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Physical Review X

Rotational Coherence Encoded in an “Air-Laser” Spectrum of Nitrogen Molecular Ions in an Intense Laser Field

Haisu Zhang, Chenrui Jing, Jinping Yao, Guihua Li, Bin Zeng, Wei Chu, Jielei Ni, Hongqiang Xie, Huailiang Xu, S. L. Chin, Kaoru Yamanouchi, Ya Cheng, Zhizhan Xu

Published Oct 29, 2013• 101 citations
Publisher

Abstract

When exposed to intense infrared laser pulses of femtosecond duration, molecules such as N${}_{2}$, CO${}_{2}$, and H${}_{2}$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 N${}_{2}$ gas and uncover both quantum coherence in the rotational wave packets of the lasing molecular ions and its footprint in the air-laser spectrum.

DOI
10.1103/physrevx.3.041009

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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.
A cavity-free laser in the sky could lead to revolutionary improvements in optical remote sensing for atmospheric science. Abundant in air, nitrogen molecules are prime candidates as an active medium for such a laser. Nitrogen molecules, either neutral or singly ionized, exhibit a significant optical gain in the UV regime when pumped by an intense femtosecond laser. Despite numerous works, the physical origin of this lasing is still not understood. Here we offer a consistent interpretation of all experimental facts by introducing a concept drawn from the field of quantum optics: transient lasing without inversion. Our experiments and simulations identify the crucial role of macroscopic coherence, instead of population inversion, in the nitrogen ions lasing system. Manipulating the macroscopic coherence provides a new level of control over nitrogen air lasing and offers new perspectives for the generation of very short wavelength laser where population inversion is difficult to achieve.
In standard lasers, light amplification requires population inversion between an upper and a lower state to break the reciprocity between absorption and stimulated emission. However, in a medium prepared in a specific superposition state, quantum interference may fully suppress absorption while leaving stimulated emission intact, opening the possibility of lasing without inversion. Here we show that lasing without inversion arises naturally during propagation of intense femtosecond laser pulses in air. It is triggered by the combination of molecular ionization and molecular alignment, both unavoidable in intense light fields. The effect could enable inversionless amplification of broadband radiation in many molecular gases, opening unusual opportunities for remote sensing. © 2020 Optical Society of America
In standard lasers, light amplification requires population inversion between an upper and a lower state to break the reciprocity between absorption and stimulated emission. However, in a medium prepared in a specific superposition state, quantum interference may fully suppress absorption while leaving stimulated emission intact, opening the possibility of lasing without inversion. Here we show that lasing without inversion arises naturally during propagation of intense femtosecond laser pulses in air. It is triggered by the combination of molecular ionization and molecular alignment, both unavoidable in intense light fields. The effect could enable inversionless amplification of broadband radiation in many molecular gases, opening unusual opportunities for remote sensing.
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.