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Time- and space- resolved ultrafast dynamics in molecular-plasmonic hybrid systems

Periodic Reporting for period 3 - QUEM-CHEM (Time- and space- resolved ultrafast dynamics in molecular-plasmonic hybrid systems)

Reporting period: 2021-05-01 to 2022-10-31

The aim of the project is to develop and apply methods and techniques to describe and investigate the complex time- and space-resolved dynamics of molecular-plasmonic hybrid systems interacting with external light fields. Interaction of such systems with external light fields leads to a complex interplay of physical processes: for the plasmonic system, the excitation of collective electron dynamics inside the metallic nanoparticles leads to strongly modified electromagnetic near-fields with complex polarization and spatial structure. Of course, also the molecular system could be independently polarized and excited, with induced electronic and/or vibrational dynamics. However, the combined system cannot necessarily be partitioned into two individual subsystems (molecule and plasmonic nanoparticle) as it features additional astonishing phenomena which only occur when both systems are in close proximity and may interact: the nanoparticle’s enhanced near field interacts with the molecular system and contributes to its polarisation and/or excitation. The induced dipole of the molecular system, in turn, interacts with a mirror dipole in the plasmonic nanoparticle, and this interaction will self-consistently modify the electromagnetic nearfield. Besides these electromagnetic (EM) aspects, the close proximity of the metal near the molecule modifies the molecular electronic structure at the quantum level (QU): new, charge-transfer states between the molecular and the metallic system may arise. These ingredients (QU+EM) lead to a plethora of new and exciting physical and chemical phenomena, such as the formation of new quasi-particles, new mechanisms for chemical reactions or the ultra-high spatial resolution and selectivity in molecular detection.

In order to model and investigate this exciting physics and chemistry, a combined description of the (static) quantum mechanical electronic structure of the molecular system in close vicinity of the plasmonic system (QU), the interaction with the electromagnetic field (EM) and also the induced complex dynamics of the hybrid system is necessary. Existing approaches to describe and investigate these complex physics and chemistry commonly focus on either an accurate description of the electronic structure of the hybrid systems or the electromagnetic properties of these systems. Only few approaches and works aim at a description of both, the quantum mechanical electronic structure and the simulation of the spatially inhomogeneous, time-dependent near field. And almost nothing is known about the field-induced complex time- and space-resolved dynamics of the hybrid systems – which lies at the heart of this project (QUEM-CHEM).

Besides being of fundamental interest, this interplay between near-fields and molecules is very promising for applications, potentially enabling revolutionary breakthrough in new emerging technologies in a broad range of research fields, such as nanophotonics, energy and environmental research, biophotonics, light-harvesting energy sources, highly sensitive nano-sensors etc. Thus, within this project, new approaches and methods beyond the state of the art are being developed, aiming at a synergistic description of existing but typically independently applied approaches and employ these to certain prototype applications and scientific questions in sensing/spectroscopy, plasmon-induced catalysis and the dynamics of new quasiparticles.
In order to reach the above described goal, novel multi-scale multi-physics methods need to be developed (part I). In part II, these methods are applied to pre-selected application questions. Concerning the methods section, part I, in the initial phase, we have parallelly investigated different “ingredients”: as planned, we have compared different existing methods to describe the electronic and/or vibrational dynamics induced by the interaction of the molecular system with strong(er) light fields: the whole spectrum of dynamical methods, purely quantum dynamical, classical and mixed quantum-classical dynamical methods has been employed to describe and simulate electronic and/or vibrational dynamics. In parallel, we have investigated and compared different existing program packages providing access to the calculation of the quantum chemical electronic structure of the combined molecular-plasmonic hybrid system. For comparison, we have compared different a large variety of quantum chemical methods and applied them already to some of the planned and envisioned application cases.

At the same time, we have started to investigate the first three exemplary science cases (part II): (1) the dynamics of new quasi-particles occurring due to the strong coupling of the plasmonic moiety and an excitonic system (the so-called “plexcitons”); (2) Plasmon catalysis: Two different plasmon-induced catalytic reactions have been investigated; the selection of which has been motivated by our collaboration partners. As observable, the systems’ characteristic Raman frequencies including the enhancement effect have been calculated. (3) Tip-enhanced Raman spectroscopy (TERS): we have investigated in detail the chemical effects contributing to the resolution in TERS, which suggest sub-nanometre spatial resolution, and extended it towards resonance excitation. We could show that resonance excitation not only further enhances the Raman signal by several orders of magnitude compared to non-resonant case.
Most of the work sketched above constitutes progress beyond the state of the art. Specifically, concerning the application part, a few highlights are selected in what follows:

Experimental evidence suggests an extremely high, possibly even sub-molecular, spatial resolution of tip-enhanced Raman spectroscopy (TERS). In a recent publication, we have presented a fully quantum mechanical description including non-resonant, resonant and charge-transfer chemical contributions. Our computational approach reveals that unique – non-resonant and resonant – chemical interactions among the tip and the molecule significantly alter the TERS spectra and are mainly responsible for the high, possibly Angstrom spatial resolution.

We expect the methods being currently developed and partially already applied to be of high importance for a broader science community. These methods and approaches will be generally applicable for a variety of scientific questions and may enable detailed investigation of astonishing physical and chemical phenomena, such as the plasmon-induced catalysis or the formation of new quasi-particles. The approaches and methods to be developed in this project will enable for the first time a self-consistent description of both, the quantum nature of the molecule-nanoparticle moiety and the complexity of near-field electrodynamics. The simulation of these effects is essential not only for the fundamental research but also in application to develop highly sensitive nano-shaped detection schemes and for broad range of applications in such fast growing and critically important research fields like nanophotonics, biophysics, light harvesting energy sources etc. Thus, the possible outcome of this truly interdisciplinary project will provide new knowledge both in physics and chemistry, and might have impact on large variety of new arising critical technologies.
Visualization of the molecular-plasmonic hybrid systems and their interaction with external light fi