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After several revolutions in spatial resolution and spectroscopic imaging capabilities, the field of electron microscopy is now entering a new frontier: studying dynamic ultrafast processes in materials at the nanoscale. In this symposium, some of the newest developments in time-resolved electron microscopy will be discussed by experts from three companies active in this field, namely Delmic, DrX Works, and Amsterdam Scientific Instruments! In particular, the symposium will cover time-resolved cathodoluminescence imaging, temporal electron beam shaping with microwave cavities, and electron detection using hybrid pixel detectors.
Abstracts:
Dr. X. Works:
Speakers: Jom luiten – TU Eindhoven, Jim Franssen – Doctor X Works BV
Title: Microwave cavities for Time-resolved EM and CL
We are developing microwave cavities for ultrafast blanking of continuous 30 - 300 keV electron beams creating ultrashort pulses, which find application in time-resolved electron microscopy and potentially in time resolved cathodoluminescence. Our method is based on the use of 3 GHz cavities operating in the TM110 beam ‘deflection’ mode. The ultrashort electron pulses generated can be synchronized to laser pulses, enabling pump-probe experiments. The simplest configuration is a single-mode cavity, generating few-100-fs electron pulses at 3 GHz rep rate by sweeping the beam across a slit. The so-called ‘Lissajoux’ cavity operates with two perpendicular TM110modes simultaneously, at two different resonant frequencies (e.g. two modes having a 75 MHz difference frequency). This creates a more complicated deflection pattern with a repetition period equal to the difference frequency. These pulses can be synchronized to a mode-locked femtosecond laser oscillator which generally operates at the same 75 MHz frequency. The Lissajoux cavity has been installed in 200 keV and 300 keV TEMs and is used for pump-probe experiments on materials, research into quantum interaction between light and electrons in vacuum, and for measurement of the beam statistics of continuous electron beams. The latest development is the so-called ‘Clocx’ cavity, which employs a rotating TM10 mode enabling almost arbitrary temporal structuring of the electron beam. This technique has an even broader application since the pulse lengths can be varied from >100 ps to <10 fs at repetition rates varying from MHz to THz frequencies.
Amsterdam Scientific Instruments:
Speaker: Penghan Lu , Staff Scientist @ ER-C, FZ Juelich
Title: Low dose and in situ 4D STEM powered by real-time sparse array analytics on an event-driven pixelated detector
Four-dimensional scanning transmission electron microscopy (4D STEM) measures 2D reciprocal space scattering information from an electron probe laterally shifted across a 2D real space specimen area [1]. This high-dimensional dataset allows much richer information to be retrieved than conventional STEM, ranging from crystallographic orientation and phase, strain, short and medium range ordering, to differential phase contrast (DPC), ptychography as well as other new imaging modes. The past decade has particularly witnessed the advance of this technique because of the introduction of fast direct electron-counting detectors. These detectors usually run at 1-10 kHz continuous readout, which corresponds to 1000-100 μs pixel dwell time in 4D STEM measurements, while the conventional STEM usually runs in few μs pixel dwell time. Such a 2-3 order-of-magnitude gap in speed would prevent applying the versatile 4D STEM measurements to at least two application fields, including low-dose atomic-resolution high-contrast imaging of sensitive specimens as well as in situ time-resolved studies of dynamic behavior upon external stimuli.
In order to tackle this challenge, we installed an event-driven CheeTah T3 detector with 2x2 chips (in total 512x512 pixels) from Amsterdam Scientific Instruments (ASI) on a monochromated and probe-corrected Thermo Fisher Scientific Titan STEM for 4D STEM applications. This detector retrieves precise timing of scanning line trigger signal from the microscope with the same clock as the electron impact events on the detector, which facilitates a reliable assignment of the electron events to each scanned pixel. Thanks to its 1.56 ns Time of Arrival resolution, this detector also permits recording electron events continuously in an effectively much higher frame rate for 4D STEM. Due to its inherent sparse nature, this high-dimensional data can be compressed and computed in real time efficiently in the compressed sparse row (CSR) format based on the collaborative development between ASI and LiberTEM team [4]. 4D STEM measurements at effectively MHz frame rate with live image reconstruction has been demonstrated based on these developments. This unique workflow enables the user to navigate the specimen, adjust focus or astigmatism, and eventually visualise sensitive specimens or dynamics without changing between different detectors or optical modes. A few application examples and future prospects for improvement will be discussed during this presentation.
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