对激光等离子体瞬态电场时空演化过程的高分辨探测研究

The high resolved study of transient dynamics of transient electromagnetic fields associated with laser plasmas is essential to understand the nature of laser-plasma interaction and eventually control its dynamic progress. However, light is not sensitive to electromagnetic fields, which is frequently applied to the dynamics of laser plasma. Therefore, the light probe could not provide the information of the self-generated electromagnetic fields, one of the most critical parameters in laser plasma evolution. Transient electric and magnetic fields can now be diagnosed experimentally by our ultrafast time-resolved electron schlieren method developed recently with combined ultrafast temporal resolution and field sensitivity. Its picoseconds temporal and microns spatial resolutions provide unprecedented chances to simultaneously record both the global and local field features. Here, we propose to develop a DC accelerated femtosecond photoelectron gun specially designed for laser plasma diagnosis and generate ultrashort electron probe. Upon the interaction between intense laser pulses (<10^18 W/cm^2) and targets such as metals and semiconductors, we plan to investigate the temporal and spatial evolution of the associated medium to low density laser plasmas by such pulsed electron probe. The evolution of the 3D transient electric field structure and the corresponding charge distribution will be directly achieved by analyzing electron deflection patterns by Abel inversion. We will further explore the formation and evolution mechanism of such field structure combining with numerical simulations, which is expected to help us understand the dynamics of energy transfer in light induced charge separation field, Coulomb Explosion, nonlinear photoelectric process and so on.

对激光等离子体瞬态电磁场分布时空演化过程的高分辨探测研究，是深入理解激光等离子体相互作用物理本质和对其动力学过程进行调控的基础。然而由于常用的光探针对电磁场变化不敏感，无法给出对激光等离子体动力学过程起关键作用的自生电磁场信息。我们近期发展的超快时间分辨电子纹影方法同时具有皮秒量级时间分辨和微米量级空间分辨能力，为诊断全局和局域的激光等离子体的电磁场时空演化过程信息提供了可能。本项目将发展适用于激光等离子体诊断的直流加速光阴极飞秒电子枪，针对非相对论强度激光(<10^18 W/cm^2)与金属、半导体等各类靶材相互作用过程，利用超短脉冲电子束对中低密度等离子体的时空演化过程开展实验研究；进而通过阿贝反演获得三维瞬态电场结构和电荷分布以及它们的演化过程，并结合理论模拟分析其形成和演化机制，以期有助于准确理解光致电荷分离场、库仑爆炸、非线性光电效应等能量吸收和转移的动力学过程。

对激光等离子体瞬态电磁场分布时空演化过程的实时探测，是深入理解激光等离子体相互作用物理本质和对其动力学过程进行调控的基础。在发展对激光等离子体瞬变电磁场敏感的飞秒电子探针技术的基础上，为一进步提升超快电子纹影法的时空分辨率，本项目发展了超快非等密电子纹影法，针对二维平面靶和零维针尖靶的等离子体演化动力学，分别设计了非等距排列的斜孔网格和半圆形调制网格，并完成了原理性验证实验。同时，本项目选取了具备代表性的半导体、绝缘体和金属样品体系，系统性地研究了1-100拍瓦每平方米量级中等强度飞秒激光激发条件下所产生的中低密度等离子体瞬态电场的演化。对于可用作光阴极材料和发光二极管的n型氮化镓半导体材料，通过建立探针电子束质心偏转量与瞬态电场场强的关系，探测并理解飞秒激光辐照下空间电荷效应对表面等离子体瞬态电场演化的影响；研究表明在空间电荷效应的限制下，氮化镓表面发射电荷总量和电子初始速度均达到饱和，分别为2.7pC和1.8um/ps；该研究为基于飞秒激光调控光阴极出射电子的特性调控提供了新思路。对于强关联量子材料中具有代表性的锰氧化合物体系，本项目初步探索了利用表面瞬态电场实现材料特性超快调控的可能途径；通过优化激光等离子体参数，在室温条件下，La0.67Sr0.33MnO3纳米单晶薄膜表面10微米以内的高度，可以产生高达100kV/m电场强度，持续时间约数十皮秒；这表明，瞬态电场有可能成为除太赫兹之外的新型超快调控手段；然而在液氮低温条件下，锰氧化合物表面瞬态电场的强度显著降低，相应的机理问题仍亟待研究解决。