Ultrafast Transmission Electron Microscopy


Dynamic TEM (DTEM) DTEM is also a high-speed imaging technique that uses


a laser to create the electron pulse for imaging. However, as mentioned previously, DTEM employs longer and more intense UV laser pulses to create electron pulses capable of generating a satisfactory image with a single pulse. Terefore, DTEM is the only ultrafast technique that can image irreversible processes because it can generate an image nearly instantaneously. In practice, longer laser pulses (5 nanoseconds to 1 microsecond), larger photocathodes (800 μm diameter), and larger, more intense, deeper UV laser pulses (∼1 mJ from 5ω over 50–100 μm spot sizes) are employed to generate > 106


electrons/pulse.


Te resultant electron beam typically loses some coherence due to the larger laser spot size creating an extended source. Additionally, space charge effects or even cathode depletion can cause the electron pulse to spread out in time. Terefore, the typical spatio-temporal resolution is in the tens of nanometers and tens of nanoseconds (∼10-16


ms) regime. Pictorial representations of DTEM and laser-UTEM


are shown in Figure 2, showing both the pump laser (red) to excite the sample and the probe laser (green) to generate the photoelectron pulse for the sample image. In single-frame DTEM (Figure 2a), a single probe pulse is provided any time aſter the pump pulse to capture the sample response at a specific time. For a time-lapse view of the irreversible process, movie mode (Figure 2b) is enabled by a burst of probe pulses and a synced downstream beam deflector that directs each pulse on to a separate portion of the imaging camera. Tus, images from each probe pulse are quickly recorded throughout the (irreversible) process evolution. Full control over time spacing between laser pulses allows probe pulse trains to be tailored temporally to best interrogate the intrinsic process response time. In Figure 2c, a typical laser-UTEM stroboscopic setup


is shown for a reversible process; probe pulses (green) are consistently synced with the pump pulses (red), and many probe pulses are collected to create the final image. Again, the reversible process must have a time period shorter than the pump laser repetition rate. Te first DTEM was demonstrated [9] during the 1980s


and was developed [10] through 2003, although the resolution was only slightly better than optical imaging. Researchers at Lawrence Livermore National Laboratory (LLNL) continued to improve [11] on these developments, achieving 10–20 nm resolution [12] with multi-frame movie acquisitions [13] and showing further improvements are possible through aberration-corrected [14,15] images. Examples of these DTEM capabilities are shown in the following applications. Movie mode acquisitions have been used to extensively


study the rapid solidification of Al-Si alloys developed for additive manufacturing [16,17]. DTEM captured the multiple growth domains of α-Al and coupled eutectic growth in real time aſter being melted with a Nd:YAG laser pulse (15 ns, 1064 nm, 2 μJ). Te DTEM image shown in Figure 3 represents three separate tests (a, b, and c) of an Al-Si sample exhibiting different mechanisms along the direction of solidification (white arrow): (a) mixed regions formed within 10 microseconds of the heating pulse, bounding a liquid-solid interface (between


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Figure 3: Rapid solidification of Al-Si alloy captured via DTEM. Scale bars are 5 μm. See text for explanation. Reprinted from [17], with permission from Elsevier.


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