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Chinese Physics B

Air lasing: Phenomena and mechanisms*

Helong Li, Danwen Yao, Siqi Wang, Yao Fu, Huailiang Xu

Published Sep 26, 2019• 18 citations
Publisher

Abstract

Air lasing is a concept that refers to remote no-cavity (mirrorless) optical amplification in ambient air with the air constituents as the gain media. Due to the high potential of air lasing in view of applications in atmospheric sensing, a variety of pumping schemes have been proposed so far for building up population-inverted gain media in air and producing forward and/or backward directional lasing emissions. This review paper presents an overview of recent advances in the experimental observations and physical understanding of air lasing in various pumping schemes of air molecules by intense laser fields. Special emphasis is given to the strong-field-induced N 2 + air lasing, the mechanism of which is currently still in a hot debate.

DOI
10.1088/1674-1056/ab47f5

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Air lasing is a concept that refers to remote no-cavity (mirrorless) optical amplification in ambient air with the air constituents as the gain media. Due to the high potential of air lasing in view of applications in atmospheric sensing, a variety of pumping schemes have been proposed so far for building up population-inverted gain media in air and producing forward and/or backward directional lasing emissions. This review paper presents an overview of recent advances in the experimental observations and physical understanding of air lasing in various pumping schemes of air molecules by intense laser fields. Special emphasis is given to the strong-field-induced N 2 + air lasing, the mechanism of which is currently still in a hot debate.
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.
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 N 2 + air lasing and reveal, by examining the intensities and temporal profiles of N 2 + 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 N 2 + 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 T 2 and the population inversion density n , determine which of these two mechanisms dominates the N 2 + 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 N 2 + lasing under different pressures and provide a deeper understanding of N 2 + air lasing not only in the establishment of optical gain but also in the lasing emission process.
Lasing refers to the generation of highly directional laser-like emission remotely pumped by laser pulses in a medium such as ambient air or a flame.It has been intensively investigated because of its potentials to revolutionize the field of remote sensing.Earlier studies were carried out in flames, where lasing effect has been observed in various radicals, for example H, O, N etc., through two-photon resonant excitation.This scheme was later applied to generate lasing of O and N in ambient air, which can be dramatically improved by predissociation of the air constituents.However, the underlying physical mechanism of two-photon-excited lasing effect is still unsettled although it has been studied for more than one decade.Currently there are three different proposals: (1) Amplified Spontaneous Emission (ASE), in which case population inversion is responsible for optical gain;(2) Superfluorescence (SF), where a macroscopic dipole occurs in the population inverted medium; (3) Both hyper-Raman scattering and four-wave mixing are involved in the generation of lasing emission.Here, we experimentally studied 656 nm lasing emission of atomic hydrogen via two-photon resonant excitation by focusing 125-fs laser pulses of 205-nm wavelength in a methane/oxygen welding flame (see Fig. 1), where hydrogen atoms are naturally present.Lasing occurs in both forward and backward directions, and the forward signal strength is almost one order of magnitude stronger than the backward one.It has been found that the durations of lasing pulses are around 20 ps, and decreases with increasing pump laser energies.Furthermore, the delay of the lasing pulse with respect to the pump pulse decreases with increasing pump energies.These results show high similarities with the behaviors of SF, suggesting that the femtosecond two-photonexcited lasing emission of atomic hydrogen might be SF.
Abstract The research topic of air lasing is of fundamental and practical importance, which has grown rapidly in the past decade. Air lasing, or a laser‐like emission from air species including atomic oxygen, atomic argon, atomic nitrogen, molecular nitrogen, and molecular nitrogen ions, has been investigated vastly in both experiment and theory. In this review, the basic concepts of air lasing are discussed and the crucial role of quantum coherence in the air‐lasing process is highlighted. As an exciting perspective, air lasing not only exhibits important fundamental physics such as superradiance, nonlinear effects, vibrational and rotational dynamics, but also shows potential practical implications toward remote sensing.