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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Standoff spectroscopy via remote generation of backward lasing and/or gain swept superradiance in air

Marlan O. Scully

Published May 1, 2011• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cleoe.2011.5943053

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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.
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.
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.
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.
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.
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.
There is nothing so practical as a good theory. As a case in point, the compelling need for standoff detection of hazardous gases and vapor indicators of explosives has motivated the development of remotely pumped, scheme(s) which produce radiation in the backward direction [1,2]. Moving from conceptualization to theoretical analysis and experimental verification, we demonstrate that high gain can be achieved in air. Backward air lasing provides possibilities for remote detection [3] as will be discussed.