For (1), we time-propagated the electromagnetic field as it interacts with an Archimedean nanospiral. It is known that the interaction of circularly polarized pulses with Archimedean spirals generate light with OAM that resembles a Bessel beam, where the final OAM depends on the pulse helicity and the geometry of the spiral. However, the way angular momentum is transferred from one system to the other remains unknown. Here, we found that it is a combination of geometrical factors and electromagnetic properties of the material that define the OAM of the resulting field, which we resolve in real space, time and frequency domains. Understanding the microscopic details of this phenomenon opens several avenues for light shaping and electromagnetic design by optimizing the structures needed to maximize given properties of the fields. These results will be presented in 2 conferences this year and a manuscript is under preparation.
In application (2), we tackled different projects motivated by collaborations with experimental groups. In particular, we simulated the near-field dynamics in STM cavities to understand the effect of matter systems in the cavity. With a group at the U Regensburg, we studied the near-field waveform probed by a molecular switch that has a conformational change that depends on the electron tunneling, which in turn is sensitive to the near field. We explored the effect of molecular polarization and electron tunneling in the probed near field. These results have been published in Nature Photonics. Then, together with colleagues at the FHI, we explored the tip-enhanced Raman scattering (TERS) in a plasmonic STM cavity with a single silver adatom, to understand the interplay between near-fields, light-driven current and atomic vibrations. We found that not only the atomistic structure of the tip apex but also its chemical interaction with the substrate strongly affect the near-field properties and the TERS intensity. The article accounting for these findings has been already submitted. Finally, we developed a method that combines TDDFT with density functional perturbation theory to calculate the TERS spectra and TERS images in a fully atomistic way, calculating the frequency-dependent near field produced by the tip. The size and symmetry of the near field determines which vibrational modes of the molecule are enhanced. The first manuscript is under preparation, and in subsequent works we will explore applications of this tool as well as improvements by going beyond some of the approximations used.
Finally, for (3) we partnered with collaborators at Columbia University to understand conductance measurements at milikelvin temperatures, that indicate a quantum point contact in a monolayer of WSe2. We modeled a nanosheet of WSe2 consisting of 1200 atoms by density functional tight-binding and applied a parabolic potential produced by split-gates. We studied the electronic states responsible for the conductance plateaus and we are preparing a first manuscript and exploring the effects of spin-orbit coupling and doping. These results could have large implications in quantum materials and quantum simulators.
In addition to the aforementioned articles, the researcher presented these results in several online events. Dr. Bonafé will be presenting this results after the end of the fellowship in conferences and invited talks postponed due to the COVID.19 pandemic.
