Kikuchi ultrafast nanodiffraction in four-dimensional electron microscopy
Coherent atomic motions in materials can be revealed using time-resolved X-ray and electron Bragg diffraction. Because of the size of the beam used, typically on the micron scale, the detection of nanoscale propagating waves in extended structures hitherto has not been reported. For elastic waves of complex motions, Bragg intensities contain all polarizations and they are not straightforward to disentangle. Here, we introduce Kikuchi diffraction dynamics, using convergent-beam geometry in an ultrafast electron microscope, to selectively probe propagating transverse elastic waves with nanoscale resolution. It is shown that Kikuchi band shifts, which are sensitive only to the tilting of atomic planes, reveal the resonance oscillations, unit cell angular amplitudes, and the polarization directions. For silicon, the observed wave packet temporal envelope (resonance frequency of 33 GHz), the out-of-phase temporal behavior of Kikuchi's edges, and the magnitude of angular amplitude (0.3 mrad) and polarization  elucidate the nature of the motion: one that preserves the mass density (i.e., no compression or expansion) but leads to sliding of planes in the antisymmetric shear eigenmode of the elastic waveguide. As such, the method of Kikuchi diffraction dynamics, which is unique to electron imaging, can be used to characterize the atomic motions of propagating waves and their interactions with interfaces, defects, and grain boundaries at the nanoscale.
Additional Information© 2011 National Academy of Sciences. Contributed by Ahmed H. Zewail, December 14, 2010 (sent for review December 2, 2010). This work was supported by the National Science Foundation and the Air Force Office of Scientific Research in The Gordon and Betty Moore Center for Physical Biology at the California Institute of Technology. Author contributions: A.H.Z. and A.Y. designed research, performed research, and wrote the paper. The authors declare no conflict of interest. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1018733108/-/DCSupplemental.
Published - Yurtsever2011p13068P_Natl_Acad_Sci_Usa.pdf
Supplemental Material - pnas.1018733108_SI.pdf