Coherent optical excitations in two-dimensional (2D) materials, 2D polaritons, can generate a plethora of optical phenomena that arise from the extraordinary dispersion relations that do not exist in regular materials. Probing of the dynamical phenomena of 2D polaritons requires simultaneous spatial and temporal imaging capabilities and could reveal unknown coherent optical phenomena in 2D materials. Here, we present a spatiotemporal measurement of 2D wave packet dynamics, from its formation to its decay, using an ultrafast transmission electron microscope driven by femtosecond midinfrared pulses. The ability to coherently excite phonon-polariton wave packets and probe their evolution in a nondestructive manner reveals intriguing dispersion-dependent dynamics that includes splitting of multibranch wave packets and, unexpectedly, wave packet deceleration and acceleration. Having access to the full spatiotemporal dynamics of 2D wave packets can be used to illuminate puzzles in topological polaritons and discover exotic nonlinear optical phenomena in 2D materials.

二维 (2D) 材料中的相干光激发，即 2D 极化子，可以产生大量光学现象，这些现象是由常规材料中不存在的非凡色散关系引起的。探测二维极化子的动力学现象需要同时进行空间和时间成像，并且可以揭示二维材料中未知的相干光学现象。在这里，我们使用由飞秒中红外脉冲驱动的超快透射电子显微镜对二维波包动力学进行了时空测量，从其形成到衰减。相干激发声子极化波包并以非破坏性方式探测其演化的能力揭示了有趣的色散相关动力学，包括多分支波包的分裂，并且出乎意料地，波包减速和加速。获得 2D 波包的完整时空动力学可用于阐明拓扑极化子中的难题，并发现 2D 材料中的奇异非线性光学现象。