Optical near-fields of nanostructures probed with electron interactions at femtosecond scales

Near-field electron microscopy is an invaluable technique for visualizing nanoscale structural features that may be smaller than optical wavelengths. The ultrafast scanning electron microscope (USEM) is an ideal tool for such applications because it allows for precise control of the femtosecond laser pulses that excite interactions of the electron beam with the near fields. With its submicron focus at ultrashort time scales, researchers report an experimental setup to study inelastic electron scattering of free electrons in the optical near-fields of nanostructures in the Journal of Applied Physics.

The team’s setup used a USEM equipped with a heated Schottky field-emission tip cathode, with UV laser pulses inducing electron emission and IR laser pulses exciting the optical near-fields. They studied the inelastic scattering of the emitted electrons in the interaction region of a nonresonant silicon nanograting. The system’s vacuum chamber also included an electromagnetic spectrometer and a microchannel plate detector to measure the electron spectra as a function of the time delay between the electron and optical pulses.

The authors could control the resonance between electrons and near-fields, and thus change the electron spectra, with a lower limit of the temporal resolution down to 410 femtoseconds. Introducing a pulse-front tilt to the IR laser beam increased the interaction distance between the electrons and optical near-fields. Adding a linear chirp to the structure’s period, meanwhile, increased the final electron energy to an observed 3.8 keV, demonstrating the resonance relationship between optical phase velocity and the electron’s propagation velocity.

The experimenters note that this setup could be developed for advanced applications including laser-driven electron accelerators and ultrafast diffraction and microscopy experiments.

Source: “Ultrafast scanning electron microscope applied for studying the interaction between free electrons and optical near-fields of periodic nanostructures,” by M. Kozak, J. McNeur, N. Schonenberger, J. Illmer, A. Li, A. Tafel, P. Yousefi, T. Eckstein, and P. Hommelhoff, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5032093 .