Direct visualization of electromagnetic wave dynamics by laser-free ultrafast electron microscopy

Integrating femtosecond (fs) lasers to electron microscopies has enabled direct imaging of transient structures and morphologies of materials in real time and space, namely, ultrafast electron microscopy (UEM). Here we report the development of a laser-free UEM offering the same capability of real-time imaging with high spatiotemporal resolutions but without requiring expensive fs lasers and intricate instrumental modifications. We create picosecond electron pulses for probing dynamic events by chopping a continuous beam with a radiofrequency (RF)-driven pulser, where the repetition rate of the electron pulses is tunable from 100 MHz to 12 GHz. A same broadband of electromagnetic wave is enabled for sample excitation. As a first application, we studied the GHz electromagnetic wave propagation dynamics in an interdigitated comb structure which is one of the basic building blocks for RF micro-electromechanical systems. A series of pump-probe images reveals, on nanometer space and picosecond time scales, the transient oscillating electromagnetic field around the tines of the combs, and time-resolved polarization, amplitude, and nonlinear local field enhancement. The success of this study demonstrates the feasibility of the low-cost laser-free UEM in real-space visualizing of dynamics for many research fields, especially the electrodynamics in devices associated with information processing technology.