Ultrafast Transmission Electron Microscopy


the microscope can be switched from ultrafast modulation to continuous. For a microscope employing laser-UTEM it can take a bit longer, as the photocathode must, at a minimum, be heated or the applied field ramped to convert it back to a thermionic or a field emission beam source. In a typical case not requiring a vacuum vent, up to an hour may be needed for the source temperature to stabilize, and, of course, the beam characteristics (current density, energy spread, divergence, etc.) will be changed relative to pulsed operation. Although not extensively demonstrated in the literature,


the sample excitation for pump-probe experiments is widely possible across all four instruments. Sub-ps triggering and timing techniques are well known, and researchers have combined various sample excitation methods with all beam modulation methods. A key parameter to consider is the maximum probe repetition rate, as it defines how long an acquisition time is needed to obtain a clear image for a reversible process. Te response and recovery times of the system ultimately determine the maximum usable probe rate with a given sample, thus for characterizing a wide range of processes (that is, UTEM as a shared resource for a diverse community), an adjustable probe rate would be ideal. System repetition rates are easily adjustable for laser or UFPTM


,


although limited by the maximum laser repetition (100 kHz, typical) rate for laser-UTEM and the maximum RF frequency for RF-UTEM (12 GHz, typical). In terms of performance, laser-UTEM can achieve atomic


resolution, sub-eV energy spread, and high coherence when using the best nano-photoemitters but suffers when using larger photo-thermionic cathodes. For RF UTEM, atomic resolution has not been fully demonstrated in the literature, although it is expected. A caveat for the RF UTEM techniques discussed is that while the RF frequency is fixed for the RF resonator design, it is compatible with any beam energy. Due to the phase-match requirement between the incident electron velocity and the propagation of the traveling wave in the UFPTM design, the beam energy for its RF traveling wave design is essentially fixed, although the repetition rate is widely variable. Commercial availability has improved as these techniques


are being adopted across multidisciplinary fields. Te initial laser-based techniques that had been typically only laboratory- built research tools have become commercially available [30] as well as the RF-based UFPTM


that can be retrofitted to electron microscopes by all manufacturers.


Conclusion As TEMs have been adopted by diverse communities in


multidisciplinary fields, additional EM markets have grown, and TEMs have become the standard in high-resolution imaging. Complementary techniques have developed for a broad range of ultrafast spatio-temporal imaging (10-15 10-23 ms). While the process details play a role in identifying


to


the ideal ultrafast approach, these lines continue to be blurred as sample excitation/pump methods expand and the techniques mature. Hybrid pump-probe approaches are likely to accelerate the adoption of ultrafast techniques, as users identify key performance requirements and avoid needless complexities. Te intention of this article is to supply new


2021 September • www.microscopy-today.com


adopters with unbiased information to make informed choices as they progress along their individual research paths. Additional references on UTEM techniques related to this


article and a webinar-based presentation of the materials can be found at https://www.euclidtechlabs.com/supplementary- utem-refs.


Acknowledgements Te authors (EM and DL) acknowledge support from NIH


SBIR Phase 2 (1R43GM133267-01) and the US Department of Energy (DOE) Basic Energy Sciences SBIR Phase 2a (DE- SC0013121). Tey also thank Dr. Spencer Reisbick (Brookhaven National Lab) for the unpublished images in Figure 4. Tis work was performed in part (support of JR) under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.


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