超快强光场诱导的背向空气激光产生与远程分子探测研究

During the propagation of intense laser pulses in atmosphere, air molecules can be excited to establish population inversion, which in turn initiates lasing actions. Air laser, as a coherent light source, shows promising applications in nonlinear spectroscopy and remote sensing due to its unique characteristics of low divergence, narrow bandwidth, long-distance propagation and the ability of remote manipulation. However, the practical application of air laser is mainly hampered by generation of the backward air laser in few species of atmospheric gain media. In the previous studies, it is found that the generation of backward air laser strongly depends on dynamics of population inversion. In this proposal, we will attempt to improve the gain coefficient, extend the lifetime of population inversion, and eliminate self-seeding effects in the forward direction by adopting suitable gain media and pumping methods, and finally realize the backward operation of high-brightness, remote air laser in atmosphere. Furthermore, we will combine the air laser with nonlinear spectroscopy techniques to enhance signal-to-noise ratio and sensitivity in remote sensing of molecules, based on unique characteristics of high coherence, low divergence and remote propagation for air laser and the ability of fingerprint identification for several nonlinear spectroscopy techniques.

强激光脉冲在大气中自由传输可以激发空气分子形成粒子数反转，从而产生“空气激光”。空气激光作为一种相干光源，具有指向性好、单色性高、传输距离远、可远程操控等优点，因此在非线性光谱、远程探测领域具有诱人的应用前景。然而，在有限的大气增益介质中实现空气激光的背向运转一直是制约其用于远程探测的主要技术瓶颈，而产生背向空气激光的关键在于调控粒子数反转动力学。本项目拟采用合适的增益介质和泵浦技术，有效提高增益系数、延长粒子数反转寿命、消除前向种子效应的影响，在大气环境中产生高亮度、远距离传输、背向运转的空气激光。进一步，将空气激光技术与非线性光谱技术结合，利用空气激光相干性高、指向性好、传输距离远的优点和某些非线性光谱技术的指纹识别能力，有效提高远程分子探测的信噪比和灵敏度。

空气激光是以空气为增益介质产生的相干辐射，具有高准直度、高相干性、高强度以及自由空间传输等优点，因此为远程探测提供了全新的技术途径。本项目拟利用空气激光独特的光学特性，并结合超强超短激光脉冲远距离、无衍射传输的优势，开展远程分子探测研究，提升远程探测的信噪比和灵敏度。围绕这一研究主题，本项目开展了一系列实验和理论研究，内容包括：探索提升空气激光强度的方法以及产生高亮度背向空气激光的途径；揭示强场电离诱导的空气激光的物理机制；探索远程分子探测技术；研究强场诱导的共振非线性效应。取得的关键研究结果可归结为以下几点。一，利用双波长共振激发方案，将氮气离子激光强度提高了三个数量级，并揭示了氮气离子振动布居对粒子数反转和光学增益的影响。二，提出了三态耦合模型，阐明了氮气离子激光的粒子数反转机制，并通过研究氮气离子激光强度与双色圆偏振光场旋向的关系，表明电子再碰撞不是主要的增益机制。三，发展了基于空气激光的转动拉曼散射技术，在自由空间条件下产生了近60阶的级联拉曼散射，并利用共振效应极大地增强了振动拉曼散射，这些研究为远程分子探测提供了可行的技术途径。四，在强场共振条件下，观测到分子离子的超强非线性效应。通过本项目的研究，我们对空气激光所涉及的基础科学问题和关键技术有了更为清晰、全面地认识，为开展后续研究奠定了基础。