Observation of the relativistic reversal of the ponderomotive potential

TL;DR

This study experimentally demonstrates how relativistic effects alter the ponderomotive potential experienced by electrons in a laser field, revealing velocity and polarization dependencies that enable advanced electron beam control.

Contribution

First experimental observation of relativistic reversal of the ponderomotive potential, confirming theoretical predictions and expanding understanding of electron-laser interactions at high velocities.

Findings

Phase shift depends on electron velocity and wave polarization.

At relativistic speeds, phase shift at electric field nodes can surpass that at antinodes.

Electron beam manipulation capabilities are enhanced by these relativistic effects.

Abstract

The secular dynamics of a non-relativistic charged particle in an electromagnetic wave can be described by the ponderomotive potential. Although ponderomotive electron-laser interactions at relativistic velocities are important for emerging technologies from laser-based particle accelerators to laser-enhanced electron microscopy, the effects of special relativity on the interaction have only been studied theoretically. Here, we use a transmission electron microscope to measure the position-dependent phase shift imparted to a relativistic electron wave function when it traverses a standing laser wave. The kinetic energy of the electrons is varied between $80\,\mathrm{keV}$ and $300\,\mathrm{keV}$, and the laser standing wave has a continuous-wave intensity of $175\,\mathrm{GW/cm}^2$. In contrast to the non-relativistic case, we demonstrate that the phase shift depends on both the electron velocity and the wave polarization, confirming the predictions of a quasiclassical theory of the interaction. Remarkably, if the electron's speed is greater than $1/\sqrt{2}$ of the speed of light, the phase shift at the electric field nodes of the wave can exceed that at the antinodes. In this case there exists a polarization such that the phase shifts at the nodes and antinodes are equal, and the electron does not experience Kapitza-Dirac diffraction. Our results thus provide new capabilities for coherent electron beam manipulation.