Precision Measurements Using Nonlinear Thomson Scattering

高强度激光在基础科学、工业和国防应用中有着广泛的用途。为了达到最高的强度，激光必须聚焦到尽可能小的光斑尺寸。为了实现小焦点，必须测量和优化激光焦点内的电磁场结构。在这个项目中，研究人员将电子放置在一个非常高强度的激光焦点内，然后研究这些电子发出的光（非线性汤姆逊散射），以了解焦点内的场结构。这种非线性汤姆逊散射的空间结构和偏振模式包含了关于激光焦点中的场结构的信息，这些信息很难用其他方式测量。研究人员将研究光发射模式，同时改变激光的焦点特性和焦点内电子的分布，目的是学习如何测量、理解和改善激光焦点。这些都是非常具有挑战性的测量，以前的大多数工作都局限于理论和计算预测。该研究项目将涉及并培训大约12名本科生和3名研究生，他们将用于驱动电子的高强度激光方法和用于测量散射光的单光子检测方法结合在一起。研究人员将把该项目的结果纳入一本免费的在线光学教科书中，该教科书在大学水平上广泛使用。该项目研究激光焦点中非常漫射（近真空密度）的自由电子的非线性汤姆逊散射，并使研究这种基本的相对论现象成为可能，而不受等离子体动力学的混淆影响。这些测量将在参考系中对几个低阶谐波进行，该参考系允许测量光电发射的角结构。研究人员将：（1）首次表征实际上在整个发射球上的各个谐波阶的偏振分辨角发射图案，（2）研究角光电发射图案中电子从焦点的有质动力喷射的特征以及这如何受焦点参数和脉冲持续时间的影响，（3）研究了原子发射的电子之间的相干性如何改变非线性汤姆逊散射的角模式和强度。这些效应中的每一个都有可能对整体非线性汤姆逊散射信号贡献可测量的量。对强激光聚焦下相对论电子轨迹的模拟表明，电子轨迹对场矢量的细节很敏感。总体目标是能够更好地将激光聚焦的矢量场与观测到的非线性汤姆逊散射相关联。该奖项反映了NSF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。

High intensity lasers have a wide range of uses in basic science, industry, and national defense applications. To achieve the highest intensities, a laser must be focused to the smallest spot size possible. To achieve a small focus, the structure of the electromagnetic fields within a laser focus must be measured and optimized. In this project, the researchers place electrons within a very high intensity laser focus, and then study the light these electrons emit (nonlinear Thomson scattering) to understand the field structure within the focus. The patterns in the spatial structure and polarization of this nonlinear Thomson scattering contain information about the structure of the fields in a laser focus that is hard to measure in other ways. The researchers will study light emission patterns while varying focal properties of the laser and the distribution of electrons within the focus, with a goal of learning how to measure, understand, and improve the laser focus. These are very challenging measurements to make, and most previous work has been restricted to theoretical and computational predictions. The research project will involve and train approximately a dozen undergraduate students and three graduate students in the challenging blend of high-intensity laser methods used to drive the electrons and the single-photon detection methods used to measure the scattered light. The researchers will incorporate results from this project into a free online optics textbook that is widely used at the university level.This project studies nonlinear Thomson scattering from very diffuse (near-vacuum density) free electrons in a laser focus and enables the study of this fundamental relativistic phenomenon without the confounding influence of plasma dynamics. These measurements will be carried out for several lower-order harmonics in a reference frame that allows the angular structure of the photoemission to be measured. The researchers will: (1) characterize for the first time the polarization-resolved angular emission patterns of individual harmonic orders over virtually the entire emission sphere, (2) investigate the signature of the ponderomotive ejection of electrons from the focus in the angular photoemission patterns and how this is influenced by focal parameters and pulse duration, and (3) study how coherence between electrons emitted from the same atom modifies nonlinear Thomson scattering, both the angular pattern and the strength. Each of these effects has the potential to contributes measurable amounts to the overall nonlinear Thomson scattering signal. Simulations of relativistic electron trajectories within a tight laser focus show that electron trajectories are sensitive to the details of the field vectors. An overarching goal is to be able to better correlate the vector fields of the laser focus with observed nonlinear Thomson scattering.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.