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Applied Physics Letters

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

Rostyslav Danylo, Xiang Zhang, A. Lisova, Aurélien Houard, Yi Liu, Vladimir T Tikhonchuk, A. Mysyrowicz

Published Mar 13, 2023• 1 citation
Publisher

Abstract

Lasing of N2+ 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 B2Σu+(0) and X2Σg+(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.

DOI
10.1063/5.0132475

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Lasing of N2+ 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 B2Σu+(0) and X2Σg+(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.
Cavity-free lasing of N 2 + induced by a femtosecond laser pulse at 800 nm is nearly totally suppressed by a delayed twin control pulse. We explain this surprising effect within the V-scheme of lasing without population inversion. A fast transfer of population between nitrogen ionic states X 2 Σ g + and A 2 Π u , induced by the second pulse, terminates the conditions for amplification in the system. The appearance of short lasing bursts at delays corresponding to revivals of rotational wave packets is explained along the same lines.
Coherent control of the multiple wavelength lasing of N2+{\rm N}_2^ +N2+​: coherence transfer and beyond
Nitrogen molecules pumped by intense femtosecond laser pulses give rise to coherent emission in the forward direction at a series of wavelengths, coined “air lasing.” We demonstrate the coherent control of these emissions via a pair of seeding pulses at two different frequencies at low pressures, revealing a coherence transfer through vibrational motion. It is found that the injection of a 427.8 nm (391.4 nm) seeding pulse results in its amplification at the expense of the 391.4 nm (427.8 nm) signal, demonstrating a competition between the two spectral components of the emission from the upper level population. Moreover, when the delay between the seeding and pump pulses is finely tuned, the coherent control of both transitions is observed via the coherence transfer. A microscopic molecular relaxation model reproduces these observations, highlighting the crucial role of electronic and vibrational coherences, as well as their coupling, during the lasing action.
We investigate the pump wavelength dependence of population inversion in a nitrogen ion when nitrogen gas is irradiated by a femtosecond laser pulse in the wavelength range from ultraviolet to near-infrared. It is found that the population inversions corresponding to the transitions at 391 and 428 nm are very sensitive to the laser wavelength, with extreme values near 400 and 1000 nm. For the ultraviolet pump, the population inversions for 391 and 428 nm transitions occur at different wavelengths: a maximum inversion for the first corresponds to a minimum for the second. This is attributed to the near-resonance single-photon transition combined with the Raman process. A significantly stronger inversion is observed for the near-infrared laser, with the maximum depending on the laser intensity, ascribed to the ac Stark effect. The Floquet theory confirms this conclusion and reveals that the three-photon transition assisted by the strong laser field leads to this enhanced population inversion. This study provides the optimal conditions for air lasing based on ionic transitions of nitrogen molecules.
We experimentally demonstrate the existence of long-lived coherent polarizations coupling simultaneously ground state X (X2Σg+) to excited states A (A2Πu) and B (B2Σu+) of N2+ inside a plasma created by a short intense laser pulse at 800 nm. This three-level V-scheme arrangement is responsible for a strong optical gain without population inversion at the B–X transition at 391.4 nm. Simulations based on Maxwell-Bloch equations reproduce well the kinetics and the pressure dependence of the gain.
Multiple gain routes complicate the amplification behaviors of N2+ lasing. A direct comparison of the amplification processes of various lasing lines of N2+ is still lacking to date, mainly because the efficient generation of different lasing lines requires different experimental conditions. In this work, to overcome the limitation, we use an intense polarization-modulated femtosecond laser pulse to simultaneously produce high-intensity N2+ lasing signals at 391 nm and 428 nm, permitting us readily performing their time-domain characterizations. Our results show that the primary amplification of 428-nm lasing is always retarded by a few picoseconds with respect to the probe pulse while the amplification of 391-nm lasing can be basically completed within the probe pulse duration at a relatively high gas pressure, and besides, the time duration of the former sustains several times longer than that of the latter. These observations indicate the non-negligible contribution of the initial electronic coherence established by the pump laser, apart from the external triggering pulse. For the 391-nm lasing, the seed amplification and superradiance can fiercely interplay within the probe pulse duration. However, for the 428-nm lasing, its amplification is in essence Raman-assisted superfluorescence due to the lacking of the initial electronic coherence and a direct triggering pulse. Our findings not only shed light on the physics of N2+ lasing but also promote the relevant studies on the collective emissions in the cascaded multi-level molecular ion system.
We report a combined theoretical and experimental investigation on the unambiguous role of electronic coherence underlying the generation of laserlike coherent emission from N2+{{\mathrm{N}}_{2}}^{+}N2​+ ions launched in a strong laser field. The coherence manifests itself by giving rise to a dramatic enhancement of N2+{{\mathrm{N}}_{2}}^{+}N2​+ laserlike radiation with two temporally separated pump laser pulses centered at 800- and 1580-nm wavelengths, respectively. We further show that coherent control of N2+{{\mathrm{N}}_{2}}^{+}N2​+ lasing in a subfemtosecond timescale can be achieved by replacing the single 800-nm laser pulse with a pair of 800-nm laser pulses of adjustable time delay. The experimental results are well reproduced in theory using a three-level quantum model of N2+{{\mathrm{N}}_{2}}^{+}N2​+ ions under the density matrix representation.