Transient Optical Nonlinearities Engendered by Femtosecond Laser Filamentation in Gases

在强、超短的激光脉冲中，电场大小可以达到或超过特征原子值，从而在脉冲尾迹中产生部分电离的非平衡等离子体。这种脉冲与气体介质的相互作用为直接控制系统中的电子提供了机会，并使系统参与具有持久结果的高度非线性过程。本研究项目研究的是在激光脉冲期间由强场电离释放的电子与邻近原子积极相互作用的过渡状态。该项目将集中研究可控灯丝尾流通道中的瞬态非线性光学；预期的结果将与若干目前和即将进行的实验活动联系起来。该研究将推进强激光物质相互作用的知识，这是物理学、化学和相干控制社区相当感兴趣的话题。它也有许多技术应用，如大气中的远程激光，以及开发新的阿秒和x射线脉冲源。天普大学高级光子学研究中心（CAPR）的研究活动吸引了大量研究生和本科生，包括广泛参与代表性不足的群体。这些学生接受飞秒激光系统的高技术领域和大量并行计算相关领域的培训。该项目旨在探索和利用在细丝尾流通道中产生和控制介质瞬态替代状态的物理机制，从而确定其非线性光学表现。延迟非线性效应在最近的许多测量中都很明显，包括高次谐波的产生，点火器-加热器过程，以及来自通道的巨大拉比边带发射。该任务将结合单个离子非线性响应的从头计算、非均匀激发介质演化的动力学描述和探测激光与该介质耦合的密度矩阵计算来解决。目标包括：(i)在相对致密的气体介质中，由反轫致辐射、冲击电离和碰撞激发等竞争过程驱动的灯丝通道形成的预测描述，以探索由此产生的激发系统的脉冲形状控制；（ii）追踪细丝尾迹通道中电子自由度的演变，包括具有有限电离/激发光栅的结构化通道，这些通道由参与碰撞过程的热化电子驱动，并受到Penning电离的影响，在分子气体的情况下也受到解离重组和振动激发的影响；预期的输出是不断变化的离子密度分布和分子/原子激发的时空模式；（iii）采用辅助场法和从头计算，计算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），得到灯丝尾迹通道和电离光栅中动态二次和四次非线性折射率的演化；（iv）研究探针脉冲与尾迹通道的标志性非线性相互作用，并预测由瞬态非绝热电荷重分配机制诱导的分子旋转恢复模式、离子旋转恢复的频域映射模式和结构通道动态拉比边带发射的可控空间光谱模式。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。

In an intense, ultrashort laser pulse, the electric field magnitude can reach and exceed characteristic atomic values to produce a partially ionized, nonequilibrium plasma in the pulse wake. Interaction of such pulses with gas media offers an opportunity to take direct control of electrons in the system, and to engage the system in highly nonlinear processes with lasting outcomes. This research project addresses the transitional regime in which the electrons released by the strong-field ionization during the laser pulse become actively interacting with neighbor atoms. The project will concentrate on transient nonlinear optics in the controllable filament-wake channels; the expected results will be linked to a number of current and upcoming experimental activities. The research will advance knowledge of intense laser-matter interactions, a topic of considerable interest for physics, chemistry, and coherent control communities. It also has many technological applications, such as remote lasing in the atmosphere, and developing new sources of attosecond and X-ray pulses. The research activity at the Center for Advanced Photonics Research (CAPR) at Temple University attracts a large number of students at graduate and undergraduate levels, including broad participation of underrepresented groups. These students receive training in high-technology areas of femtosecond laser systems and the related fields of high-volume parallel computations. The project aims at exploring and harnessing the physical mechanisms that create and control a transient alternative state of medium in a filament wake channel and thus determine its nonlinear-optical manifestations. Delayed nonlinear effects are manifest in many recent measurements, including higher harmonics generation, igniter-heater processes, and giant Rabi sideband emission from the channels. The task will be addressed using a combination of ab initio calculations for the nonlinear response of individual ions, kinetic description of the evolution of the inhomogeneous excited medium, and density matrix calculations for probe laser coupling with this medium. The objectives include: (i) developing a predictive description of filament channel formation in a relatively dense gas medium, as driven by the competing processes of inverse Bremsstrahlung on neutrals, impact ionization, and collisional excitation, toward exploring pulse-shape control of the resulting excited system; (ii) tracing evolution of electronic degrees of freedom in the filament wake channels, including structured channels with finite ionization/excitation gratings, as driven by thermalized electrons engaged in collisional processes and affected by Penning ionization and also by dissociative recombination and vibrational excitation in the case of molecular gases; the expected output being the spatio-temporal patterns of the evolving ion density profiles and molecular/atomic excitation; (iii) calculating dynamic polarizability and hyperpolarizability coefficients of ions in the wake of the laser pulse (when perturbative approaches become applicable) by implementing the auxiliary-field approach and ab initio calculations to obtain the evolving dynamic quadratic and quartic nonlinear refractive indices in filament wake channels and ionization gratings; and (iv) investigating hallmark nonlinear interactions of probe pulses with the wake channels and predicting the patterns of molecular rotational revival induced via the transient nonadiabatic charge redistribution mechanism, the patterns of frequency-domain mapping of ionic rotational revivals, and controllable spatial-spectral patterns of dynamic Rabi sideband emission from structured 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.