[Paper Review] Direct visualization of electromagnetic wave dynamics by laser-free ultrafast electron microscopy

Xuewen Fu, Erdong Wang+8 more|arXiv (Cornell University)|Jun 4, 2020

Advanced Electron Microscopy Techniques and Applications43 references4 citations

TL;DR

This paper presents a laser-free ultrafast electron microscopy (UEM) technique that achieves real-time, nanoscale spatiotemporal imaging of electromagnetic wave dynamics without requiring femtosecond lasers. By using an RF-driven pulser to generate tunable picosecond electron pulses from a continuous electron beam, the method visualizes GHz electromagnetic wave propagation and field enhancement in interdigitated comb structures with sub-nanometer and picosecond resolution, demonstrating a low-cost, scalable alternative to laser-based UEM.

ABSTRACT

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.

Motivation & Objective

• To develop a low-cost, laser-free alternative to conventional ultrafast electron microscopy that maintains high spatiotemporal resolution.
• To eliminate the need for complex femtosecond laser systems and intricate optical alignment in ultrafast electron microscopy.
• To enable direct visualization of transient electromagnetic field dynamics in nanoscale devices, particularly in RF micro-electromechanical systems.
• To demonstrate the feasibility of this method for studying electrodynamics in information-processing technologies.

Proposed method

• A continuous electron beam is modulated using a radiofrequency (RF)-driven electronic pulser to generate picosecond electron pulses.
• The repetition rate of the electron pulses is tunable from 100 MHz to 12 GHz, enabling flexible pump-probe timing.
• A broadband electromagnetic wave is excited in the sample simultaneously with electron pulse generation, ensuring synchronization.
• The method uses standard transmission electron microscopy hardware with minimal modifications, avoiding the need for laser-based optical systems.
• Electron probe pulses image the transient electromagnetic field distribution in real time, capturing dynamic field evolution.
• Time-resolved imaging enables extraction of polarization, amplitude, and local field enhancement characteristics at nanoscale resolution.

Experimental results

Research questions

• RQ1 Can electromagnetic wave dynamics in nanoscale RF devices be visualized in real time without using femtosecond lasers?
• RQ2 How does the electromagnetic field evolve in time and space around interdigitated comb structures under GHz excitation?
• RQ3 What is the degree of local field enhancement and polarization behavior in such nanostructures during dynamic excitation?
• RQ4 Can a laser-free UEM system achieve sufficient temporal and spatial resolution for meaningful electrodynamics studies?
• RQ5 Is the RF-driven electron pulsing method scalable and reliable for broader applications in materials and device physics?

Key findings

• The laser-free UEM successfully visualized transient oscillating electromagnetic fields around the tines of an interdigitated comb structure with picosecond temporal and nanometer spatial resolution.
• Time-resolved images revealed dynamic field oscillations at GHz frequencies, confirming the propagation and interference of electromagnetic waves in the structure.
• Local field enhancement of up to 10-fold was observed at the comb tines due to geometric confinement and resonant effects.
• Polarization of the electromagnetic field was directly measured and found to vary spatially and temporally, indicating complex field vector dynamics.
• The system demonstrated stable operation across a wide range of repetition rates (100 MHz to 12 GHz), enabling flexible pump-probe timing.
• The method achieved comparable spatiotemporal resolution to laser-based UEM but at significantly reduced cost and complexity.