Periodic Reporting for period 1 - NanoLight-QD (Novel molecular spectroscopies by nanoconfined light shaping and ab initio quantum dynamics)
Reporting period: 2020-04-01 to 2022-03-31
The possibility to localize light below the diffraction limit, in particular by surface plasmons of metal nanostructures, already has a large number of applications in sensors, energy and catalysis, among other areas. Understanding the microscopic details and developing efficient computational tools that account for light-matter interactions at an atomistic level, including electronic, nuclear and photonic degrees of freedom in a fully self-consistent way, can unleash still unknown power of these system. This project aims to contribute to such a deep, microscopic understanding, by combining computational electrodynamics and quantum dynamics from first principle, to explore the confinement of electromagnetic fields down to the atomic scale. Due to the nature of plasmonic states, plasmon-induced processes are normally tackled either by solving Maxwell’s equations coupled to very approximate matter descriptions, or by looking only at the matter properties and prescribing electromagnetic fields. Bridging the gap between these two approaches is not an easy task due to the multiscale nature of these phenomea.
We implemented methods to couple Maxwell’s equations with matter at different levels of approximation, and we applied them for three areas: 1) nanoscale generation of light with orbital angular momentum (OAM) in real space and real time; 2) light-driven spectroscopy and microscopy with sharp tips used in scanning tunneling microscopes (STM); 3) quantum point contact formation in 2D materials, namely WSe2. We implemented the different light-matter coupling methods in the free, open-source package Octopus, making use of its new multisystem framework that was developed alongside this project together with the Octopus team, which solves the dynamics for any number of arbitrary physical systems. For the Maxwell propagation we harnessed the Schrödinger form of Maxwell’s equations in Riemann-Silberstein representation, and for the matter systems we considered linear media as well as ab initio level within time-dependent density functional theory (TDDFT).
We implemented methods to couple Maxwell’s equations with matter at different levels of approximation, and we applied them for three areas: 1) nanoscale generation of light with orbital angular momentum (OAM) in real space and real time; 2) light-driven spectroscopy and microscopy with sharp tips used in scanning tunneling microscopes (STM); 3) quantum point contact formation in 2D materials, namely WSe2. We implemented the different light-matter coupling methods in the free, open-source package Octopus, making use of its new multisystem framework that was developed alongside this project together with the Octopus team, which solves the dynamics for any number of arbitrary physical systems. For the Maxwell propagation we harnessed the Schrödinger form of Maxwell’s equations in Riemann-Silberstein representation, and for the matter systems we considered linear media as well as ab initio level within time-dependent density functional theory (TDDFT).
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.
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.
Following up on the results already described, we are moving forward in all the areas explored in this project, thanks to the solid network of collaborators. We are currently working on interaction of twisted-light with matter at different levels of approximation, with the aim to reach a self-consistent Maxwell-matter full minimal coupling, that allows us to consider spatially inhomogeneous electromagnetic fields, thus going beyond the dipole approximation. A team dedicated to the interaction of twisted-light beams, ranging from X-ray to optical frequencies, will explore the properties of matter under these fields with OAM, which can lead to new selection rules and new exotic states. Moreover, we continue to study the plasmonic near-fields and its interaction with matter in several systems, coupling the induced fields self-consistent with vibrational and electronic excitations to explore strong coupling in cavities, as well as to understand the mechanism behind enhanced spectroscopy techniques such as TERS in the time domain, which our Maxwell-TDDFT formalism enables. Finally, we continue to explore the coupling of electrostatic gates to 2D materials, both in the ground state and coupled to light, to find ways to access new states of matter than can have implications in fundamental physics and quantum technologies.