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Infrared PINEM developed by diffraction in 4D UEM

Significance One of the most significant advances made is the ability to visualize nanoscale objects and to determine their shapes (through imaging) and their atomic-scale structure (through diffraction). Furthermore, for optical excitation of the nanostructure the photon-induced near-field EM (PINEM) imaging results in the mapping of nanostructure plasmonics with high spatial–temporal resolution. Here, we show that PINEM can be exploited in the infrared region with greater sensitivity, and that diffraction can be implemented for structural dynamics. Just as importantly, the time resolution of ultrafast EM (UEM), which already far exceeds, by 10 orders of magnitude, that of conventional EM, may be further enhanced by IR PINEM and photon gating with the unprecedented energy resolution (0.63 eV) reported here for 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.