In the first year of the project, we have made progress towards all of the three objectives listed above.
Objective 1
On the theoretical side, a roadmap was developed for achieving the targeted sub-meV, sub-fs and sub-Ångström resolution. We have also formulated a fully quantum-mechanical description of light, free electrons, and their interactions. On the experimental side, we demonstrated an energy resolution exceeding previous technologies by typically three orders of magnitude through ultrahigh-resolution electron energy gain spectroscopy. Moreover, in the low energy far-infrared range mapping of surface phonons was demonstrated by electron energy loss spectroscopy (EELS). On the cathodoluminescence (CL) side, we are developing software for spatially-resolved polarimetry and explored limits of pump-probe CL spectroscopy as well as CL photon correlations for continuous and pulsed electron beams. Using an event-based EELS detector, we demonstrated ns dwell-time EELS and consequently could implement EELS-CL coincidence experiments with sub-10 nm spatial resolution. Furthermore, EELS-CL coincidence measurements on photonic cavities demonstrated phase-matched electron light interaction and efficient electron phase modulation using low-power, continuous-wave excitation. We also realized energy-dispersive X-ray spectroscopy (EDX)-EELS coincidence measurements.
Objective 2
We demonstrated that CL can be used as a probe of coherent excitations in semiconducting materials, thereby extending the application area of CL. We also collaborated on the potential of correlating photon-induced near-field electron microscopy (PINEM), EELS and CL for plasmonic nanomaterials. Moreover, nanothermometry measured by electron excitation has been explored theoretically and experimentally opening the possibility to measure temperatures and thermal conductivity with high spatial resolution. By using alternative scan patterns in scanning transmission electron microscopy (STEM), we could significantly reduce beam damage in beam-sensitive samples opening up the possibility to measure atypical materials with electron microscopy. We also extended the information level that can be extracted for transition metal dichalcogenides by resolving the correlation between upper and lower polariton branches.
Objective 3
On the instrumentation front, we designed and implemented a laser incoupling module for a CL system. In addition, a retarding field analyzer and sensitive TimePix electron detector were installed and tested at the same SEM. The combination of components will enable novel PINEM experiments in a scanning electron microscope (SEM). Moreover, we are developing a system based on a beam modulator and fast electron detector. Coupled to a spectrum analyser, radio-frequency modulated electron excitation will become possible for spectral signatures in the ultralow-energy range. In addition, electron-driven photon sources were designed and implemented in a SEM allowing for correlative electron-photon measurements without using an external light source.
