Polarization-Resolved Single-Photon Measurements of Nonlinear Thomson Scattering

激光技术的进步使得在激光束聚焦的微小区域内产生异常明亮的光成为可能。本项目将开辟一种新的方法来研究这种激光聚焦附近的光场。这种极端光线下的电场和磁场可以将电子从任何物质中撕裂出来，然后以超过千万亿g的速度加速电子，其中1 g是由于地球引力而产生的加速度。然后，自由电子被推进到非常接近光速的速度，因此需要爱因斯坦的相对论方程来描述它们的运动。在这些极端条件下，电子将波长偏移的光从激光焦点中散射出去（称为非线性汤姆逊散射），这可以揭示电子运动的动力学，从而揭示激光场的结构。这个研究小组已经开发出了一种技术，可以将散射光子与构成驱动电子的激光脉冲的背景光子区分开来。在这个项目中，该团队将使用散射光作为探针来绘制焦点内和周围光场的强度和偏振。这样的激光聚焦是具有挑战性的，以任何其他方式表征，因为焦点往往是小到一个单独的像素在相机上。此外，在全功率下，任何放置在焦点上的相机都会被摧毁。分析一个非常强但很小的激光焦点内的电场和磁场，将有助于理解用世界上最强大的激光器研究的激光与原子、分子和固体之间的相互作用。该项目还将为学生提供在光学和光子学技术方面进行培训的机会，并为基础科学的进步做出贡献。测量激光聚焦中仅由少数电子散射的单个光子的能力，使得在非常低密度（本质上是真空）下观察基本相对论现象成为可能。这将使该团队能够避免高密度等离子体动力学的混淆影响。他们将研究非线性汤姆逊散射在散射基频、二次谐波和三次谐波光的空间分布中的偏振。极化包含了一个额外的信息维度，这是以前没有被研究或测量过的。偏振分辨散射图中丰富的结构为电子动力学提供了新的见解。该团队在紧密激光聚焦中对相对论电子轨迹的模拟表明，电子轨迹对激光场矢量的细节很敏感。可变形镜的使用将使他们能够创建理想的和奇异的焦点形状，以测试非线性汤姆逊散射对焦点中矢量场分布细节的灵敏度。

Advances in laser technology have made it possible to create exceptionally bright light inside a tiny region where a laser beam is focused. This project will develop a new way to study the optical fields near such a laser focus. The electric and magnetic fields in such extreme light can rip electrons away from any substance and then accelerate the electrons at more than sextillion (thousand billion billion) g's, where one g is the acceleration due to gravity on Earth. The freed electrons are then propelled to very near the speed of light, so Einstein's equations of relativity are required to describe their motion. Electrons under these extreme conditions scatter wavelength-shifted light out of the laser focus (called nonlinear Thomson scattering), and this can reveal the dynamics of the electron motion and thus the structure of the laser fields. This research team has developed techniques to distinguish the scattered photons from the quintillion background photons that comprise the laser pulse that drives the electrons. With this project, the team will use the scattered light as a probe to map the strength and polarization of the optical fields in and around the focus. Such a laser focus is challenging to characterize in any other way, since the focus is often as small as an individual pixel on a camera. Furthermore, at full power any camera placed in the focus would be destroyed. Analysis of the electric and magnetic fields inside a very intense but tiny laser focus will be helpful for understanding interactions between laser light and atoms, molecules, and solids that are studied with the world's most powerful lasers. This project will also provide an opportunity for students to be trained on optics and photonics technologies and contribute to progress in fundamental science.The ability to measure individual photons scattered from only a few electrons in a laser focus enables observation of fundamental relativistic phenomena at very low densities (essentially a vacuum). This will allow this team to avoid the confounding influence of plasma dynamics that occur for higher densities. They will study the polarization of nonlinear Thomson scattering in the spatial distribution of the scattered fundamental, second, and third harmonic light. The polarization contains an added dimension of information, which has not previously been investigated or measured. The rich structure in polarization-resolved scattering plots promises new insights into electron dynamics. This team's simulations of relativistic electron trajectories within a tight laser focus show that electron trajectories are sensitive to the details of the field vectors of the laser. The use of a deformable mirror will allow them to create ideal and exotic focal shapes to test the sensitivity of nonlinear Thomson scattering to the details of the vector field distributions in the focus.