Transient Optical Nonlinearities Engendered by Femtosecond Laser Filamentation in Gases

在强、超短激光脉冲中，电场大小可以达到并超过特征原子值，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提供了一个机会来直接控制系统中的电子，并使系统参与到具有持久结果的高度非线性过程中。这个研究项目研究了在激光脉冲中由强场电离释放的电子变得活跃地与邻近原子相互作用的过渡机制。该项目将集中在可控制的灯丝尾迹通道中的瞬时非线性光学；预期的结果将与当前和即将进行的一些实验活动相联系。这项研究将促进对强烈激光-物质相互作用的了解，这是物理、化学和相关控制界非常感兴趣的一个主题。它还具有许多技术应用，例如在大气层中进行远程激光，以及开发阿秒和X射线脉冲的新来源。坦普尔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，包括代表不足的群体的广泛参与。这些学生在飞秒激光系统的高科技领域和相关的大容量并行计算领域接受培训。该项目旨在探索和利用在灯丝尾迹通道中创造和控制介质的瞬时交替状态的物理机制，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，包括高次谐波产生，点火器-加热器过程，以及通道的巨大拉比边带发射。这项任务将使用单个离子的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，以及探测激光与该介质耦合的密度矩阵计算的组合来解决。其目标包括：(I)发展在相对稠密的气体介质中的细丝通道形成的预测性描述，该描述由中性粒子上的反向韧致辐射、碰撞电离和碰撞激发的竞争过程驱动，以探索所产生的激发系统的脉冲形状控制；(Ii)跟踪细丝尾迹通道中电子自由度的演变，包括具有有限电离/激发栅格的结构化通道，其由参与碰撞过程的热化电子驱动，受Penning电离的影响，以及在分子气体的情况下受解离复合和振动激发的驱动；预期的输出是演变的离子密度分布和分子/原子激发的时空模式；(3)利用辅助场方法和从头计算方法，计算离子在激光脉冲作用下(当微扰方法适用时)的动态极化率和超极化率系数，得到丝状尾迹通道和电离栅中动态二次和四次非线性折射率的演化；以及(Iv)研究探测脉冲与尾迹通道的标志性非线性相互作用，并预测由瞬时非绝热电荷重新分配机制引起的分子旋转复苏模式，离子旋转复苏的频域映射模式，以及来自结构化通道的动态Rabi边带发射的可控空间频谱模式。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.