John H. Gaida

Affiliation: Max Planck Institute for Multidisciplinary Sciences and Georg August University of Göttingen (Germany)

Research interests: Physics

Title: Attosecond electron microscopy using Lorentz PINEM and free-electron homodyne detection

Abstract:  Transmission electron microscopy (TEM) offers high spatial resolution and is widely used to characterize optical ex citations and near fields by electron energy loss spectroscopy (EELS). This method probes self – induced electric fields in the nanostructure, using spontaneous inelastic scattering of electrons. Importantly, it is difficult to experimentally access near – fie ld phase information. Using external laser excitation, strong optical near fields can induce stimulated inelastic electron – light scattering, as employed in photon – induced near – field electron microscopy (PINEM). In this case, optical modes populated at the laser frequency are excited, providing enhanced spectral resolution and polarization sensitivity. Importantly, in this process, the induced near field is phase – locked to the exciting laser, which translates to a coherent phase modulation of the electron wa ve function. In this talk , we present two phase contrast techniques for imaging optical near fields at nanostructures. Specifically, we implement Lorentz – mode PINEM and free – electron homodyne detection (FREHD). Using these techniques, we image optical fields with subwave length (10 nm) spatial and sub – cycle temporal resolutions. Lorentz microscopy is an in – line holography technique, employing Fresnel diffraction at a small defocus. The electron wave function is locally mixed, and contrast arises near phase gradients. Sensitivity to the light field is obtained by energy filtering T EM (EFTEM) of PINEM – induced sidebands in the electron spectrum. We obtain complementary information about the phase profile by exploiting the conjugate symmetry of the energy gain and loss sidebands. From two complementary measurements, we reconstruct the light – imprinted phase using an iterative reconstruction algorithm. In the newly developed FREHD technique, on the other hand, we measure the phase profile by means of a phase – controlled reference interaction. The near field at a sample modulates the phase of the electron wavefunction. This wavefunction modulation is amplified or attenuated for in – phase and anti – phase reference interactions, respectively, which allows for a coherent read – out of a position – dependent phase. Overall, FREHD transfers concepts from homodyne detection in optics and amplitude (AM) and frequency modulation (FM) in radio techno logy to the realm of electron microscopy. Combining free electrons with phase – locked laser excitation offers fascinating new possibilities to image local attosecond and phase – resolved responses on the nanometer scale.