Transient lensing from a photoemitted electron gas imaged by ultrafast electron microscopy

Omid Zandi,Allan E Sykes,Brandon S Zerbe,Ryan D Cornelius,Francis M Alcorn,Phillip M Duxbury,Bryan W Reed,Renske M Van Der Veen

doi:10.1038/s41467-020-16746-z

Abstract

Understanding and controlling ultrafast charge carrier dynamics is of fundamental importance in diverse fields of (quantum) science and technology. Here, we create a three-dimensional hot electron gas through two-photon photoemission from a copper surface in vacuum. We employ an ultrafast electron microscope to record movies of the subsequent electron dynamics on the picosecond-nanosecond time scale. After a prompt Coulomb explosion, the subsequent dynamics is characterized by a rapid oblate-to-prolate shape transformation of the electron gas, and periodic and long-lived electron cyclotron oscillations inside the magnetic field of the objective lens. In this regime, the collective behavior of the oscillating electrons causes a transient, mean-field lensing effect and pronounced distortions in the images. We derive an analytical expression for the time-dependent focal length of the electron-gas lens, and perform numerical electron dynamics and probe image simulations to determine the role of Coulomb self-fields and image charges. This work inspires the visualization of cyclotron dynamics inside two-dimensional electron-gas materials and enables the elucidation of electron/plasma dynamics and properties that could benefit the development of high-brightness electron and X-ray sources.

Highlights

Understanding and controlling ultrafast charge carrier dynamics is of fundamental importance in diverse fields of science and technology
Quantum effects arising from Landau levels are dominant when the mean thermal energy of the gas is smaller than the energy level separation, which means experiments are often performed at low temperatures and under strong magnetic fields
We performed our experiments using a modified environmental transmission electron microscope (TEM) operating at 300 keV (Fig. 1a), which is interfaced with a high repetition rate, fs laser system

Summary

Introduction

Understanding and controlling ultrafast charge carrier dynamics is of fundamental importance in diverse fields of (quantum) science and technology. Carrier motion often unfolds on ultrafast time scales and requires tools that can directly visualize the dynamics with appropriate spatial and temporal resolutions, i.e. Ångstroms–micrometers (Å–μm) and femtoseconds–nanoseconds (fs–ns), respectively In this regard, ultrafast electron microscopy (UEM) has recently emerged as a powerful technique for the study of ultrafast photoinduced processes in nanoscale systems[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. Confined electron gases[16] can exhibit intriguing properties such exceptionally high electron mobilities[17], quantum Hall effects[18,19], Shubnikov–de Haas oscillations[20], anomalous de Haas–van Alphen effects[21], and superradiant damping[22] Understanding and controlling these phenomena are of fundamental importance in diverse fields of quantum science and technology[23,24]. Systematic variations of the laser fluence and wavelength, and adding a bias to the sample, will enable obtaining valuable insight into the electron emission process and the subsequent processes that affect electron beam properties such as emittance

Methods

Results

Conclusion