Transient Optical Nonlinearities Engendered by Femtosecond Laser Filamentation in Gases

由强超短激光脉冲引发和控制的原子和分子过程是等离子体科学和相关技术中不断增长的兴趣的主题。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又引起各种非线性光学现象。一种这样的现象，即激光诱导，通过激光束的自聚焦来实现，这导致激光脉冲的急剧收缩和在可控的间隔距离处产生延伸的细丝。部分电离的等离子体被留在激光脉冲的尾流中，它可以被随后的激光脉冲探测，并对各种实际应用开放。这项理论研究解决了瞬态非线性光学在不断发展的湍流尾流通道的一个新领域，它探讨了较长时间的强场电离发生在一个相对密集的等离子体的光学后果。这项研究将促进对微观尺度上控制强场光-物质相互作用的方法的更广泛理解。它还直接讨论了最近发展的基于双折射的、复杂混合物远程检测的多重方法，这是许多技术应用的一个挑战。这一研究涉及到以大气中的远程激光为目标的发射动力学的控制，以及开发新的阿秒和X射线脉冲源，这一研究涉及到飞秒激光丝尾流通道中离子、高能自由电子和激发中性粒子的相互作用动力学，以及演化系统与探测激光脉冲的相互作用。预期的结果将形成有效控制的非线性光学效应的基础。该任务将解决使用组合的从头计算的非线性响应的单个离子，动力学建模的非均匀电离气体介质演化，和密度矩阵计算探针激光耦合与介质。其目标包括：（1）建立由碰撞电离、中性粒子和离子的碰撞电子激发、离解复合、振动激发和弹性散射等竞争过程驱动的灯丝尾流通道和电离栅中离子密度分布和分子/原子激发演化的预测模型; 2）计算了电离激光脉冲尾流中离子的动态极化率和超极化率系数（当微扰方法适用时）通过实施微扰场方法和从头计算，并应用该结果获得丝状体尾流通道中动态二次和四次非线性折射率的演变空间分布;预测探测激光脉冲与尾流通道的非线性相互作用的结果，包括电离光栅介导的双光束耦合、离子转动回复和动态Rabi边带发射的可控空间光谱模式。预期的结果将应用于实验电离相关的偏振旋转，旋转复兴光谱，并在灯丝尾通道中的动态拉比边带在中心高级光子学研究（CAPR）在坦普尔大学。此外，电离特定的非线性效应与最近在高度电离气体中产生明亮的高次谐波（高达X射线）和灯丝尾流通道中点火器加热器过程的实验有关。该奖项反映了NSF的法定使命，并被认为值得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来支持。

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.