Invited Talk

The direct visualization of structural dynamic processes in matter occurring on femtosecond time scales over sub-nanometer (even atomic) spatial dimensions has long been a goal in science. Transmission electron microscopy (TEM) is a powerful tool to observe directly the image from specimen with high spatial resolution. When coupled with time resolution, it, which called ultrafast electron microscopy (UEM), would be the strongest tool for the study of ultrafast dynamics in materials. Currently, the UEM with the time-spatial resolution of 10 ns and 10 nm has been achieved in conventional TEM through the use of photo-activated electron source driven by a nanosecond laser in the non-space-charge-limited regime with ns-long pulse length. A large number of important phenomena, i.e. phase transformations, melting, resolidification, nucleation and growth of damage in nanosecond time region, have been investigated. For the high time resolution development, a multi-shot (or stroboscopic) UEM, where as few as one electron in a pulse being sufficient for a ~100 MHz repetition rate laser system, has been proposed. However, to accumulate more signals, one need merely cycle the experiment for a longer time, within the limits of the stability of the microscope and the sample.

To overcome the space-charge limitation, we have proposed and designed a femtosecond time-resolved relativistic-energy electron microscopy using a photocathode radio-frequency (RF) electron gun. We started the femtosecond time-resolved electron microscopy project from 2010: (1) in 2012, a first prototype of RF gun based relativistic-energy TEM has been constructed at Osaka University. Both the static measurements of both relativistic-energy electron diffraction and image have been succeeded. (2) We have developed a new RF gun under the collaboration with KEK to generate a low-emittance femtosecond-bunch electron beam: 100 fs and 0.2 mm-mrad, which are essential for the achievement of nm-fs space-time resolution in future. (3) We have succeeded to develop a RF gun based ultrafast relativistic-energy electron diffraction, which has been used for the ultrafast dynamic study on phase transformation with femtosecond time resolution.

In this talk, the activities on UED and UEM are introduced. The requirements and limitations of the beam parameters in UEM are reviewed. The concept and design of RF gun based relativistic-energy TEM prototype are reported. The beam dynamics and challenges in femtosecond RF gun will be discussed. Finally, some demonstrations of the relativistic-energy TEM images, the single-shot and time-resolved UED measurements are reported.

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