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Laser Physics

Analytical study of the spiky feature in a two-photon driven lossy ladder system

Luojia Wang, Jingping Xu, Luqi Yuan, Yaping Yang

Published Sep 2, 2019• 1 citation
Publisher

Abstract

Abstract There has been an experimental report (Traverso et al 2012 Proc. Natl Acad. Sci. USA 109 15185) that oxygen lasing in ambient air exhibits a spiky feature. We theoretically explore a ladder-type three-level atomic system driven by a two-photon resonant pump pulse in a lossy medium, which corresponds to that oxygen experiment. We study the Maxwell–Bloch equations both numerically and analytically and discover that a long pump field brings competition between two mechanisms of coherent pumping and coherent emission, leading to the spiky emission, which cannot be produced using a short pump. The results indicate that atomic coherence occurs in the experiment. Our study provides a clear understanding of the oxygen experiment using a nanosecond pump laser and could lead to the potential application of this oxygen lasing in future experiments.

DOI
10.1088/1555-6611/ab3c48

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Abstract There has been an experimental report (Traverso et al 2012 Proc. Natl Acad. Sci. USA 109 15185) that oxygen lasing in ambient air exhibits a spiky feature. We theoretically explore a ladder-type three-level atomic system driven by a two-photon resonant pump pulse in a lossy medium, which corresponds to that oxygen experiment. We study the Maxwell–Bloch equations both numerically and analytically and discover that a long pump field brings competition between two mechanisms of coherent pumping and coherent emission, leading to the spiky emission, which cannot be produced using a short pump. The results indicate that atomic coherence occurs in the experiment. Our study provides a clear understanding of the oxygen experiment using a nanosecond pump laser and could lead to the potential application of this oxygen lasing in future experiments.
We present a detailed theoretical study of a recent experiment [A. J. Traverso et al., Proc. Natl. Acad. Sci. USA 109, 15185 (2012)] in which a laserlike source is created in air by pumping with a nanosecond pulse. The source generates radiation in the forward and backward directions. The temporal behavior of the emitted pulses is investigated for different pump shapes and durations. Our analysis indicates that the spiky emission is due to quantum coherence via cooperation between atoms of an ensemble, which leads to strong-oscillatory superfluorescence. We show that these cooperative nonadiabatic coherence effects cannot be described by rate equations and instead a full set of the Maxwell-Bloch equations must be used. We consider a range of parameters and study transitions between various regimes. Understanding these coherence-brightened processes in air should lead to improvements in environmental, atmospheric remote sensing and other applications.
A strong ultraviolet pumping laser propagating through the atmosphere could activate the medium and produce the forward and backward coherent air lasing. In this work, we present the theoretical analyses of forward and backward lasing dynamics in the long gain medium consisting of oxygen atoms. By numerically solving the semiclassical Maxwell-Bloch equations, we demonstrate that the atom density, the gain medium length, and the collisional decoherence rate substantially regulate the forward and backward air laser radiation at 845 nm. Especially, our study points out that lower atom density and moderate collisional decoherence rate are favorable for the backward radiation amplification in a long medium, since such conditions balance the competing processes of forward and backward radiation fields. The simulation shows that long medium backward air lasing becomes challenging in the oxygen atom system via double-photon pump, because the amplification can only occur around z=0z=0z=0, and the amplified backward radiation does not carry the spectral information of atoms throughout the medium. On the other hand, certain mechanisms such as variation of backward-seeding delay could possibly alleviate the suppression of backward amplification.
We have studied coherent emission from ambient air and demonstrated efficient generation of laser-like beams directed both forward and backward with respect to a nanosecond ultraviolet pumping laser beam. The generated optical gain is a result of two-photon photolysis of atmospheric O(2), followed by two-photon excitation of atomic oxygen. We have analyzed the temporal shapes of the emitted pulses and have observed very short duration intensity spikes as well as a large Rabi frequency that corresponds to the emitted field. Our results suggest that the emission process exhibits nonadiabatic atomic coherence, which is similar in nature to Dicke superradiance where atomic coherence is large and can be contrasted with ordinary lasing where atomic coherence is negligible. This atomic coherence in oxygen adds insight to the optical emission physics and holds promise for remote sensing techniques employing nonlinear spectroscopy.
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Two-photon amplification and lasing in laser-driven potassium atoms is investigated theoretically using a rigorous model that takes into account the hyperfine level structure of the atoms for arbitrary intensity drive and lasing fields. The model correctly predicts most features of the recent experiments, such as the characteristics of the multiphoton gain resonances and the two-photon laser emission intensity and line shape. In addition, it constitutes a new tool for exploring behaviors that are difficult to address experimentally, such as interference effects between different quantum pathways. In contrast to single-photon lasers, the laser emission line shape is frequency pushed and has the form of a closed curve composed of a stable and an unstable branch.