Photon-induced near field electron microscopy

Abstract

Ultrafast electron microscopy in the space and time domains utilizes a pulsed electron probe to directly map structural dynamics of nanomaterials initiated by an optical pump pulse, in imaging, diffraction, spectroscopy, and their combinations. It has demonstrated its capability in the studies of phase transitions, mechanical vibrations, and chemical reactions. Moreover, electrons can directly interact with photons via the near field component of light scattering by nanostructures, and either gain or lose light quanta discretely in energy. By energetically selecting those electrons that exchanged photon energies, we can map this photon-electron interaction, and the technique is termed photon-induced near field electron microscopy (PINEM). Here, we give an account of the theoretical understanding of PINEM. Experimentally, nanostructures such as a sphere, cylinder, strip, and triangle have been investigated. Theoretically, time-dependent Schrödinger and Dirac equations for an electron under light are directly solved to obtain analytical solutions. The interaction probability is expressed by the mechanical work done by an optical wave on a traveling electron, which can be evaluated analytically by the near field components of the Rayleigh scattering for small spheres and thin cylinders, and numerically by the discrete dipole approximation for other geometries. Application in visualization of plasmon fields is discussed.

Electron-light interactions beyond the adiabatic approximation: recoil engineering and spectral interferometry

“…In PINEM, a pulsed laser excitation is synchronized by the electrons emitted from a photoemission electron gun, within a photon-pump and electron-probe time-resolved spectroscopy apparatus (see Figure 6a). Observations of energy gain and loss processes in a series of spectacular experiments carried out in the group of Zewail and co-workers, demonstrated a series of loss and gain peaks in the spectra of carbon nanotubes (see Figure 6b) [9,12,104]. Indeed, those experiments demonstrated the possibility of multiple-photon gain and loss processes in addition to the single-photon processes.…”