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Proceedings of the National Academy of Sciences

Coherence brightened laser source for atmospheric remote sensing

Andrew J. Traverso, Rodrigo Sánchez-González, Luqi Yuan, Kai Wang, Alexander A. Voronin, Aleksei M. Zheltikov, Yuri V. Rostovtsev, Vladimir A. Sautenkov, Alexei V. Sokolov, Simon W. North, Marlan O. Scully

Published Sep 4, 2012• 73 citations
Publisher

Abstract

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.

DOI
10.1073/pnas.1211481109

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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.
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.
This work is focused on quantum coherence and superradiant emission in a dense pencil-like multi-level medium where many novel effects appear such as transient lasing without inversion, coherence-brightened sky laser, and quantum amplification by superradiant emission of radiation. \n\nWe start from an interesting cascade model where quantum coherence effects can lead to surprising phenomena, gain without population inversion and gain suppression, under different parameters. We further show superradiant emission inside helium plasma. The population evolution shows the decay is significantly faster than collisional decoherence and spontaneous decay rates. This indicates superradiant coherent behavior of the atomic system inside the plasma.\n\nBased on these results, we demonstrate lasing without inversion on a time scale shorter than the decoherence time. The possibility of transient lasing without inversion holds promise for lasing in the extreme-ultraviolet/x-ray regime. We propose experiments to demonstrate this in helium or helium-like ions plasma. \n\nWe also study coherent emission from ambient air and demonstrate efficient generation of laser-like beams directed both forward and backward with respect to a nanosecond ultraviolet pumping laser beam. The emission process exhibits nonadiabatic quantum coherence, which is similar in nature to Dicke superradiance. This coherence-brightened backward light source in air provides possibility for atmospheric remote sensing through a phase-matched coherent Raman scattering process.\n\nFinally, we present a new kind of quantum amplifier, the QASER (quantum amplification by superradiant emission of radiation), based on collective superradiant emission which does not require initial population in the excited state. We show that parametric resonance between the driving field and collective superradiant oscillations of the atomic polarization can yield light amplification at high frequencies. The resulting superradiant amplifier is many orders of magnitude more efficient than nonlinear multiphoton excitation and holds promise as a new way to generate high-frequency coherent radiation.
New approaches for generating coherent light are of keen interest to the scientific community due to the increasing demand for unique sources tailored to specific applications. In fields like microscopy, remote sensing, and soft X-ray/XUV lasers this interest is especially pronounced as novel techniques would have a profound impact. Here, we present new methods for generating coherent light by inducing collective coherence in the gain medium. In particular, we demonstrate coherent emission produced through four-wave mixing on a dressed state, stimulated Raman by inducing coherence between two vibrational levels via 2-photon coupling, and finally, a remote atmospheric coherent source via strong-oscillatory superfluorescence in atomic oxygen. Four-wave mixing on a dressed state could be used to generate XUV coherent radiation or remote atmospheric sensing, as it produces both a forward and backward propagating field. We observe this in Rb vapor with emission occurring at detunings of up to 30 nm from the transition. Similarly, 2-photon enhanced stimulated Raman could be implemented for XUV generation as well, due to the asymmetric gain observed on the anti-Stokes emission relative to the Stokes in Methane. It is also a great platform for microscopy as it is background-free unlike other conventional methods like CARS and CSRS. Atmospheric lasing via oxygen is a prime candidate for nextgeneration remote sensing techniques, and here we study its emission mechanisms in depth demonstrating that superfluorescence serves as the main driving process for the observed 845 nm generation. This intense strong-oscillatory emission lends itself for use in coherent Raman techniques. Furthermore, a dual Raman-Brillouin microscope is also presented and demonstrated, which could prove to be a powerful new tool in biomedical and material characterizations applications.
Abstract form only given. The continuous monitoring of the atmosphere for traces of gases and pathogens at kilometer-scale distances is an important and challenging problem. We have demonstrated that by tailored pairs of pulses of different colors (e.g., red and blue) it is possible to invert an ensemble of molecules such that lasing and/or gain-swept superradiance is realized in a direction toward the observer. In recent experiments, we have observed strong backward lasing of oxygen-ionized molecules in air, thus enabling a transient gain for an optical field propagating toward the observer. This technique results in the generation of a strong, coherent, counter-propagating optical probe pulse. Such a probe, combined with a wavelength-tunable laser signal(s) propagating in the forward direction, provides a tool for various remote-sensing applications. The proposed technique can be enhanced by combining it with the gain-swept excitation approach as well as with beam shaping and adaptive optical techniques.