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Physical Review A

Comparative study of enhancement and suppression of N2+ lasing induced by coherent molecular rotational wave packets of N2 and N2+

Hongqiang Xie, Shu‐Ting Wu, Yihong Huang, Junda Lv, Jiayi Cai, Chenrui Jing, Zaicheng Xiao, Zhiming Chen, Xinghao Wang, Guihua Li

Published Apr 1, 2024• 1 citation
Publisher

Abstract

${\mathrm{N}}_{2}^{+}$ lasing action involves the creation, evolution, and radiation of ions in intense laser fields. In this paper, we demonstrate that ${\mathrm{N}}_{2}^{+}$ lasing intensity can be enhanced by several times or totally suppressed by controlling the ionic creation and radiation processes through manipulation of the coherent molecular rotational wave packets of ${\mathrm{N}}_{2}$ and ${\mathrm{N}}_{2}^{+}$ with a pump-control scheme. Our results show that if a weak control pulse arrives at the revival moments of the ionic rotational wave packet, the lasing signal can be notably promoted, which is in contrast to observations in previous studies. Meanwhile, the modulation profiles of the lasing signals caused by molecular alignments of ${\mathrm{N}}_{2}$ and ${\mathrm{N}}_{2}^{+}$ are analyzed at length, revealing the double roles of the weak control pulse in inducing molecular alignment of ${\mathrm{N}}_{2}$ or monitoring the amplification of the ${\mathrm{N}}_{2}^{+}$ system by controlling electronic and rotational coherence. Moreover, by performing a Fourier transformation of the delay-dependent ${\mathrm{N}}_{2}^{+}$ lasing, the rotational wave-packet information of ${\mathrm{N}}_{2}$ and ${\mathrm{N}}_{2}^{+}$ is simultaneously retrieved.

DOI
10.1103/physreva.109.043101

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N2+{\mathrm{N}}_{2}^{+}N2+​ lasing action involves the creation, evolution, and radiation of ions in intense laser fields. In this paper, we demonstrate that N2+{\mathrm{N}}_{2}^{+}N2+​ lasing intensity can be enhanced by several times or totally suppressed by controlling the ionic creation and radiation processes through manipulation of the coherent molecular rotational wave packets of N2{\mathrm{N}}_{2}N2​ and N2+{\mathrm{N}}_{2}^{+}N2+​ with a pump-control scheme. Our results show that if a weak control pulse arrives at the revival moments of the ionic rotational wave packet, the lasing signal can be notably promoted, which is in contrast to observations in previous studies. Meanwhile, the modulation profiles of the lasing signals caused by molecular alignments of N2{\mathrm{N}}_{2}N2​ and N2+{\mathrm{N}}_{2}^{+}N2+​ are analyzed at length, revealing the double roles of the weak control pulse in inducing molecular alignment of N2{\mathrm{N}}_{2}N2​ or monitoring the amplification of the N2+{\mathrm{N}}_{2}^{+}N2+​ system by controlling electronic and rotational coherence. Moreover, by performing a Fourier transformation of the delay-dependent N2+{\mathrm{N}}_{2}^{+}N2+​ lasing, the rotational wave-packet information of N2{\mathrm{N}}_{2}N2​ and N2+{\mathrm{N}}_{2}^{+}N2+​ is simultaneously retrieved.
We investigate the formation of rotational coherence of N2+ resonantly interacting with an intense femtosecond laser field by numerical simulations based on a strong-field ionization-coupling model described with the density matrix formalism. The created N2+ system is unique in many aspects: the variable total population within the pump duration due to the intensity-dependent ionization injection, the readily accessible resonance owing to the effect of Stark shift, and the involvement of a few dozen of quantum states. By regarding the N2+ system as an open and non-stationary Λ-type cascaded multi-level system, we quantitatively studied the dependence of rotational coherence in different electronic-vibrational states of N2+ on the alignment angle θ and the pumping intensity. Our simulation results indicate that the quantum coherence between the neighbouring rotational states of J, J+2 in the vibrational state ν=0, 1 of the ground state of N2+ can be changed from a negative to a positive. The significant contribution of rotational coherence to inducing an extra gain or absorption of N2+ air lasing is further verified by solving the Maxwell's propagating equation. The finding provides crucial clues on how to manipulate N2+ lasing by controlling the rotational coherence and paves the way to studying strong-field quantum optics effects such as lasing without inversion and electromagnetically induced transparency in molecular ionic systems.
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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.
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.
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When exposed to intense infrared laser pulses of femtosecond duration, molecules such as N2{}_{2}2​, CO2{}_{2}2​, and H2{}_{2}2​O not only become ionized but can also produce lasers as a result of simultaneous population inversion. Scientists look deep into this ``air lasing'' phenomenon in N2{}_{2}2​ gas and uncover both quantum coherence in the rotational wave packets of the lasing molecular ions and its footprint in the air-laser spectrum.