Exploring nanoscale ultrafast dynamical phenomena by 4D electron microscopy
In the past decade, four-dimensional electron microscopy (4D EM), which enables the direct observation of transient morphologies, structures and carrier dynamics of materials in real time and space, has attracted increasing interest to the research community due to its powerful capability in the interdisciplines of physics, material science, chemistry, and biology [1-2]. In this presentation, I will firstly give a brief introduction of the development of 4D-EM, several cutting-edge technologies, and the state-of-the-art of its applications. Then I will present our most recent development of two-color photon induced near field electron microscopy (PINEM) in 4D EM based on “photon gating” effect, which enhances the temporal resolution of 4D-EM by an order of magnitude . Such high temporal resolution enables visualizing the ultrafast electronic dynamics of a single nanostructure. Using this methodology, we reveal the nanoscale-femtosecond dielectric response of a single VO2 nanowire in the insulator-to-metal phase transition. After that, I will talk about our development of liquid-phase 4D EM and its first application in imaging Brownian dynamics and photochemical reaction dynamics of nanoparticles in liquid on the nanometer-nanosecond time scale [4-6]. Both the translational and rotational dynamics of individual nanoparticles were imaged in both diffusion and ballistic regimes, and a full transition from diffusive to superdiffusive, and further to ballistic rotation was revealed with increasing the asymmetry of the particles. With increasing laser fluence, photoinduced photomorphic reaction dynamics such as agglomeration, coalescence, and fusion dynamics of plasmonic nanoparticles in liquid were directly unraveled. This advanced liquid-phase 4D EM opens a promising possibility for future study of numerous physical, chemical and biological dynamical processes in native environments. In the end, I will introduce the recent development of low-cost, laser-free 4D-EM and its application in visualization of electromagnetic wave propagation dynamics in high frequency miniature electronic devices at nanometer and picosecond scales, which extends the application of 4D EM to the field of electromagnetic dynamics in nanoscale electronic devices .
 B. Barwick et al., Nature 462, 902-906 (2009)
 A. H. Zewail, Science 328, 187-193 (2010).
 X. W. Fu et al., Nature Communications 11(1), 1-11(2020).
 X. W. Fu et al., Science 355, 494-498 (2017).
 X. W. Fu et al., Science Advances 3 (8), e1701160 (2017).
 X. W. Fu et al., Science Advances 4 (7), eaat3077 (2018).
 X. W. Fu et al., Science Advances 6 (40), eabc3456 (2020).
付学文，南开大学物理科学学院教授，博士生导师，海外引进青年人才，天津市杰出青年基金获得者，南开大学“百名青年学科带头人”，国家重点研发计划青年项目首席科学家。2014年获北京大学凝聚态物理博士学位（导师：俞大鹏院士），曾荣获北京市优秀博士毕业生、北京大学优秀博士毕业生和优秀博士论文奖。先后在美国加州理工学院（诺贝尔奖得主Ahmed Zewail教授研究组）和美国布鲁克海文国家实验室（Yimei Zhu教授研究组）从事研究工作。2019年受聘于南开大学物理科学学院担任教授、博导，牵头建立了南开大学超快电子显微镜实验室。长期从事4D超快电子显微镜、超快阴极荧光等超高时空分辨电子成像与探测技术开发及其在低维量子功能材料的结构、载流子及自旋等动力学中的应用研究。在Science、Science Advances（3篇）、Nature Communications、Advanced Materials、PNAS、ACS Nano（4篇）、Nano Letters等知名国际期刊发表学术论文近40篇，获授权发明专利1项。研究成果多次被 Science、Phys.org、Physicsword、Nanotechweb、Advances in Engineering等科学媒体选为研究亮点进行报道。