Transient Optical Nonlinearities Engendered by Femtosecond Laser Filamentation in Gases

由强、超短激光脉冲发起和控制的原子和分子过程是等离子体科学和相关技术中一个不断增长的兴趣课题。在等离子体介质中，单个原子和分子的激光诱导电离和激发依次产生各种非线性光学现象。一种这样的现象，激光成丝，是由激光束的自聚焦实现的，这导致激光脉冲的剧烈收缩，并在可控的距离上产生延长的灯丝。部分电离的等离子体在成丝的激光脉冲后留下；它可以被后续的激光脉冲探测，并开放各种实际应用。这一理论研究解决了一个新的领域瞬态非线性光学在不断发展的灯丝尾流通道；它探索了在相对密集的等离子体中发生的强场电离的持久光学后果。该研究将促进对在微观尺度上控制强场光-物质相互作用的方法的更广泛的理解。它还直接解决了基于长丝的复杂混合物远程检测的多重方法的最新发展，这是许多技术应用的挑战。它涉及到在大气中实现远程激光的成丝动力学控制，以及开发新的阿秒和x射线脉冲源。本文研究了飞秒激光尾迹通道中离子、高能自由电子和受激中性的相互作用动力学，以及演化系统与探测激光脉冲的相互作用。预期的结果将为有效控制灯丝-尾迹非线性光学效应奠定基础。该任务将结合单个离子非线性响应的从头计算、非均匀电离气体介质演化的动力学建模以及探针激光与介质耦合的密度矩阵计算来解决。目标包括：1)建立由冲击电离、中性离子和离子的冲击电子激发、解离重组、振动激发和弹性散射等竞争过程驱动的离子束尾流通道和电离光栅中离子密度分布和分子/原子激发演变的预测模型；2)采用辅助场法和从头计算方法计算电离激光脉冲尾迹中离子的动态极化系数和超极化系数（当微扰方法适用时），并应用计算结果得到了灯丝尾迹通道中动态二次和四次非线性折射率的空间演化剖面；3)预测探测激光脉冲与尾迹通道非线性相互作用的结果，包括电离光栅介导的两束耦合、离子旋转恢复和动态拉比边带发射的可控空间光谱模式。预期的结果将应用于坦普尔大学高级光子学研究中心（CAPR）的电离相关极化旋转、旋转恢复光谱和灯丝尾流通道中动态拉比边带的实验。此外，电离特异性非线性效应与最近在高电离气体中产生的明亮高谐波（高达x射线）和灯丝尾流通道中的点火器-加热器过程的实验有关。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Atomic and molecular processes initiated and controlled by intense, ultrashort laser pulses are a subject of steadily growing interest in plasma science and related technology. The laser-induced ionization and excitation of individual atoms and molecules beget in turn various nonlinear optical phenomena in the plasma medium. One such phenomenon, laser filamentation, is enabled by self-focusing of the laser beam, which results in a drastic contraction of the laser pulse and generation of an extended filament at a controllable stand-off distance. Partially ionized plasma is left in the wake of the filamenting laser pulse; it can be probed by subsequent laser pulses and is open to various practical applications. This theoretical research addresses a new realm of transient nonlinear optics in evolving filament-wake channels; it explores longer-lasting optical consequences of strong-field ionization occurring in a relatively dense plasma. The research will advance broader understanding of means to control strong-field light-matter interaction on a microscopic scale. It also directly addresses recent developments of filament-based, multiplex methods of remote detection of complex mixtures, which is a challenge with many technological applications. It is related to control of filamentation dynamics with the aim of remote lasing in the atmosphere, and to developing new sources of attosecond and X-ray pulses.This research concerns the interrelated dynamics of ions, energetic free electrons, and excited neutrals in wake channels of femtosecond laser filaments, as well as the interaction of the evolving system with probe laser pulses. The expected outcomes will form a basis for effective control of filament-wake nonlinear optical effects. The task will be addressed using a combination of ab initio calculations for nonlinear response of individual ions, kinetic modeling of inhomogeneous ionized-gas medium evolution, and density matrix calculations for probe laser coupling with the medium. The objectives include:1) developing predictive models of the evolution of ion density profiles and molecular/atomic excitation in filament wake channels and ionization gratings, which is driven by competing processes of impact ionization, impact electronic excitation of neutrals and ions, dissociative recombination, vibrational excitation, and elastic scattering; 2) calculating dynamic polarizability and hyperpolarizability coefficients of ions in the wake of the ionizing laser pulse (when perturbative approaches become applicable) by implementing the auxiliary-field approach and ab initio calculations, and applying the results to obtain evolving spatial profiles of dynamic quadratic and quartic nonlinear refractive indices in filament wake channels; and3) predicting outcomes of nonlinear interactions of probe laser pulses with the wake channels, including two-beam coupling mediated by ionization gratings, ionic rotational revivals, and controllable spatial-spectral patterns of dynamic Rabi sideband emission. The expected results will be applied to experiments on ionization-related polarization rotation, rotational-revival spectroscopy, and dynamic Rabi sidebands in filament wake channels at the Center for Advanced Photonics Research (CAPR) at Temple University. Further, the ionization-specific nonlinear effects are related to recent experiments on bright higher harmonic generation (up to X-rays) in highly ionized gases and on igniter-heater processes in filament wake channels.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.