Transient Optical Nonlinearities Engendered by Femtosecond Laser Filamentation in Gases

气体中飞秒激光丝产生的瞬态光学非线性

• 批准号: 2309247
• 负责人: Dmitri Romanov
• 金额: $ 20万
• 依托单位: Temple University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2023
• 资助国家: 美国
• 起止时间: 2023-08-01 至 2026-07-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2309247&HistoricalAwards=false
• 关键词: Transient; Optical; Nonlinearities; Engendered; Femtosecond

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.

在强烈的超短激光脉冲中，电场强度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直接控制系统中电子的机会，并使系统参与高度非线性过程并产生持久的结果。该研究项目研究了激光脉冲期间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。该项目将集中于可控灯丝尾流通道中的瞬态非线性光学；预期结果将与一些当前和即将进行的实验活动联系起来。这项研究将增进对强激光与物质相互作用的了解，这是物理、化学和相干控制界非常感兴趣的一个话题。它还具有许多技术应用，例如大气中的远程激光发射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，其中包括代表性不足群体的广泛参与。这些学生接受飞秒激光系统高科技领域和大容量并行计算相关领域的培训。该项目旨在探索和利用在灯丝尾流通道中创建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。延迟非线性效应在许多最近的测量中都很明显，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来解决。目标包括：（i）在相对致密的气体介质中，由逆轫致辐射、碰撞电离和碰撞激发的竞争过程驱动，开发对细丝通道形成的预测描述，以探索由此产生的脉冲形状控制兴奋系统； (ii) 追踪灯丝尾流通道中电子自由度的演化，包括具有有限电离/激发光栅的结构化通道，由参与碰撞过程的热化电子驱动，并受到潘宁电离以及解离重组和振动激发的影响分子气体的情况；预期输出是不断演变的离子密度分布和分子/原子激发的时空模式； (iii) 通过实施辅助场方法和从头计算来计算激光脉冲尾迹中离子的动态极化率和超极化系数（当微扰方法适用时），以获得灯丝尾迹中不断变化的动态二次和四次非线性折射率通道和电离光栅； (iv) 研究探针脉冲与尾流通道的标志性非线性相互作用，并预测通过瞬态非绝热电荷重新分布机制引起的分子旋转复兴模式、离子旋转复兴的频域映射模式以及可控空间光谱模式该奖项反映了 NSF 的法定使命，并通过使用基金会的智力优点和更广泛的影响审查标准进行评估，被认为值得支持。