Periodic Reporting for period 4 - eNANO (FREE ELECTRONS AS ULTRAFAST NANOSCALE PROBES)
Berichtszeitraum: 2023-06-01 bis 2023-11-30
In the eNANO project, we aim at improving our capabilities for using electron beams to explore the nanoworld with ever better resolution in space and time, but also in the excitations supported by different nanostructures, such as optical modes that are relevant in their interaction with light. Given the complexity of this subject, the project has a theoretical nature, whereby we employ state-of-the-art analytical and computational methods to extend our understanding of the interaction between free electrons and nanomaterials to make it possible the design of improved electron microscopes. Specifically, we target the following specific aspects of such interaction: (1) For relatively slow electrons, their behaviour is affected by quantum aspects of the sampled materials that can help us learn about their dynamics, but in this regime a brand new theory is needed to account for the evolution of the complex electron-sample system; (2) We aim to push the limits of resolution with which we can image nanostructures down to the atomic scale, and specifically the resolution in space down to sub-Angstrom details, in time below the femtosecond, and in energy below the millielectronvolt; (3) We are interested in exploring the possibility of using a single electron to excite and at the same time sample the evolution of the created excitations in a specimen, thus pushing the sensitivity of electron microscopes to the single-electron level; (4) Free electrons provide an excellent tool to explore relatively inaccessible aspects of vacuum, which is in fact a complex entity that displays fluctuations and affects objects placed in it through quantum effects emanating as forces and non-contact energy transfer; (5) Besides free electrons in traditional electron microscopes, we envision the use of ballistic electrons in two-dimensional materials to realize an integrated version of electron energy-loss spectroscopy, with potential application in ultrasensitive detection in all-electrical devices. Beyond their intrinsic interest from a fundamental viewpoint, the results from this research project should improve our ability to design more precise microscopes, which ultimately will enable our ability to understand the nanoworld, and therefore also our potential for producing better devices in consumer products that should impact the society at large.
In the initial phase of eNANO, we achieved significant milestones in our research. In particular, (1) we developed an advanced quantum-mechanical theory elucidating the interaction among free electrons, light, and optical excitations in nanostructures. This comprehensive theory surpassed prior art, revealing surprising effects related to electron wave function coherence in transverse and longitudinal directions relative to the electron beam. These effects are crucial for designing time-resolved spectral imaging systems. (2) Our theory was applied to explore novel approaches for enhancing resolution in electron microscopy, specifically through combined ultrafast electron and light pulses. (3) We successfully showed that the nonlinear response of nanomaterials can be retrieved using light-electron interactions with unprecedented spatial resolution provided by the electron beam. (4) We proposed a new avenue of research, predicting nonlinear effects due to strong interaction with slow electrons. This presents a unique opportunity to study nonlinear optical response independently of light. (5) We investigated the interaction between free electrons and quantum light, yielding exciting conclusions to probe photon statistics of quantum optical fields. (6) We introduced a novel theory describing the interaction between photon polaritons and free electrons from first principles. (7) We designed a compact chemical sensing device utilizing ballistic electrons in a two-dimensional semiconductor. (8) Collaboration with leading experimental groups facilitated the interpretation of new measurements and the explanation of physical effects related to free electron-matter interactions. In the subsequent phase, we made significant progress toward the outlined objectives. (9) We explained experimental results on ultrafast electron pulses interacting with laser-induced plasma in collaboration with specific groups. (10) Predictive simulations demonstrated the potential of current ultrafast electron microscopy for simultaneous spatiotemporal compression of electrons in the sub-fs and sub-nm regime. (11) In collaboration with the group of Kociak we achieved microelectronvolt energy resolution in EEGS. Finally, we extended theoretical methods to describe the production of electron-positron pairs from polaritons and gamma-rays. These achievements required the development of new theories at the intersection of many-body physics, quantum electrodynamics, and electromagnetism.
The project has progressed extremely well, with all aspects of the proposed research satisfactorily achieved and having produced results that go beyond the state of the art. In particular, (1) we have investigated slow electrons, for which we are developing a brand new theory that is needed to account for the evolution of the complex electron-sample system environment; (2) We have proposed several combinations of ultrafast electron and light pulses to gain resolution in electron microscopy, but also to explore previously inaccessible information, such as the nonlinear response of nanostructures with sub-nanometer precision; in this direction, we continue exploring new possible strategies for improving the spatial-temporal-spectral resolution of electron microscopes by breaking the uncertainty principle through the use of different particles to excite (electron/light) and probe (light/electron) the sampled materials; (3) Thanks to our recent theoretical developments, we now understand the role played by the free electron wave function in the interaction with optical excitations in nanomaterials, which can be summarized in the independence of the excitation probability on the wave function, but this effort has also generated the surprising result that the phase imprinted on the excitation does depends on wave function and therefore can be exploited to pursue the proposed pump-probe interaction at the single-electron level; (4) We have investigated the interaction of free electrons with vacuum fluctuations and concluded that these can lead to substantial reshaping of the electron wave function; (5) We have designed and theoretically explored a two-dimensional version of electron microscope with energy-loss analysis capabilities based on the use of ballistic electrons in two-dimensional semiconductors. Besides these goals, we have further explained experimental results for the interaction of ultrafast electron pulses with laser-induced plasma in collaboration with the groups of Fabrizio Carbone and Ido Kaminer; we have demonstrated through predictive simulations that current ultrafast electron microscopy can be used to produce a simultaneous spatiotemporal compression of electrons in the sub-fs and sub-nm regime; we have collaborated with the group of Mathieu Kociak to demonstrate microelectronvolt energy resolution in EEGS, following ideas expressed in the project; and we have extended the theoretical methods noted above to describe the production of electron-positron pairs out of polaritons and gamma-rays.