Coherently amplified ultrafast imaging using a free-electron interferometer

TL;DR

This paper introduces Free-Electron Ramsey Imaging (FERI), a novel electron microscopy technique that coherently amplifies optical near-fields, enabling high-resolution, phase-resolved imaging of ultrafast dynamics in materials with significantly enhanced sensitivity.

Contribution

FERI provides a new method for coherent amplification of near-fields in electron microscopy, allowing simultaneous high-resolution, phase, and time-resolved imaging of ultrafast material dynamics.

Findings

Achieved 20-fold coherent amplification of near-field signals.

Visualized sub-cycle dynamics of 2D polariton wavepackets.

Revealed vortex-anti-vortex singularities and traveling wave phenomena.

Abstract

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 while simultaneously disentangling amplitude and phase information. Here, we present Free-Electron Ramsey Imaging (FERI), a microscopy approach based on light-induced electron modulation that enables 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 2D polariton wavepackets therein. The phase-resolved measurements reveals vortex-anti-vortex singularities on the polariton wavefronts, together with an intriguing phenomenon of a traveling wave mimicking the amplitude profile of a standing wave. Our experiments show a 20-fold coherent amplification of the near-field signal compared to conventional electron near-field imaging, resolving peak field intensities in the order of ~W/cm2, corresponding to field amplitudes of a few kV/m. As a result, our work paves the way for spatio-temporal electron microscopy of biological specimens and quantum materials, exciting yet delicate samples that are currently difficult to investigate.