling manipulation of the transition from triggered super-radiance to seed amplification or vice versa. Our findings clarify the emission mechanism of N2 + lasing under different pressures and provide a deeper understanding of N2 + air lasing not only in the establishment of optical gain but also in the lasing emission process.

Transition from triggered super-radiance to seed amplification in N2 + lasing.

Cavity-free air lasing offers a promising route towards the realization of atmospheric lasers for various applications such as remote sensing and standoff spectroscopy; however, achieving efficient generation and control of air lasing in ambient air is still a challenge. Here we show the experimental realization of a giant lasing enhancement by three to four orders of magnitude in ambient air for the self-seeded N2+ lasing at 428 nm, assigned to the B2Σu+(ν'=0) and X2Σg+(ν''=1) emission, by modulating the spatiotemporal overlap of ultrashort near-infrared control-pump pulses in a filamentary plasma grating; meanwhile, the spontaneous emission from the same transition is only enhanced by three to four times. We find that this enhancement is sensitive to the relative polarization and interference time of the two pulses, and reveal that the formation of the plasma grating induces different population variations in the B2Σu+(ν'=0) and X2Σg+(ν''=1) levels, resulting in an enormous population inversion between the two levels, thereby a higher gain for the giant enhancement of N2+ lasing in ambient air.

Extremely enhanced N2+ lasing in a filamentary plasma grating in ambient air.

We investigate lasing of a nitrogen gas induced by intense femtosecond laser pulses at around 400 nm. By examining both the self-induced and externally seeded forward emission spectra, we unambiguously identify the ${{\mathrm{N}}_{2}}^{+}$ lasing actions at 427.8 and 423.6 nm assigned respectively to the $B\phantom{\rule{0.16em}{0ex}}{}^{2}\mathrm{\ensuremath{\Sigma}}{{}_{u}}^{+}\ensuremath{-}X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ (0, 1) and (1, 2) emissions and show that the lasing mechanism is totally different from the lasing induced by near-infrared (800 nm) laser pulses, in which the population transfer from $X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ to $A^{2}\mathrm{\ensuremath{\Pi}}_{u}$ proceeds through the resonant $X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}\ensuremath{-}A^{2}\mathrm{\ensuremath{\Pi}}_{u}$ transition, facilitating the population inversion between the $B\phantom{\rule{0.16em}{0ex}}{}^{2}\mathrm{\ensuremath{\Sigma}}{{}_{u}}^{+}\ensuremath{-}X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ states [Phys. Rev. Lett. 123, 203201 (2019)]. We simulate the population distributions among the vibrational levels in the three lowest electronic states of ${{\mathrm{N}}_{2}}^{+}$ by the 400-nm laser pumping and find that the population is efficiently transferred between the $X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ state and the $B\phantom{\rule{0.16em}{0ex}}{}^{2}\mathrm{\ensuremath{\Sigma}}{{}_{u}}^{+}$ state by a Rabi oscillation combined with a Raman-type transition, leading to the population inversion so that the lasing occurs at the $B\phantom{\rule{0.16em}{0ex}}{}^{2}\mathrm{\ensuremath{\Sigma}}{{}_{u}}^{+}\ensuremath{-}X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{\phantom{\rule{0.28em}{0ex}}+}$ (0, 1) and (1, 2) emissions.

Jincheng Cao

Papers

Extremely enhanced N2+ lasing in a filamentary plasma grating in ambient air.

Population inversion in N 2 + by vibrationally mediated Rabi oscillation at 400 nm