The Evolution of Ultrafast Electron Microscope Instrumentation

Abstract

Extrapolating from a brief survey of the literature, we outline a vision for the future development of time-resolved electron probe instruments that could offer levels of performance and flexibility that push the limits of physical possibility. This includes a discussion of the electron beam parameters (brightness and emittance) that limit performance, the identification of a dimensionless invariant figure of merit for pulsed electron guns (the number of electrons per lateral coherence area, per pulse), and calculations of how this figure of merit determines the trade-off of spatial against temporal resolution for different imaging modes. Modern photonics' ability to control its fundamental particles at the quantum level, while enjoying extreme flexibility and a very large variety of operating modes, is held up as an example and a goal. We argue that this goal may be approached by combining ideas already in the literature, suggesting the need for large-scale collaborative development of next-generation time-resolved instruments.

Complex crystallization dynamics in amorphous germanium observed with dynamic transmission electron microscopy

“…The recent developments in dynamic transmission electron microscopy (DTEM) at LLNL [19][20][21][22] have provided an experimental platform capable of following the nanometer scale evolution of the microstructure associated with this process in-situ from its earliest stages (~10 ns) to completion (~10 µs) in unprecedented detail.…”

Outrun radiation damage with electrons?

“…Even so, pulsed electron beams are attractive because of their ability to provide time resolution in both repetitive (stroboscopic) and non-repetitive (single-shot) modes, allowing such things as 'molecular movies' and the study of phase transitions [43,51]. Incoherent contrast will suffice for many of these applications, and single-pulse dark-field images with subnanometer resolution may be possible with an RF gun source and aberration-corrected lenses [43].…”

“…Both ε and B are conserved in an ideal optical system but are degraded by Coulomb repulsion between electrons, which reduces B and increases ε. An even more useful measure is the coherent fluence: N c = N p λ 2 /ε 2 = πλ 2 B T/e, which is unaffected by electron acceleration, beam focusing or apertures, space charge, or pulse compression but is degraded by lens aberrations and stochastic interaction [43]. For diffraction-contrast images, N c ≈ 1 suffices, but for phase-contrast or diffractive imaging, N c > 10 is required, whereas current UED systems have N c << 1 (Bryan Reed, personal communication), hence the need for electron gun development, which includes maximizing the extraction field at the photocathode, ensuring low capacitance (for short pulses), and reducing the thermal emittance by using a low cathode temperature.…”

Development of an Ultrafast Low-Energy Electron Diffraction Setup

“…Time and angle-resolved photoemission spectroscopy (trARPES) [125] and THz-STM can be, for example, employed to map the electronic structure with atomic resolution and high surface sensitivity. For the investigation of the atomic structure at surfaces, mainly two pathways are currently followed: On the one hand, extremely thin films are probed by ultrafast TEM (UTEM) [131] and ultrafast electron diffraction (UED) [132] in transmission. To obtain a strong surface signal, a grazing incidence geometry is alternatively applied as in time-resolved XRD [133] and RHEED [44] studies.…”

“…In order to further increase the experimental capabilities of both approaches, much effort is currently devoted into the development and modification of novel electron sources [43,131,153].…”