Accessing the low-energy non-equilibrium dynamics of materials and their polaritons with simultaneous high spatial and temporal resolution has been a bold frontier of electron microscopy in recent years. One of the main challenges lies in the ability to retrieve extremely weak signals and simultaneously disentangling the amplitude and phase information. Here we present free-electron Ramsey imaging—a microscopy approach based on light-induced electron modulation that enables the coherent amplification of optical near fields in electron imaging. We provide simultaneous time-, space- and phase-resolved measurements of a micro-drum made from a hexagonal boron nitride membrane, visualizing the sub-cycle dynamics of two-dimensional polariton wavepackets therein. The phase-resolved measurement reveals vortex–anti-vortex singularities on the polariton wavefronts, together with an intriguing phenomenon of a travelling wave mimicking the amplitude profile of a standing wave. Our experiments show a 20-fold coherent amplification of the near-field signal compared with conventional electron near-field imaging, resolving peak field intensities in the order of a few watts per square centimetre, corresponding to field amplitudes of a few kilovolts per metre. As a result, our work paves the way for the spatiotemporal electron microscopy of biological specimens and quantum materials, exciting yet delicate samples that are currently difficult to investigate.

近年来，以高空间和时间分辨率获取材料及其极化子的低能非平衡动力学一直是电子显微镜的一个大胆前沿。主要挑战之一在于检索极弱信号并同时解开幅度和相位信息的能力。在这里，我们提出了自由电子拉姆齐成像——一种基于光诱导电子调制的显微镜方法，可以实现电子成像中光学近场的相干放大。我们提供由六方氮化硼膜制成的微鼓的同步时间、空间和相位分辨测量，可视化其中二维极化子波包的子周期动力学。相位分辨测量揭示了极化子波前的涡旋-反涡旋奇点，以及行波模仿驻波振幅分布的有趣现象。我们的实验表明，与传统电子近场成像相比，近场信号的相干放大达到了 20 倍，峰值场强度达到每平方厘米几瓦的量级，对应于每米几千伏的场幅。因此，我们的工作为生物样本和量子材料的时空电子显微镜铺平了道路，这些样本令人兴奋但目前难以研究。