Infrared PINEM developed by diffraction in 4D UEM
The development of four-dimensional ultrafast electron microscopy (4D UEM) has enabled not only observations of the ultrafast dynamics of photon–matter interactions at the atomic scale with ultrafast resolution in image, diffraction, and energy space, but photon–electron interactions in the field of nanoplasmonics and nanophotonics also have been captured by the related technique of photon-induced near-field electron microscopy (PINEM) in image and energy space. Here we report a further extension in the ongoing development of PINEM using a focused, nanometer-scale, electron beam in diffraction space for measurements of infrared-light-induced PINEM. The energy resolution in diffraction mode is unprecedented, reaching 0.63 eV under the 200-keV electron beam illumination, and separated peaks of the PINEM electron-energy spectrum induced by infrared light of wavelength 1,038 nm (photon energy 1.2 eV) have been well resolved for the first time, to our knowledge. In a comparison with excitation by green (519-nm) pulses, similar first-order PINEM peak amplitudes were obtained for optical fluence differing by a factor of more than 60 at the interface of copper metal and vacuum. Under high fluence, the nonlinear regime of IR PINEM was observed, and its spatial dependence was studied. In combination with PINEM temporal gating and low-fluence infrared excitation, the PINEM diffraction method paves the way for studies of structural dynamics in reciprocal space and energy space with high temporal resolution.
Additional Information© 2016 National Academy of Sciences. Contributed by Ahmed H. Zewail, January 8, 2016 (sent for review December 23, 2015; reviewed by Fabrizio Carbone, Archie Howie, and X. Sunney Xie). Published online before print February 4, 2016. This work was supported by the National Science Foundation Grant DMR-0964886 and the Air Force Office of Scientific Research Grant FA9550-11-1-0055 for research conducted in The Gordon and Betty Moore Center for Physical Biology at the California Institute of Technology. Author contributions: H.L., J.S.B., and A.H.Z. designed research; H.L. performed experiments; H.L. and J.S.B. analyzed data; and H.L., J.S.B., and A.H.Z. wrote the paper. Reviewers: F.C., École Polytechnique Fédérale de Lausanne; A.H., University of Cambridge; and X.S.X., Harvard University. The authors declare no conflict of interest.
Published - PNAS-2016-Liu-2041-6.pdf