Firstly, a number of technical implementations had to be performed to prepare the experiments designed by the research project.
On the instrumentation front, we needed to enhance the performances of our Transmission Electron Microscope (TEM) by installing a high-sensitivity CMOS camera and a new laser system capable of generating radiation with tunable wavelengths.
Regarding the materials to be investigated, our project planned to explore a variety of samples, ranging from simple metals and graphene to strongly correlated insulators and superconductors. We designed nanostructures of these materials to achieve efficient coupling with electromagnetic radiation.
According to the original plan, the first two and a half years were allocated for upgrading and characterizing the instrument, including the installation of the new laser and detector. With these new tools in place, we scheduled initial experiments during this period on metallic surfaces, the topological magnet Cu2OSeO3, and VO2 nanowires.
The new laser system has been successfully installed in our laboratory. The infrared (IR) radiation beam path has been constructed, shielded, and channeled through vacuum pipes to prevent water absorption at sensitive wavelengths.
We successfully installed and characterized the K2 CMOS detector from Gatan, significantly enhancing the Lorentz imaging capabilities of our instrument.
In the new upgraded instrument, implemented as described in the original project, we carried out the following experiments:
Skyrmions Manipulation by Light:
1) We demonstrated that individual circularly polarized light pulses, tuned below the bandgap of the Mott insulator Cu2OSeO3, can induce controlled rotation of the skyrmion lattice in nanofilms of the material. These results have been published in Phys. Rev. X and have been a subject of a press release. We presented these data as an invited seminar at a Gordon conference and other international venues and summer schools.
2) We also have shown that specific light excitation conditions can create skyrmions in regions of the phase diagram where they do not naturally exist in equilibrium. These results have important implications for the potential application of skyrmions in spintronic devices. These results have been published in Advanced Materials and have been a subject of a press release. We presented these data as invited seminars at international conferences and summer schools.
Engineering Light Fields Confined on Surfaces:
Nanoconfined light is essential for providing spatially selective excitation in this project. Therefore, characterizing the interaction between the excitation laser, imaging electrons, and metallic surfaces is crucial.
3) During this period, we collected data to model the transient evolution of photoinduced charges at the interface between a metallic surface and vacuum. Unexpected dynamics of photoemitted charges were observed, and we reported two articles describing the nanoscale ultrafast dynamics of photoexcited carriers at the surface of a metal and the emitted plasma interacting with them. These manuscripts were published in ACS nano and have been presented at international conferences.
Using Engineered Light to Switch Quantum Materials:
4) In VO2, the metal-insulator transition can be triggered by infrared light pulses, with changes in conductivity corresponding to optical property changes within a few hundred femtoseconds. VO2 nanostructures have been considered for ultrafast switches, but the switching behavior of individual nanoparticles has never been characterized due to the need for both femtosecond and nanometer resolution. By confining a plasmonic near-field on the surface of an individual VO2 nanowire using visible light pulses, we characterized the field using the PINEM effect in our TEM, providing insights into the dielectric properties of the nanowire. We then performed a three-pulse experiment, using one light pulse in combination with an electron pulse to characterize the nanostructure's dielectric function, while another pulse induced the metal-insulator transition. This approach allowed us to achieve the first characterization of the switching properties of an individual VO2 nanowire. These results have been published in Nature Communications
