Photon gating in four-dimensional ultrafast electron microscopy
Ultrafast electron microscopy (UEM) is a pivotal tool for imaging of nanoscale structural dynamics with subparticle resolution on the time scale of atomic motion. Photon-induced near-field electron microscopy (PINEM), a key UEM technique, involves the detection of electrons that have gained energy from a femtosecond optical pulse via photon–electron coupling on nanostructures. PINEM has been applied in various fields of study, from materials science to biological imaging, exploiting the unique spatial, energy, and temporal characteristics of the PINEM electrons gained by interaction with a "single" light pulse. The further potential of photon-gated PINEM electrons in probing ultrafast dynamics of matter and the optical gating of electrons by invoking a "second" optical pulse has previously been proposed and examined theoretically in our group. Here, we experimentally demonstrate this photon-gating technique, and, through diffraction, visualize the phase transition dynamics in vanadium dioxide nanoparticles. With optical gating of PINEM electrons, imaging temporal resolution was improved by a factor of 3 or better, being limited only by the optical pulse widths. This work enables the combination of the high spatial resolution of electron microscopy and the ultrafast temporal response of the optical pulses, which provides a promising approach to attain the resolution of few femtoseconds and attoseconds in UEM.
Additional Information© 2015 National Academy of Sciences. Freely available online through the PNAS open access option. Contributed by Ahmed H. Zewail, September 9, 2015 (sent for review August 24, 2015; reviewed by Andrea Cavalleri and Chong-Yu Ruan). Published online before print October 5, 2015. We thank Dr. S. T. Park for very helpful discussion and Dr. B-K. Yoo for his help in the initial studies of photon-induced nearfield electron microscopy (PINEM) on ZnO nanowires. This work was supported by the National Science Foundation Grant DMR-0964886 and the Air Force Office of Scientific Research Grant FA9550-11-1-0055S for research conducted at The Gordon and Betty Moore Center for Physical Biology at the California Institute of Technology. Author contributions: M.T.H., H.L., J.S.B., and A.H.Z. designed research, performed research, contributed new reagents/analytic tools, analyzed data, and wrote the paper. Reviewers: A.C., Max Planck Institute; and C.-Y.R., Michigan State University. The authors declare no conflict of interest. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1517942112/-/DCSupplemental.
Published - PNAS-2015-Hassan-12944-9.pdf
Supplemental Material - pnas.201517942SI.pdf